QUEENSLAND UNIVERSITY OF TECHNOLOGY

SCHOOL OF PHYSICAL AND

CHEMICAL SCIENCES

Radiological Aspects of Petroleum Exploration

and Production in the Sultanate of Oman

Afkar Nadhim Al-Farsi

BSc (Hons), PGDip, MSc

A thesis submitted in partial fulfilment of the requirements of the degree of

Doctor of Philosophy

2008

i

Dedicated to my mother Sheikha Issa, and

To my late grant parents Jokha Ali and Issa Said

ii

Key words

NORM, radiological, petroleum, mining, dating, sludge farming, separation tanks, radium, thorium, radon, lead-210, actinium, potassium, gamma spectroscopy, gamma dose rate, 222Rn exhalation, sludge, oil scales, gas scales, evaporation pond sediment, ambient soil, Oman

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Abstract

This thesis is a study of naturally occurring radioactive materials (NORM)

activity concentration, gamma dose rate and radon (222Rn) exhalation from the

waste streams of large-scale onshore petroleum operations. Types of activities

covered included; sludge recovery from separation tanks, sludge farming,

NORM storage, scaling in oil tubulars, scaling in gas production and

sedimentation in produced water evaporation ponds. Field work was conducted

in the arid desert terrain of an operational oil exploration and production region

in the Sultanate of Oman.

The main radionuclides found were 226Ra and 210Pb (238U - series), 228Ra and

228Th (232Th - series), and 227Ac (235U - series), along with 40K. All activity

concentrations were higher than the ambient soil level and varied over several

orders of magnitude. The range of gamma dose rates at a 1 m height above

ground for the farm treated sludge had a range of 0.06-0.43 µSv h-1, and an

average close to the ambient soil mean of 0.086 ± 0.014 µSv h-1, whereas the

untreated sludge gamma dose rates had a range of 0.07-1.78 µSv h-1, and a mean

of 0.456 ± 0.303 µSv h-1. The geometric mean of ambient soil 222Rn exhalation

rate for area surrounding the sludge was 7.90.11.3 mBq m-2 s-1. Radon exhalation

rates reported in oil waste products were all higher than the ambient soil value

and varied over three orders of magnitude.

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This study resulted in some unique findings including: (i) detection of

radiotoxic 227Ac in the oil scales and sludge, (ii) need of a new empirical

relation between petroleum sludge activity concentrations and gamma dose

rates, and (iii) assessment of exhalation of 222Rn from oil sludge. Additionally

the study investigated a method to determine oil scale and sludge age by the use

of inherent behaviour of radionuclides as 228Ra:226Ra and 228Th:228Ra activity

ratios.

v

Contents Key words .................................................................................................................................... ii

Abstract ....................................................................................................................................... iii

Contents ....................................................................................................................................... v

List of Figures ........................................................................................................................... viii

List of Tables ............................................................................................................................... x

Statement of original authorship ............................................................................................. xii

Acknowledgements ................................................................................................................... xiii

CHAPTER 1 INTRODUCTION ............................................................ 1

1.1 Origin of petroleum ...................................................................................................... 1

1.2 History of NORM in the petroleum industry ............................................................. 2

1.3 Distribution of radioactivity in the petroleum exploration and production processes ........................................................................................................................ 3

1.4 Onshore operations ..................................................................................................... 11

1.5 Gaps in knowledge ...................................................................................................... 15

CHAPTER 2 LOCALITY AND OIL MINING ...................................... 17

2.1 The Sultanate of Oman ............................................................................................... 17

2.2 Mining sites .................................................................................................................. 18

2.3 The surrounding area ................................................................................................. 20

2.4 The oil mining process ................................................................................................ 21

2.5 Implications of Oman’s aging reservoirs .................................................................. 22

2.6 The future of oil exploration in Oman ...................................................................... 24

CHAPTER 3 SAMPLING AND MEASUREMENT TECHNIQUES .... 27

3.1 Introduction ................................................................................................................. 27

3.2 Dating of petroleum scale and sludge ........................................................................ 28

3.3 In-situ gamma spectroscopy ....................................................................................... 30

3.4 Laboratory gamma spectroscopy .............................................................................. 32 3.4.1 Sample collection and preparation ........................................................................... 32 3.4.2 Gamma spectroscopy measurement procedure ........................................................ 34

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3.5 Comparison between in-situ and laboratory gamma spectroscopy measurements .. ...................................................................................................................................... 37

3.6 In-situ gamma dose-rate measurements ................................................................... 40

3.7 Radon activity flux measurements using charcoal cups .......................................... 41

3.8 Radon exhalation rate measurements using the emanometer ................................. 43

CHAPTER 4 RADIOACTIVITY CONCENTRATION OF SCALE, SLUDGE AND SOIL SEDIMENT, FROM THE OIL FIELDS OF THE SOUTHERN OMAN DIRECTORATE .................................................... 47

4.1 Introduction ................................................................................................................. 47

4.2 Radioactivity in sludge ................................................................................................ 48 4.2.1 Sludge farming ......................................................................................................... 49 4.2.2 Radioactivity in ambient soil .................................................................................... 53 4.2.3 Radioactivity in the sludge recovered from a separator tank ................................... 55 4.2.4 Radioactivity in untreated piles at sludge farms ....................................................... 58 4.2.5 Radioactivity in treated sludge strips ....................................................................... 68

4.3 Bahja NORM store yard ............................................................................................ 76 4.3.1 Oil industry scales .................................................................................................... 78

4.3.1.1 Oil scale formation and removal ..................................................................... 78 4.3.1.2 Radioactivity in oil scales ............................................................................... 81

4.3.2 Gas industry scales ................................................................................................... 87 4.3.2.1 Gas scale formation and removal .................................................................... 87 4.3.2.2 Radioactivity in gas scales .............................................................................. 88

4.3.3 Comparison between oil and gas scales ................................................................... 91 4.3.4 Radioactivity in the sludge stored in barrels ............................................................ 92

4.4 Radioactivity in the sediments of Al-Noor evaporation ponds ................................ 96

4.5 Discussion and conclusions ....................................................................................... 100

CHAPTER 5 GAMMA DOSE RATES AT SLUDGE FARMS IN OILFIELDS OF THE SOUTHERN OMAN DIRECTORATE ................ 105

5.1 Introduction ............................................................................................................... 105

5.2 Terrestrial and cosmic gamma dose rates ............................................................... 105 5.2.1 Gamma dose rates in the petroleum industry ......................................................... 109

5.3 Materials and Methods ............................................................................................. 111

5.4 Results and discussion .............................................................................................. 112 5.4.1 Correlation between measured and predicted gamma dose rate ............................. 112 5.4.2 Development of a new gamma dose rate empirical model ..................................... 113 5.4.3 Gamma dose rate measurements ............................................................................ 117 5.4.4 Combining synthesised and measured gamma dose rates ...................................... 127

5.5 Conclusions ................................................................................................................ 130

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CHAPTER 6 RADON-222 EXHALATION FROM PETROLEUM INDUSTRY SCALE, SLUDGE AND SEDIMENT ................................ 133

6.1 Introduction ............................................................................................................... 133

6.2 Materials and methods ............................................................................................. 137

6.3 Results and discussion .............................................................................................. 138 6.3.1 Rn-222 exhalation rates .......................................................................................... 138

6.4 Conclusions ................................................................................................................ 153

CHAPTER 7 SUMMARY AND CONCLUSIONS ............................. 155

7.1 Summary .................................................................................................................... 155

7.2 Future directions ....................................................................................................... 167

References ................................................................................................................................ 170

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List of Figures Figure 1.1 Schematic on the precipitation of scales and sludge in production plant

and equipment where T: tubular, V: valves, W: wellheads, P: pumps, S: separation tank, H: water treatment vessel, G: gas treatment, O: oil storage tank ........................................................................................................................ 4

Figure 1.2 Primordial radioactive decay series (a) 238U, (b) 232Th and (c) 235U ........ 7 Figure 1.3 Produced water disposal in shallow, deep and producing reservoir wells

.............................................................................................................................. 10 Figure 1.4 Map of the Sultanate of Oman with Petroleum Development Oman’s

concession land ................................................................................................... 12 Figure 2.1 The five major oilfields studied during this research, all located within

PDO’s Southern Oman Directorate .................................................................. 19 Figure 2.2 Typical bedew rooming on their camels (picture courtesy of Trek Earth

http://www.trekearth.com/gallery/Middle_East/Oman/page19.htm) ............ 20 Figure 2.3 Daily oil, condensate and gas production in Oman over the last 11

years. .................................................................................................................... 23 Figure 3.1 The relative activity ratio of 228Th/228Ra and the relative decay of 228Ra

.............................................................................................................................. 29 Figure 3.2 (a) In situ gamma spectroscopy, and (b) A lead shield (designed and

poured at QUT) for shielding the NaI(Tl) probe of the portable gamma spectroscopy system. ........................................................................................... 31

Figure 3.3 Correlation between field portable NaI(Tl) and laboratory HPGe activity concentration readings for: (a) 226Ra Field vs 226Ra Lab, (b) 228Th Field vs 228Th Lab, (c) 40K Field vs 40K Lab and (d) 228Th Field vs 228Ra Lab............................ 39

Figure 3.4 Charcoal cups planted on a sludge pile ................................................... 42 Figure 3.5 (a) Schematic diagram of the emanometer, and (b) The emanometer in

its wooden box housing ...................................................................................... 44 Figure 3.6 (a) Emanometer’s airtight PVC sample chambers with FESTO valves

and Perspex cover, and (b) a sludge sample wrapped in perforated textile material, labelled and ready for 222Rn counting .............................................. 45

Figure 4.1 The sludge farming process: (a) sludge removed from a separation tank, (b) untreated sludge piles, (c) sludge piles after transport to the farming area, (d) a typical sludge strip, (e) watering the sludge strips, and (f) tilling the sludge strips. ................................................................................................. 52

Figure 4.2: Relation between 228Ra:226Ra and 228Th:228Ra activity ratios ............... 58 Figure 4.3: Relation between 226Ra and 228Ra for Marmul, Bahja and Nimr

untreated sludge piles. ........................................................................................ 65 Figure 4.4: 228Ra:226Ra and 228Th:228Ra mean activity ratios for Bahja, Nimr and

Marmul sludge farms. ........................................................................................ 66 Figure 4.5: Sludge pile activity concentration versus age (a) 226Ra, and (b) 228Ra. 67 Figure 4.6: NORM store yards: ................................................................................. 77 Figure 4.7: Average activity concentrations for radionuclides found in oil and gas

scales. ................................................................................................................... 92

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Figure 4.8: Section 2 of Al-Noor evaporation pond [picture courtesy of Mohammad Al-Masri] ....................................................................................... 97

Figure 5.1 Relation between measured and predicted gamma dose rates using UNSCEAR (2000) dose conversion factors. ................................................... 113

Figure 5.2 Relation between measured and empirically determined gamma dose rates. ................................................................................................................... 116

Figure 5.3 3D graph of gamma dose rate relation with both 226Ra and 228Ra activity concentrations. .................................................................................... 126

Figure 5.4 Measured and predicted gamma dose rate profiles at a 1 m height for untreated sludge piles and treated strips at Bahja, Nimr and Marmul sludge farms (where n denotes the total number of samples at each location). ...... 128

Figure 6.1 Emanometer to charcoal cup readings correlation .............................. 139 Figure 6.2 222Rn exhalation rate versus 226Ra activity concentration ................... 144 Figure 6.3 222Rn exhalation rate range and averages for the various sample types

............................................................................................................................ 150 Figure 6.4 Ratio of 222Rn exhalation rate to 226Ra activity concentration range and

averages for the various sample types ............................................................ 151

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List of Tables Table 1.1 Typical 226Ra and 228Ra activity concentrations for various primary

production and power generation industries according to APPEA activity concentrations (kBq kg-1) .................................................................................. 14

Table 4.1: Bahja, Nimr and Marmul sludge farm locations, estimated volume of untreated sludge in piles and number of treated sludge Strips at the time of this study (Jan 2006 – June 2007) ..................................................................... 50

Table 4.2: Activity concentration (Bq kg-1) for ambient soils of Bahja, Al-Noor, Nimr and Marmul and the world average (UNSCEAR, 2000) (uncertainties represent counting error). .................................................................................. 54

Table 4.3: Activity concentration (Bq kg-1) and individual reading error of freshly removed sludge from a Nimr station separator tank ...................................... 56

Table 4.4: Sludge activity concentrations (Bq kg-1) and radioisotope ratios from Bahja, Nimr and Marmul untreated sludge piles ............................................ 60

Table 4.5: Activity concentrations (Bq kg-1) of untreated Bahja sludge piles ........ 61 Table 4.6 Activity concentrations (Bq kg-1) of untreated Nimr sludge piles

(analysed using the HPGe gamma spectroscopy system). ............................... 63 Table 4.7 Activity concentrations (Bq kg-1) of untreated Marmul sludge piles

(using the HPGe gamma spectroscopy system). .............................................. 64 Table 4.8 Activity concentration (Bq kg-1) of Bahja sludge strips. ......................... 71 Table 4.9 Activity concentrations (Bq kg-1) of Nimr sludge strips. ......................... 72 Table 4.10 Activity concentrations (Bq kg-1) of Marmul sludge strips. .................. 74 Table 4.11 Committed effective dose coefficients (µSv Bq-1) of selected

radionuclides likely to be present in petroleum scales (ICRP68) ................... 84 Table 4.12 Activity concentrations (Bq kg-1) of oil scale samples. .......................... 86 Table 4.13 Activity concentrations (Bq kg-1) of gas scale samples. ......................... 90 Table 4.14 Range of sludge 226Ra and 228Ra activity concentrations (kBq kg-1) for

oil exploration operations of several countries of the world ........................... 94 Table 4.15 Activity concentrations (Bq kg-1) for sludge stored in barrels. ............. 95 Table 4.16 Activity concentrations (Bq kg-1) of Al-Noor evaporation pond soil

sediments. ............................................................................................................ 99 Table 5.1 Air Kerma rate at 1 m height per disintegration rate (nGy h-1 per

Bq kg-1) of the parent nuclide per unit soil weight for natural sources uniformly distributed in the ground (adapted from Saito and Jacob). ....... 108

Table 5.2 Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul untreated sludge piles .......... 120

Table 5.3 Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul treated sludge strips ............ 122

Table 5.4 Summary of field measured gamma dose rates (µSv h-1) at 1 m height for Bahja, Nimr and Marmul petroleum sludge treatment farms’ .............. 125

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Table 5.5 Number of samples, mean, median and range of the dose rates for untreated and treated sludge at Bahja, Nimr and Marmul obtained by both direct measurement and empirical relation. .................................................. 129

Table 6.1(a): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for various petroleum industry radioactive waste. ............................... 140

Table 6.1(b): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for various petroleum industry radioactive waste (emanometer measurements only). ......................................................................................... 142

Table 6.2: Arithmetic mean, geometric mean maximum and minimum 222Rn exhalation rates, and arithmetic and geometric means of 222Rn:226Ra ratio for the various samples........................................................................................... 148

Table 7.1 (a): Range median and mean activity concentrations of 226Ra, 210Pb, 228Ra, 228Th, 227Ac and 40K in Bq kg-1, for the various sample types analysed in this study ........................................................................................................... 159

Table 7.1 (b): Mean (± standard deviation), median and range of gamma dose rates in µSv h-1 for untreated and treated sludge in Bahja, Nimr and Marmul sludge farms, and ambient soil readings ........................................................ 162

Table 7.1 (c): Maximum, minimum and geometric mean of 222Rn exhalation rates in mBq m-2 s-1 and the geometric mean of radon exhalation to radium concentration ratio in mBq m-2 s-1/Bq kg-1 for the various sample types analysed in this study ...................................................................................... 164

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Statement of original authorship

The work contained in this thesis has not been previously submitted for a degree or a diploma at any other higher education institution. To the best of my knowledge and belief, the thesis contains no materials previously published or written by another person except where due reference is made. Signature: Date: 17 December 2008

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Acknowledgements In the name of God, most Gracious, most Merciful. I initially would like to start by thanking God for the gift of life and for empowering his powerless creatures to realise (with hard work and dedication) their aspirations and dreams. Then I would like to have the honour of mentioning in my humble thesis the name of the greatest human being ever to walk on earth; “Mohammad” peace be upon him, and use one of his wise quotes: “He does not thank Allah, who does not thank people”. The success in carrying out various aspects of this work would not have been possible without the generous contributions in funding and volunteer support provided by many people. My heartfelt appreciation is given to all the persons involved. In the event of these acknowledgements failing to mention the names of any particular person(s) or organisation(s), I offer my sincerest apologies. I would like to thank Sultan Qaboos University for granting me the sabbatical to embark on the doctorate research degree, and also for allowing me use of Laboratory Instruments to analyse field collected samples of this research work. I would like to thank Queensland Univesity of Technology (QUT) for granting me the PhD Fee Waiver Scholarship, and for bearing the financial costs of the research’s instruments purchase and development along with international shipment between Brisbane (Australia) and Muscat (The Sultanate of Oman). I also would like to extend my thanks to Petroleum Development Oman (PDO) for the collaboration and logistic support during the research work, which included air transportation to and from the petroleum mining sites, land transportation within the mining sites, accommodation and meals. Not forgetting the special transportation arrangements made for the instruments by a cargo truck between Muscat and the desert mining locations. QUT academics, professionals and fellow students: Special thanks and gratitude are due to my principal supervisor: Dr. Riaz Akber for being my mentor, for his guidance, patience, expert feedback and enthusiastic help and support. Thanks are also due to my associate supervisor: Professor Lidia Morawska for being my mentor, for her guidance and encouragement. I am truly highly indebted to A/Professor Brian Thomas for his instrumental role in my enrolment at QUT, continuous encouragement, advice, help and support throughout my Master and Doctorate degrees. A lot of thanks are also due to my Master degree principle supervisor: Dr David Thiele for being a great mentor and bringing me up to the PhD degree doorstep. Thanks are also due to my Master degree associate supervisor: Dr Gregory Michael for his expert advice and support. Thanks and gratitude are also due to Ms. Rachael Robinson for the professional proof reading and Ms. Kerry Kruger for the final styling organisation of the thesis. Thanks are also due to the professional support of Mr. Jaya Dharmasiri, Mr. John Barrett, Ms. Elizabeth Stein, Ms. Magaret McBurney, Mr. Robert Organ and Mr. James Drysdale. I would also like to thank all my fellow students for their friendship, support and sharing educational experiences and especially: Hussein Kanaani, Yudi Wardoyo, Sade Fatokun, Diane Keogh, Mohammad Al-Roumy, Ezzat Abu Azza and Javaid Khan.

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SQU academics and professionals: Many thanks are due to A/Professor Fadhil Mahdi Saleh, for his advice and recommendations on my career planning and development. Thanks are also due to Professor Lamk Al-Lamki, for his continuous support, help and encouragement. Many thanks to my colleagues; Dr Haddia Bererhi, Dr Nadir Atari, Mr Kirthi Jayasekara, Mr Hilal Al-Zheimi, Mr Mohammad Al-Subhi, Mrs Fatma Al-Maskery, Mrs Ibtisam Al-Maskery and Mrs Amaal Al-Rasby for their support and understanding with sharing the department’s facilities during this study’s sample analysis work. PDO professionals: Many thanks are due to Mr. Naaman Al-Naamany, Mr. Brett Young, Mr. Ahmed Al-Sabahi for their logistic support of the project and planning, providing transportation, accommodation and meals during the field work. The list of names for the people who offered volunteer help during the field work visits on the different locations is too long; to name a few: Mr. Said Saud Al-Maawaly, Mr. Salim Al-Rawahy, Mr. Nasser Abdullah Al-Kitany, Mr. Rashid Said Shinoun, Mr. Salim Al-Riyami, Mr. Rashid Al-Zakwani, Mr. Ahmed Al-Jabri and Mr. Seif Al-Habsy. To all the above and the others, I would like to say thank you for your help and support during the field visits. Other professionals: Many thanks are also due to Professor Mohammad Al Masri (Syrian Atomic Energy Agency) for advice and academic enrichment on the study during the field work, and collection of the sediment samples. Thanks are also due to Mr. Gert Jonkers (Shell Company) for providing us with bibliography on petroleum industry NORM. I would also like to thank Mr. Ibrahim Awad for his advice and help during sample collection. Family and friends: Thanks are also due to all my friends. Special thanks to all my family members for their support and encouragement. Finally, I would like to offer my sincere appreciation to my beloved wife: Ahlam Al-Adawi, for her relentless support, encouragement and sacrifice through this ostensibly jam-pact period of our lives.

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Chapter 1 INTRODUCTION

1.1 Origin of petroleum

There are currently two plausible scientific theories that explain the process

of oil formation. The first is the biotic or biogenic theory, which states that oil

was formed hundreds of millions of years ago, following the extinction of

dinosaurs (i.e. terrestrial reptiles of the Mesozoic era) and algae that inhabited

the earth some 65-248 million years ago. The remaining organic matter was

then buried under many layers of sediment, and was exposed to high levels of

litho-spherical heat and pressure, which then transformed this preserved matter

into hydrocarbons (black gold or oil). According to geologists, this process is

thought to occur amid the earth’s solid rock layers, at temperatures ranging from

80-350 °C and pressures ranging from 0.8-2 kbar (Dyer and Graham, 2002,

Dutkiewicz et al., 2003). The oil then migrates and remains in porous stones

(such as limestone or sandstone, which have a porosity of about 20%) until it is

discovered.

The second theory is known as the abiotic or abiogenic theory, which states

that oil is not a fossil fuel, but that it was formed from inorganic materials deep

within the earth’s crust. According to this theory, hydrogen and carbon

molecules found in the earth’s mantle are subject to extremely high

temperatures and pressures, causing them to form hydrocarbon molecules,

which then migrate upward into oil reservoirs, through deep fracture networks

in the earth’s crust. Supporters of the abiogenic theory claim that the millions of

barrels of oil produced per day (1 barrel = 159 L) could not possibly be supplied

2

by the limited number of pre-historic animals (algae and dinosaurs) that existed.

For example, according to Morton (2004), the most productive oil well in Saudi

Arabia (Al-Ghawar) produces about 5x106 barrels of crude oil per day, with a

cumulative production of 5.5x1010 barrels of crude oil since 1951. Although

there is support for both theories, no one is mutually exclusive from the other,

and both may be equally as valid.

1.2 History of NORM in the petroleum industry

Kolb and Wojcik (1985) provide an interesting account of the discovery of

naturally occurring radioactive materials (NORM) in petroleum. The presence

of higher than background concentrations of radioactivity in crude petroleum

was reported for the first time more than a century ago, by Himstedt (1904) and

Burton (1904). In the 1920-1930’s, the presence of NORM was also reported in

numerous Russian and German research papers, however the first official

survey, from a radiation protection point of view, was not done until the early

1970’s.

Subsequent to the detection of NORM in a North Sea oil platform in 1981,

the presence of NORM in crude petroleum and petroleum industry waste has

been studied and reported by a number of authors worldwide, including Kolb

and Wajcik (1985), Smith (1987), Wilson and Scott (1992), Heaton and

Lambley (1995), Paschoa (1997), White and Rood (2001), Matta et al. (2002),

Godoy and Petinatti da Cruz (2003), Hamlat et al. (2003a), Hamlat et al.

(2003b), Smith et al. (2003), Al-Masri and Aba (2005) and Gazineu and Hazin

(2007).

3

In the petroleum industry, naturally occurring radionuclide concentrations

are often enhanced as a result of industrial operations. Whilst these materials are

formally referred to as technologically enhanced NORM (TENORM), the term

NORM is more widely used in both industry and the literature.

Examples of other industrial operations that have NORM present in their

primary material, products, by-products and waste include: uranium mining and

milling, metal mining and smelting, phosphate industries, coal mining and

power generation from coal, rare earth and titanium oxide industries, zirconium

and ceramics industries, building material disposal and the application of natural

radionuclides, such as radium and thorium. Whilst the activity levels of these

NORM are not always enhanced, simple chemical or physical changes can

sometimes take place, resulting in the radionuclides being more readily

available for transfer by various pathways (Heaton and Lambley, 1995,

UNSCEAR, 2001). For example, in uranium mining, simply bringing the

uranium to the surface can often leave radionuclide tailings, that may pose a

radiological threat to both the environment and the general public.

1.3 Distribution of radioactivity in the petroleum exploration and production processes

During the process of oil exploration and production (E&P), radioactive by-

products, such as scale and sludge, are formed and retained in the processing

equipment (Figure 1.1). Scales are formed in the electric submersible pumps,

down-hole tubular, upstream tubular and well heads (Testa et al., 1994, Al-

Masri and Aba, 2005, Othman et al., 2005), while its brittle nature can also

4

cause it to dislodge from the pipe walls and migrate to the oil-water separation

tanks.

Sludge, on the other hand, is found in the downstream tubular, water-oil

separation vessels, slops tanks of oil production facilities and storage tanks, and

as a result of the processes of tubular cleaning using ‘pig’ devices. It contains a

mixture of hydrocarbon, mud, natural radionuclides, sediments, bacterial

growth, corrosion particles and some scale debris (APPEA, 2002, Omar et al.,

2004). While the activity of radium in sludge is generally lower than that found

in scale (Vandenhove, 2002), it has still been found to be significantly above

background levels.

Figure 1.1: Schematic on the precipitation of scales and sludge in production plant and equipment where T: tubular, V: valves, W: wellheads, P: pumps, S: separation tank, H: water treatment vessel, G: gas treatment, O: oil storage tank.

S

H

O

G P

W

V

Scale

T

Formation water, oil and gas

DownstreamUpstream

Sludge

210Pb film

5

The estimated annual radium activity brought to the surface by global oil

exploration is in the order of 10 TBq (Lieser, 1995). This not only poses

significant health risks to the industry workers, but also to the community and

environment as a whole. In the United States, they are spending about

US$ 6 billion per year on the clean up and containment of such radioactive

waste (Harley, 2000).

The NORM found in scale and sludge are mainly from the 238U (T½: 4.5

billion years) and 232Th (T½: 14 billion years) natural radioactive decay series

(Figures 1.2 (a) and (b)). In contrast, Ac-227 (T½: 21.77 years) from the 235U

(T½: 0.7 billion years) natural radioactive decay series (Figure 1.2 (c)) was

detected for the first time during this research, and prior to this, it was only ever

mentioned in passing by Kolb and Wajcik (1985). The main isotopes found in

scale and sludge are those of 226Ra (T½: 1602 years; 238U primordial series), and

to a lesser degree 228Ra (T½: 5.75 years; 232Th primordial series). These two

radium isotopes are present as both sulphates and carbonates in the strontium,

barium and calcium mineral scales that develop in the tubular and other areas of

the extraction rigs (Wilson and Scott, 1992, Hamlat et al., 2001, Godoy and

Petinatti da Cruz, 2003, Al-Masri and Aba, 2005).

In the petroleum reservoirs, crude oil co-exists with underground water,

usually called ‘formation water’ or ‘formation brine’. While the initial oil

production process is usually dry (Smith, 1987), as the reservoir pressure falls

over time, water can also be co-produced with the crude oil, and this water is

given the name ‘produced water’. The amount of NORM formed in oil

6

producing fields and incorporated into the scale and sludge, is directly

proportional to the volume of produced water generated during the pumping of

the oil (Rood et al., 1998, Paranhos Gazineu et al., 2005).

238U

234Th

234Pa

234U

230Th

226Ra

222Rn

218Po

214Pb

210Po

210Bi

214Po

214Bi

206Pb 210Pb

Leach from reservoir rock into formation water

Transported with natural gas

α α

α

α

α

α α α

4.5 billion years

β β

ββ

β

β 24 days

1.2 minutes 240 years

77,000 years

1,602 years

3.8 days

3.1 minutes

27 minutes

20 minutes 160 micro-seconds 5 days

22.3 years

140 days

Precipitate on internal surfaces of petroleum equipment

(a)

7

Figure 1.2: Primordial radioactive decay series (a) 238U, (b) 232Th and (c) 235U.

(b)

310 nano- seconds

232Th 228Ac

228Ra

220Rn

224Ra

228Th

212Pb

216Po

208Tl

212Po

208Pb

212Bi

Leach from reservoir rock into formation water

Transported with natural gas

α 14 billion years

β 5.75 years

β6.1 hours

α

α

αα

α

α 1.9 years

3.7 days

56 seconds

0.15 seconds

β

β

β 11 hours

61 minutes (64%)

61 minutes (36%)

3.1 minutes

Precipitate on internal surfaces of petroleum equipment

235U

231Pa 231Th

α 700 million years

β 26 hours

1.8 milliseconds

219Rn

223Ra

227Th

211Pb

215Po

α

α

α

α 19 days

11 days

4 seconds

β36 minutes

207Tl

207Pb

211Bi

α

α

β

β22 years

(99%)

2.1 minutes

4.8 minutes

227Ac

223Fr

α

33,000 years

22 years (1%)

β22 minutes

Detected in oil and gas scales

(c)

8

The major radionuclides found in produced water include 226Ra, 228Ra, 224Ra

and 210Pb, in concentrations of up to a few hundred becquerels per litre (IAEA,

2003). This is because radium isotopes leach into the oil reservoir, as a result of

their high solubility in water, when compared to uranium and thorium. The high

temperature and pressure in the oil reservoirs also aid in the leaching of radium

from reservoir rock into the formation water. Because geological formations are

not always closed, this may allow 226Ra to migrate into the soil matrix and deposit

elsewhere outside the formation. If this takes place, the secular equilibrium of

226Ra with its parent will no longer exist, and the radium is then said to be

“unsupported”, meaning that its activity is not related to the activity of its series

predecessors (Paranhos Gazineu et al., 2005).

The produced water associated with the explored oil is usually saline,

containing potentially high levels of mineral salts, such as sodium chloride

(NaCl). They contain not only elements of low potential toxicity (Na, K, Ca, Ba,

Sr and Mg), but also more toxic elements, such as Pb, Zn, Cd and Hg. Other

minerals that may be present in the produced water include traces of oil, metals

and noxious gases. The reported salinities of produced water vary from 1-

400 g L-1 (USGS, 1997). For comparison, seawater has a salinity of around

35 g L-1 (USGS, 1997). The United States Environmental Protection Agency

(USEPA, 2005), Australia’s National Health and Medical Research Council

(NHMRC, 1996), and the World Heath Organisation (WHO, 2004) all

recommended that the level of salinity or total dissolved solids (TDS) in safe

drinking water should be less than 0.5 g L-1, though up to 1 g L-1 is palatable.

9

The salinity of the produced water may increase over the production lifetime

of an oil well, suggesting the co-production of brine. As such, the dissolution of

radium and other group II elements from the formation rock may be enhanced by

the higher salinity of the produced water, in a manner similar to that which occurs

as a result of injecting seawater to enhance recovery. This indicates that NORMs

may be absent at the start of production, but may appear in the later stages of the

well’s lifetime. The quantities of scale and sludge produced vary significantly

between reservoirs, individual wells and production conditions. Therefore, there

is no typical radionuclide concentration in NORMs from oil and gas production,

nor is there a typical quantity of scale and sludge produced annually or over the

lifetime of an oil well.

Petroleum companies have several management options with regard to the

produced water, after it has been separated from the crude oil. These include,

injection of the produced water into deep abandoned oil wells, pumping it into

evaporation ponds, or injecting it back into reservoirs to aid in the recovery of

more crude oil (Figure 1.3). The shallow disposal management option has been

consistently reduced in order to minimise produced water surface environmental

impact.

10

621 674 696 730 764 733

497550 547 575

645 732

385312 276 217

152 122

02 03 04 05 06 07

Shallow disposalDeep disposalProducing resevoir

Figure 1.3: Produced water disposal in shallow, deep and producing reservoir wells.

Another alternative was also found when a new technology called “Solar

Dew” was introduced by Petroleum Development Oman (PDO), designed to

purify the produced water. This system purifies water using a solar-driven, non-

porous membrane distillation process. Water purified using this process passes

World Health Organisation (WHO) and Oman standards for drinking water.

However, it does require the addition of a number of minerals before it is deemed

adequate for human consumption. A desert greening project has also been

successfully launched in Oman using solar dew purified water for irrigation

(PDO, 2001).

Year

Vol

ume

(mill

ion

of b

arre

ls)

11

1.4 Onshore operations

Oman discovered oil in 1962, and the first oil export consignment took place

in 1967. Oil is the country’s major source of income; it comprises 80% of the

export income, and 40% of the Gross Domestic Product (GDP). To date, the

largest oil producing company in Oman, Petroleum Development Oman (PDO),

has only been undertaking onshore oil exploration and production. It produces

90% of the country’s crude oil and almost all of the country’s natural gas.

PDO’s major shareholders are: the Oman Government (60%), Royal Dutch

Shell (34%), Total (4%) and Partex (2%).

The facilities operated by PDO cover approximately 114,000 km2 of

concession land (PDO, 2001), and the Oman government has granted PDO the

use of this land (Figure 1.4) for oil and gas exploration until 2044. Within this

area, PDO executes a range of activities, such as seismic surveys, drilling and

production. The area comprises 120 producing oil fields, 3,750 producing wells,

59 gathering and production stations and over 7,000 km of pipes and flow lines,

and these operations invariably create localised disturbances within the

environment.

12

Figure 1.4: Map of the Sultanate of Oman with Petroleum Development Oman’s

concession land.

13

An internal PDO inventory of oily waste, conducted in 2001, revealed that it

has in excess of 100,000 tonnes of oily sand/sludge, stored at 8 waste

management centres in the county’s interior regions, and a further 30,000 tonnes

was being treated at the time (PDO, 2001). These sludge volumes would have

substantially increased since then, due to the fact that an estimated 72,000

tonnes of oily sludge are produced annually (Al-Futaisi et al., 2007).

At PDO, one of the major issues they face is the handling of radioactive

sludge material and its eventual disposal. As mentioned earlier, the sludge

contains radioactivity, mainly due to radioisotopes of radium (226Ra and 228Ra).

As radium isotopes and their progeny are strong gamma emitters, the external

radiation dose in the vicinity of separation tanks increases as sludge builds up.

Moreover, frequent cleaning and replacing the lining of these tanks further

increases the external radiation dose. This work is generally carried out by

personnel accessing the interior of the tank, and because of the confined

environment within the tank, 222Rn and 220Rn tend to build up, leading to

significant air concentrations of hazardous radioactive material.

Petroleum industry sludge disposal methods are similar to those used to

dispose of tailings from the mining and milling of uranium ores. The average

ore grade at Roxby and ERA Ranger uranium mines in Australia are such that

the 226Ra activity concentrations in their tailing repositories are expected to be

6.6 and 31 kBq kg-1, respectively (Sonter et al., 2002). These values are

comparable to the PDO sludge activity concentration range of 0.15-1 kBq kg-1

(Salih et al., 2005). The Australian Petroleum Production and Exploration

14

Association (APPEA, 2002) also reported similar activity concentrations of

228Ra and 226Ra found in the NORMs of primary production and power

generation industries (Table 1.1).

Table 1.1: Typical 226Ra and 228Ra activity concentrations for various primary production and power generation industries according to APPEA activity concentrations (kBq kg-1)

Material/Grade 228Ra 226Ra

Uranium ore (1% U) - 120

Monazite 200 – 290 1.2 – 37

Xenotime 61 48

Fly ash 0.56 0.56

Phosphate rocks 4.8 0.12

To date in Oman, sludge with 226Ra activity concentrations > 1 kBq kg-1 is

contained in barrels, which are temporarily stored in confined concrete fenced

areas. The more oily sludge is stored in recently engineered hold up ponds

(100 x 10 x 1 m), lined with special geo-textile material. However, the issue of

long-term disposal of sludge remains and current methods need to be thoroughly

evaluated in order to ascertain their merits and demerits. On the other hand,

sludge with 226Ra activity concentration < 1 kBq kg-1 undergoes sludge farming

process.

The removal of scale from NORM contaminated tubular also generates

radioactive waste. This is commonly done by human operated machines that use

mechanical arms and pneumatic pressure to scrape and blow away the scale

15

from the inner surface of the pipes. However, significant amounts of dust are

also suspended during this process, resulting in the inhalation and ingestion of

radioactive particles by personnel. The total radiation dose can then be derived

from the internal dose exposure pathways and the external dose pathways.

PDO has thousands of NORM contaminated tubular and processing pieces of

equipment that require the scale removal process. This de-scaling project is still

under review by PDO’s Health, Safety and Environment (HSE) department.

1.5 Gaps in knowledge

Studies of NORM enhancement in the scale, sludge and produced water of

typical petroleum industries have been conducted in the past (Kolb and Wojcik,

1985, Heaton and Lambley, 1995, Paschoa, 1997, Spitz et al., 1997, Shawky et

al., 2001, White and Rood, 2001, Jerez Veguería et al., 2002, Paschoa and

Godoy, 2002, Al-Masri, 2006), however, most of these studies focus on the

offshore exploration and production operations conducted in the North Sea and

Brazil. According to an assessment reported by the European Commission

(Oman, 2008), the petroleum industry now contributes more radioactivity to the

North Sea than the nuclear power industry, which includes numerous power

stations and processing plants.

The main objective of this research project is to conduct a comprehensive

assessment and evaluation of the activity concentration and gamma dose rate of

large-scale onshore petroleum operations in ‘the Sultanate of Oman’. To the

best of our knowledge, no comprehensive and systematic studies on the

radiological impact of the industry have ever been carried out, particularly in the

16

Middle East and the Gulf region, which is the largest producer of oil worldwide.

This research also investigates radon gas exhalation from petroleum treated and

untreated sludge, oil and gas scales samples, and from evaporation pond

sediment soils. This assessment of radon gas exhalation in the petroleum

industry is the first of its kind; however it is somewhat limited in scope, due to

the fact that access permits were only granted to a limited number of sites in the

area.

17

Chapter 2 LOCALITY AND OIL MINING

2.1 The Sultanate of Oman

The Sultanate of Oman is one of six oil producing countries that make up the

Gulf Cooperation Council (GCC). These countries include the Sultanate of Oman,

the Kingdom of Saudi Arabia, the Kingdom of Bahrain, the United Arab

Emirates, Qatar and Kuwait. Oman is situated in the south eastern corner of the

Middle East (Arabian Peninsula), located between latitudes 16.40-26.20°N and

longitudes 51.50-59.40°E. It has an area of 309,500 km2 and a population of

2.6 million (Oman, 2008). It is bordered by the United Arab Emirates to the

northwest, the Kingdom of Saudi Arabia to the west and the Republic of Yemen

to the southwest. Oman also has a coast line of approximately 1700 km, adjoining

the Oman Gulf along its north eastern boarder and the Arabian Sea to the south

east. The capital, Muscat, is located on the country’s north eastern border, near

the Tropic of Cancer.

To date, the largest oil producing company in Oman, ‘Petroleum

Development Oman (PDO)’, produces 90% of the country’s crude oil and almost

all of the country’s natural gas. In 2006, a workforce of 5,000 people were

managing and handing the oil mining process at PDO (PDO, 2006).

18

2.2 Mining sites

PDO have divided their operations into two regions, otherwise known as the

“Northern Oman Directorate (NOD)” and the “Southern Oman Directorate

(SOD)”. The NOD accounts for half of PDO’s production and it contains four

clusters of oil fields, namely the Lekwair, Fahud, Yibal and Qarn Alam clusters.

The SOD accounts for the other half of production and it contains five clusters,

namely the Bahja, Marmul, Nimr, Harweel and Rahab Thuleilat Qaharir (RTQ)

clusters. The major oilfields studied during this research included Al Noor,

Bahja, Marmul, Nimr and Zuliya, which were all located within PDO’s SOD

(see Figure 2.1).

19

Figure 2.1: The five major oilfields studied during this research, all located within PDO’s Southern Oman Directorate.

20

2.3 The surrounding area

Inhabitants from the surrounding areas are predominantly bedews

(shepherds), who traditionally moved around with their herds of sheep and

camels to wherever water and food could be found (Figure 2.2). However, a

recent shift towards a more semi-urban lifestyle has seen many bedews

beginning to settle in permanent wooden or concrete houses. And in addition to

relying on their livestock for food, they are now beginning to rely on the

increasing number of grocery stores that have opened up in scattered locations

throughout the local area.

Figure 2.2: Typical bedew rooming on their camels (picture courtesy of Trek Earth http://www.trekearth.com/gallery/Middle_East/Oman/page19.htm).

These newly established residential areas are located in close proximity

(within 10km) to the sludge farms of Bahja, Nimr and Marmul, as well as the

21

Bahja NORM store yard. These facilities are surrounded by wire mesh boundary

fences, with guarded gates, in order to ensure that the local community, and

straying animals, are kept out of these contaminated areas.

2.4 The oil mining process

Oil mining can be divided into four major phases: exploration, drilling,

production, and rehabilitation and restoration. In the exploration phase, seismic

waves are used to detect the presence of underground hydrocarbons in the

surrounding area. Rock samples are also taken for laboratory analysis and

exploration wells are then drilled, in order to confirm the existence of oil

reservoirs. Once the presence of oil has been confirmed, the drilling of production

wells can then commence. This drilling process is a precise science, which can be

expensive and extremely hazardous, since many oil and gas reservoirs exist at

very high temperatures and pressures, ranging from 80-350 °C and 0.8-2 kbar,

respectively (Dyer and Graham, 2002, Dutkiewicz et al., 2003).

Once the wells have been drilled, they are then secured and capped, before

finally being connected to a collection of valves called the wellhead. These valves

channel the flow of crude oil (which coexists with both saline produced water and

natural gas) from the reservoir into distribution pipelines. After travelling along

these pipelines, which can range from a few kilometres to tens of kilometres long,

the crude oil eventually reaches the gathering station, where it enters large

separation tanks (1200 m3) that allow for the crude oil, natural gas and produced

water to separate. The dehydrated, degassed crude oil is then pumped to Mina

Al-Fahal sea port in Muscat (which is up to 800 km from some gathering stations)

22

for export on large oil tankers. In contrast, the natural gas component is delivered

to a number of Oman Government agencies for local use, as well as to the Oman

and Qalhat Liquefied Natural Gas (LNG) plants, near Sur sea port, for export on

LNG transporting tankers. The produced water is dealt with in a number of ways

that have already been discussed in detail in Chapter 1.

2.5 Implications of Oman’s aging reservoirs

When oil flows to the surface naturally, as a result of its own reservoir

pressure, it is by far the most convenient and cost effective means of oil

extraction. However, when an oil well begins to age and the reservoir pressure

falls, it becomes necessary to use artificial methods of extraction, such as

electrical submerged pumps or beam pumps (nodding donkeys). In general,

petroleum companies only recover 30-40% of the oil contained in a given

reservoir using this method (Chierici, 1992, Carrero et al., 2007).

While the oil production process is essentially anhydrous, with time, and as

the reservoir pressure falls, produced water is often co-produced along with the

crude oil. Hence, as a well ages, the ratio of produced water to crude oil increases,

sometimes reaching as high as 95% of the total production volume, while still

remaining economically viable (Oil & Gas_UK, 2008).

In 2002, PDO produced 3.774 million barrels (6 x 105 m3) of produced water

per day, while its daily crude oil production was 0.849 million barrels

(1.35 x 105 m3) – a total produced water volume of 82%. However, by the end of

2006, PDO’s produced water had increased to 88% of the total production volume

23

(PDO, 2006), indicating that PDO’s oil reservoirs may have already reached their

peak production capacities (see Figure 2.3).

0

300

600

900

1200

97 98 99 00 01 02 03 04 05 06 07

GasCondensate Black oil

Figure 2.3: Daily oil, condensate and gas production in Oman over the last 11 years.

This problem of aging oil wells is not restricted only to Oman. In 1993, the

United States total annual produced water volume was 25 billion barrels

(4 x 109 m3), whereas the crude oil total volume was only 2.5 billion barrels

(4 x 108 m3) – a total produced water volume of 91%. In response to this global

problem, PDO have started to investigate the use of enhanced oil recovery (EOR)

technologies, which work by flooding the reservoir with substances such as: (i)

reinjected produced water; (ii) a special water/polymer mix; (iii) gas (otherwise

known as ‘gas lifting’); or (iv) steam, in order to force the remaining, more

viscous oil into adjacent producing wells.

Year

Ave

rage

dai

ly p

rodu

ctio

n (th

ousa

nds o

f bar

rels

of o

il eq

uiva

lent

)

24

After flooding a well with reinjected produced water, the amount of

remaining oil may still be as high as 70% of the original oil volume, as the oil is

often too viscous and heavy to be moved by the water alone. However, the

addition of an alkaline surfactant polymer to the produced water, prior to

reinjecting it into the reservoir, increases the viscosity of the injected fluid and

increases the oil recovery factor, along with the final volume of oil produced

(Carrero et al., 2007).

Gas lifting, on the other hand, works by pumping natural gas into the

reservoir, which then mixes with the oil, making it less viscous and thus, more

mobile. Once the oil has been extracted, the gas is then recovered and reinjected

back to the reservoir to extract more oil. Similarly, injecting steam into the

reservoir also acts to decrease the viscosity of heavy oil, thus also increasing

yields.

Whilst the complete removal of oil from reservoirs is not possible with any of

the existing EOR technologies, together, these four methods have the potential to

enhance the average oil recovery factor to well over 50% (Doscher and Wise,

1976, Carrero et al., 2007).

2.6 The future of oil exploration in Oman

A report by the Oman Ministry of National Economics revealed that, despite

the Government’s efforts to expand the country’s range of income sources,

petroleum products remain the dominant income earner and driver of the Oman

25

economy. In 2006, oil and gas exports accounted for approximately 80% of all

export earning revenue and more than 20 international companies are now

exploring for oil and gas throughout the country. As an example, in early 2008,

a new offshore exploration contract was signed by the Oman Ministry of Oil

and Gas, which extended the Gulf of Oman offshore concession area by

23,850 km2, in addition to the 21,000km2 already allocated in Block 18 (Prabhu,

2007).

The current world oil price makes exploration of geologically challenging

reservoirs economical. The Oman Government’s pursuit of new offshore

explorations and embarking on new EOR technologies is a mark of commitment

to a sustainable oil supply (Al-Shaibany, 2007).

26

27

Chapter 3 SAMPLING AND MEASUREMENT TECHNIQUES

3.1 Introduction

Various measurements were performed on scales, sludge and soil sediment,

both in-situ and on physical samples collected from the study locations. The in-

situ analysis consisted of: (a) collecting gamma spectra measurements using a

portable gamma spectroscopy system consisting of a Pb shielded 2 ¼” NaI(Tl)

scintillation crystal connected to a multi-channel analyser (MCA, model 7000

Rainbow) with 1024 channels; (b) gamma dose rate measurements using an

energy compensated Geiger-Müller (GM) probe Mini Instrument Type 6-80;

and (c) charcoal cups analysis, for determining radon exhalation rates. On the

other hand, the physical samples underwent: (a) gamma spectroscopy analysis

using an Ortec EG&G high purity germanium detector (HPGe); and (b)

determination of radon exhalation, using a specially designed emanometer.

Gamma dose rate measurements were carried out along with gamma

spectroscopy measurements, on both treated and untreated sludge. Onsite total

gamma counts were also collected over a preset duration of 600 s and charcoal

cups were planted to determine 222Rn exhalation at the same locations. Physical

samples were also collected from piles and strips, which were then transported

to the Medical Physics Unit Laboratory in Muscat, where they underwent

gamma spectroscopy and 222Rn activity flux analysis.

28

3.2 Dating of petroleum scale and sludge

Radionuclide activity ratios can be used to date scale and sludge samples,

using two different methods. The first method makes use of the 228Ra:226Ra

activity ratio, where the former has a shorter half life than the latter. This

method is valid, provided that the radium isotopes incorporated into a radium

insoluble mineral approximate a chemically closed system (Zielinski et al.,

2001, Al-Masri and Aba, 2005). Observations made by Al-Masri (2006) and

Zielinski et al. (2001) have shown that this ratio is fairly constant for specific

formations and varies from 0.5-2 for nascent samples, which corresponds to

Th:U mass ratio of 1.5 to 6. Ahmad et al. (2003) and Al-Masri (2006) have

illustrated a potential use of this ratio, whereby the ratio which is obtained from

produced water samples can be used to determine if two oil wells are sharing the

same reservoir. Similarly, if the ratio for a certain reservoir changes over time,

this can indicate produced water breakthrough from a nearby water source. We

did not use 228Ra:226Ra activity ratio for the purpose of reservoir fingerprinting.

The second method for dating petroleum sludge and scale samples used the

228Th:228Ra ratio. In this method, a zero activity concentration of 228Th is

assumed at the start of sludge formation. This is because the produced water co-

extracted with crude oil is thorium free and an ingrowth of 228Th takes place

with the decay of 228Ra, whereby the parent-progeny transient equilibrium ratio

approaches 1.5 (Figure 3.1).

29

0.00

0.25

0.50

0.75

1.00

1.25

1.50

0 5 10 15 20 25

Th-228/Ra-228 activity ratio

Ra-228 decay

Th-228 accumulation

Figure 3.1: The relative activity ratio of 228Th/228Ra and the relative decay of 228Ra.

The first method can be used to find the age of scale and sludge samples up

to 40 years, provided the initial 228Ra/226Ra ratio is known. Beyond this, the

228Ra would have gone through more than seven half lives, and may no longer

be detectable enough to make accurate estimates of age. In contrast, the second

method is more suitable for estimating the age of relatively new scale or sludge,

typically less than 10 years. This is because as the change in relative activity

beyond 10 years approaches the 1.5 transient equilibrium ratio, it becomes more

difficult to resolve age with acceptable uncertainty.

Time (years)

Rel

ativ

e A

ctiv

ity

30

3.3 In-situ gamma spectroscopy

A portable gamma spectroscopy system was used to collect gamma spectra

measurements in the field (Figure 3.2 (a)). The spectrometer was energy

calibrated by Amersham Cs-137 and Co-60 standard reference sources at the

Queensland University of Technology (QUT) Radiological Laboratory,

Brisbane, Australia. Calibration by 232Th, 238U and 40K radionuclides was

performed at the CSIRO Core Library, Sydney, Australia. The calibration

facility comprised of five concrete circular slabs, namely background,

potassium, uranium, thorium and mixed, each being 0.8 m thick and 2.0 m

diameter.

In order to shield the detector from adjacent cosmic radiation and gamma

rays, and ensure consistent detection geometry over the measured area of

interest (being 1.5 m2 when the system was set at a height of 1 m), a Pb casket

was designed to surround the NaI(Tl) detector (Figure 3.2 (b)). At 3.5 cm thick

and 10.75 cm deep, the shield was able to attenuate approximately 82% of

2.615 MeV (208Tl ), 84% of 1.765 MeV (214Bi) and 85% of 1.461 MeV (40K) of

incident gamma radiation. Although the 18 kg shield was not able to eliminate

the effects of incident radiation completely, a larger shield would not have been

easily portable, so a compromise had to be made between practicality and

accuracy of the gamma spectroscopy measurements. A tripod was used to

mount the portable NaI(Tl) gamma spectroscopy system, including the Pb

shield, at a height of 1 m, and the spectra were recorded for a preset duration of

600 s.

31

Figure 3.2: (a) In situ gamma spectroscopy, and (b) A lead shield (designed and poured at QUT) for shielding the NaI(Tl) probe of the portable gamma spectroscopy system.

The spectra were then downloaded from the MCA to a laptop and an energy

calibration was applied. Carefully determined regions of interest (ROI) were

used to obtain total counts under the three major peaks: 1.461 MeV (40K),

1.765 MeV (214Bi) and 2.615 MeV (208Tl ). The peaks had a range of 1.43-1.49,

1.69-1.87 and 2.55-2.67 MeV, respectively. An algorithm was also developed to

find factors by a 3x3 inverse matrix method, that were applied on 40K, 214Bi and

208Tl peak counts in order to strip peak cross-talk and obtain activity

concentrations of these radionuclides in Bq kg-1.

In ambient soils, 238U and 232Th are in secular equilibrium with their

progeny, however because the 238U and 232Th series in the petroleum sludge

samples started from 226Ra and 228Ra, respectively, it was evident that the

equilibriums of the two series had been disturbed. The 238U progeny 214Po, 214Bi,

214Pb, 218Po and 222Rn were in secular equilibrium with unsupported 226Ra, while

the 232Th progeny 208Tl, 212Po, 212Bi, 212Pb, 216Po, 220Rn and 224Ra were in

(b) (a)

32

secular equilibrium with supported 228Th. Further, 228Th and 228Ac were also in

transient and secular equilibrium with 228Ra, respectively. In the above two

series, 222Rn and 220Rn radioactive gases were assumed to have been fully

retained in the surveyed samples. For ambient soil, because secular equilibrium

is assumed with 232Th, the branching ratio corrected 208Tl activity concentration

would represent all of its predecessor radionuclides. However, because the

series starts from 228Ra in sludge, transient equilibrium exists between 228Th and

228Ra, and thus the branching ratio corrected 208Tl activity concentration would

only represent predecessors up to 228Th. This makes determination of 228Ra by

the portable NaI(Tl) system for petroleum sludge complex because; (a) 228Th is

only present by ingrowth, and therefore a fresh sludge may have a significant

amount of 228Ra, but zero 208Tl due to zero 228Th, resulting in false activity

concentration for 228Ra, (b) Due to the ingrowth of 228Th into 228Ra, the ratio of

228Th:228Ra will vary according to sample age until 228Th reaches transient

equilibrium with 228Ra with an equilibrium factor of 1.5. This makes it

impossible to accurately determine the activity concentration of 228Ra unless the

ratio of 228Th:228Ra is known by other means.

3.4 Laboratory gamma spectroscopy

3.4.1 Sample collection and preparation

Sludge Samples: A number of 100-300 g surface sludge samples were

collected from freshly removed sludge, sludge piles, sludge strips, sludge

storage barrels, a sand waste and beads. Those samples that were exposed to

sunlight for an extended period of time were quite dry, while those that were

33

recently removed from separation tanks or stored in barrels, tended to be more

oily. Following collection, each sludge sample was placed in a standard 500 mL

sample bag. The bags were sealed, labelled with the date, time, sample type and

samplers name, and then transported to the laboratory for analysis. The oily

sludge samples were dried in the laboratory at 110 °C for 24 h. The dry samples

were then crushed, homogenised and passed through a 2 mm sieve. The samples

were then packed and pressed into petri dishes, which were sealed using

adhesive tape. Typical sample mass in the petri dish was about 0.1 kg. In some

cases, coarse non-crushable gravel with greater than 2 mm diameter was left

over. This material was monitored for radioactivity by a portable Mini

Instrument 900 series scintillation count rate meter and was found to emit

radioactivity at ambient levels, which may have resulted in an over estimation

of the radioactivity concentration of some sludge samples by up to 10%.

Scale Samples: Scales are generally chunky, hard, and in some cases brittle.

The scales collected in this study were from pipes previously used in the Fahud

oil fields of the Northern Oman Directorate. These pipes were used for pumping

produced water to injection pump stations, either for disposal or for re-injection

into reservoirs, as part of the EOR ‘water flooding’ process. They were

decommissioned because they became clogged with scales, and were

transported to Bahja NORM store yard in 1999. A spatula was used to collect

100-300 g scale samples from each pipe, penetrating the entire depth of the

scale deposited along the pipes internal wall. Following collection, each scale

sample was placed in a standard 500 mL sample bag, as per the methodology

outlined above. As the collected scale samples were already dry, solid and

34

brittle, they did not require drying. The samples were then crushed,

homogenised and packed in petri dishes, also according to the methodology

outlined above.

A further 12 samples of gas scale were also collected, however these

samples were only 50-100 g in weight, since many of the pipes they were

collected from were tightly capped, and thus we were unable to obtain samples

from many of the pipes. In addition, those pipes that were accessible only had

small amounts of gas scale that could be sampled. The gas scale samples

consisted of dry, flat flakes, about 1 mm thick and ranging from 0.01-20 cm2 in

area. These samples were then handled according to the same methodology used

for the original scale samples.

Sediment Samples: Six sediment samples, ranging from 100-300 g, were

collected from Al Noor evaporation pond, using a metal scooper. The samples

were placed in plastic bottles and transported to the laboratory in Muscat, where

they were dried at 110 °C for 24 h, before being crushed, homogenised and

passed through a 2 mm sieve. The homogenised samples were then packed and

handled according to the same methodology outlined above. In order to avoid

cross-contamination, sample preparation instruments were wipe-cleaned after

handling each individual sample.

3.4.2 Gamma spectroscopy measurement procedure

The high purity germanium (HPGe) detector system that was used to

measure the gamma spectra of the collected samples was housed in Laboratory

35

2043 at Sultan Qaboos University (SQU), Muscat. The system consisted of an

EG&G Ortec spectrometer, with a pop top semi-conductor detector (diameter:

59.0 mm, length: 74.2 mm), with 30% relative efficiency. It had a ‘full width at

half maximum’ (FWHM) of 0.807 keV for the 122 keV peak of 57Co, and

1.71 keV for the 1.33 MeV peak of 60Co. The detector was shielded against

background radiation by a 10 cm thick Pb castle.

The HPGe was energy calibrated daily, using a geometry reference source

standard in a 1000 mL marinelli beaker with known isotopes. This standard

complies with the requirements for traceability to the National Institute for

Standards and Technology (NIST), and was also used for efficiency calibration

of the system. However, because the samples used in this research were placed

in petri dishes for analysis, a separate geometry correction calibration was

carried out on the HPGe, using known volumes and activities of standard

radionuclides, in order to accommodate for the petri dish geometry. Background

spectra were also recorded and corrected during sample spectrum analysis.

The main radioisotopes identified by this study in petroleum NORM are

radium isotopes and their progeny, along with 40K and 227Ac. Because of the

226Ra strong overlap with the 235U 185.7 keV peak (actinium series - emission

intensity 57.5%), many gamma spectrometers do not rely on the main

186.1 keV peak of 226Ra (uranium series - emission intensity 3.5%) to determine

its activity concentration. Therefore, gamma emitting 226Ra progeny (214Pb,

T½: 26.8 minutes and 214Bi, T½: 19.9 minutes) are used to obtain 226Ra activity

concentration. However, due to the presence of the gaseous intermediate 222Rn

36

(refer to 238U decay series in Chapter 1, Figure 1.2 (a)) the samples had to be

sealed for 21 days, in order to allow the 222Rn and its progeny to reach

equilibrium with their parent radionuclide. The 226Ra activity could then be

obtained by calculating the error weighted average of gamma emitting 214Pb and

214Bi. Assuming similar activity concentrations of 238U and 232Th, and a 1:20

ratio between 235U and 238U, the contribution of 235U in the 226Ra peak was

found to be 2.7%. Due to the HPGe’s 30% relative efficiency, along with its

energy efficiency peak at 160 keV, the system was found to have a low

efficiency for radionuclides with low gamma energy, such as 210Pb (46.5 keV).

However, samples with a large activity concentration of 210Pb were an

exception, as was the case with the oil and gas scale samples collected during

this research. Activity concentrations of 210Pb, 228Th and 40K were also

determined directly from their respective gamma lines. However, activity

concentrations of radioisotopes with a weak or no gamma signal (for example;

227Ac and 228Ra) were determined by their progeny (227Th and 228Ac) gamma

lines, respectively.

The gamma spectrum for each sample was obtained over a 17 hour

timeframe. SQU’s HPGe system utilises Gamma Vision 5.1 software for

spectrum analysis. This software adjusts for the effects of interfering peaks,

should the presence of a second radionuclide be confirmed. As such, the

Gamma Vision software was able to adjust for the contribution of 235U to the

observed 186 keV peak, thus allowing for the 226Ra activity concentration to be

determined immediately. However, because SQU’s HPGe system was also used

for the University’s own work, the analysis of field samples was sometimes

37

delayed for up to three months. Even though 228Th has a relatively short half life

of 1.9 years, it is in transient equilibrium with 228Ra (T½: 5.75 years); and since

radioactivity is transported by produced water and 228Th is not water soluble, an

initial activity concentration of zero was assumed at the time of sludge/scale

formation. However, all reported gamma spectroscopy data were still needed to

be corrected for 228Ra and 228Th decay. To correct for 228Ra decay it is a straight

forward exponential relationship, however for the 228Th decay correction, the

age of the sample had to be calculated based on an analysis of the 228Ra/228Th

activity ratio at the time of collection. Thus, using the 228Ra activity

concentration at the collection date, 228Th activity concentration was calculated

for each sample (see Section 3.2).

3.5 Comparison between in-situ and laboratory gamma

spectroscopy measurements

Overall, 24 sampling locations were analysed for gamma spectra, using both

in-situ and laboratory gamma spectroscopy. The corresponding results for each

location were then compared and checked for correlation. The main

radionuclides that were compared are illustrated in Figures 3.3 (a-d). From these

figures it can be seen that the field and laboratory 226Ra, 228Th and 40K activity

concentrations compared well, giving linear fit and correlation coefficients as

follows:

ARa-226 Field = (1.05 ± 0.05) ARa-226 Lab (R2 = 0.96), (3.1)

ATh-228 Field = (1.09 ± 0.05) ATh-228 Lab (R2 = 0.96), and (3.2)

AK-40 Field = (1.10 ± 0.12) AK-40 Lab (R2 = 0.78) (3.3)

38

However, the linear fit of the field 228Th to laboratory 228Ra activity

concentration comparison was significantly further away from 1:

ATh-228 Field = (1.57 ± 0.11) ARa-228 Lab (R2 = 0.91) (3.4)

This difference is because, due to the absence of the primordial series parent

232Th in the oil industry NORM, 228Th reaches transient (and not secular)

equilibrium with 228Ra – hence the ratio approaches 1.5, instead of 1. This fact

was considered while applying in-situ gamma spectroscopy techniques for

NORM measurements.

39

0

1

2

3

4

5

6

0 1 2 3 4 5 0

300

600

900

1200

1500

1800

0 300 600 900 1200 1500

0

175

350

525

700

875

1,050

0 150 300 450 600 750 0

100

200

300

400

500

600

0 70 140 210 280 350 Figure 3.3: Correlation between field portable NaI(Tl) and laboratory HPGe activity concentration readings for: (a) 226Ra Field vs 226Ra Lab, (b) 228Th Field vs 228Th Lab, (c) 40K Field vs 40K Lab and (d) 228Th Field vs 228Ra Lab .

(a) AField = (1.05 ± 0.05) ALab R2 = 0.96, n = 24

(b)

(c) AField = (1.10 ± 0.12) ALab R2 = 0.78, n = 24

Lab. HPGe 40K (Bq kg-1) Lab. HPGe 228Ra (Bq kg-1)

Lab. HPGe - 226Ra (kBq kg-1)

Fiel

d N

aI -

226 R

a (k

Bq

kg-1

)

Fiel

d N

aI -

228 Th

(Bq

kg-1

)

Fiel

d N

aI -

40K

(Bq

kg-1

)

(d)

AField = (1.09 ± 0.05) ALab R2 = 0.96, n = 23

Fiel

d N

aI -

228 Th

(Bq

kg-1

) Lab. HPGe 228Th (Bq kg-1)

AField = (1.57 ± 0.11) ALab R2 = 0.91, n = 20

40

3.6 In-situ gamma dose-rate measurements

The Mini-Instrument 6-80 dose rate meter was used to assess gamma

radiation dose rate above piles and strips, located at the Bahja, Nimr and

Marmul sludge farms. The meter consisted of an electrometer, connected to an

energy compensated Geiger-Müller (GM) tube, which was mounted on a tripod

at a height of 1 m above ground. The electrometer provided two separate

readings, an instantaneous absorbed dose rate in air (in µGy h-1) on an analogue

scale, and pre-set time total counts at 600 s, on a digital display. The count rates

(s-1) were divided by a calibration factor of 17.1 to obtain the effective dose rate

in µSv h-1. This factor was obtained through calibrating the system against an

Amersham certified standard (Cs-137 – source number 3702GF) and the

measured dose rates were then verified, by calculating the external exposure

rates using (UNSCEAR, 2000) dose coefficients.

The 226Ra, 228Ra and 40K activity concentrations (in Bq kg-1) obtained from

both in-situ and laboratory gamma spectroscopy measurements were also used

to calculate the dose rate in air D (in µGy h-1) at a 1 m height from the ground,

according to the following equation:

D = (0.462 A[238U] + 0.604 A[232Th] + 0.0417 A[40K])/1000 (3.5)

where A[238U], A[232Th] and A[40K] are the activity concentrations (in Bq kg-1)

for 226Ra, 228Ra and 40K, respectively.

41

3.7 Radon activity flux measurements using charcoal

cups

Activated charcoal is known to adsorb radon gas onto its surface. Brass

charcoal canisters, or cups, containing 25 g of fine activated charcoal (secured

by a wire spring over metal mesh) were used to collect passive field readings of

222Rn exhalation from the sludge piles. These cylindrical cups were open at one

end, with a height and base area of 0.080 m and 0.0029 m2, respectively. Prior

to use, the charcoal cups are annealed for 8-10 h, at a temperature of 110 ºC.

The cups were then allowed to cool for 20 min, before being covered by a

polyethylene lid and sealed with adhesive tape.

Before field measurements were conducted, a ‘standard’ cup was prepared

using a 25 g sludge sample, with 226Ra and 228Ra activity concentrations of 4030

± 21 and 343 ± 7 Bq kg-1, respectively. An epoxy resin was then poured over the

seal, in order to ensure that the 222Rn would not escape. When the 222Rn reached

secular equilibrium with its parent 226Ra three weeks later, the standard was

ready for use.

In the field, the polyethylene lids were removed from the annealed cups and

they were planted upside down on the sludge piles, with the rim of the cup

buried approximately 1 cm into the ground (Figure 3.4). Two cups were planted

for each location, where the mean value was then used. The cups were left in the

field for four days, before being removed and immediately sealed. They were

then allowed to sit for a minimum of four hours, so that the 222Rn could reach

equilibrium with its short lived gamma emitting progeny 214Pb and 214Bi.

42

Figure 3.4: Charcoal cups planted on a sludge pile.

A 600 s gamma spectrum was then collected from the cups using the

NaI(Tl) detector. Planting, removal and counting dates and times were noted, in

order to calculate exposure (te), delay (td) and counting (tc) time intervals. The

net count rate was obtained from the spectra region of interest (ROI) covering

the gamma peak energies of 222Rn progeny (Pb-214 peaks at 242, 295 and

352 keV and Bi-214 peak at 609 keV). The set range was from 223-725 keV,

corresponding to channels 60-195 on the MCA. Rn-222 exhalation rate in the

charcoal cup was then interpolated from its gamma emitting progeny 214Pb (T½:

26.8 min) and 214Bi (T½: 19.9 min) using the following equation (Spehr and

Johnston, 1983):

)1()1(

2

ec

d

tt

tc

eeaetR

J λλ

λ

ελ

−− −⋅−⋅⋅⋅⋅⋅

= (3.6)

where te, td and tc are the exposure time in the field, the delay time between

retrieval from the field and counting in the laboratory, and the counting time (s);

J is the average 222Rn exhalation rate (Bq m-2 s-1) over the exposure time te; R is

the net count rate (s-1) post background subtraction obtained during tc; λ is the

43

decay constant (s-1) for 222Rn; a is the surface area covered by the charcoal

canister (m2); and ε is the counting efficiency of the detector system (s-1 Bq-1).

3.8 Radon exhalation rate measurements using the

emanometer

The emanometer was calibrated using certified Pylon radon gas (Model RN-

1025). Figure 3.5 (a) shows a schematic diagram of the emanometer, which was

housed in a secure wooden box with all of its components secured in

predetermined slots, as shown in Figure 3.5 (b). Due to time constraints during

the field visits, only a limited number of charcoal cup applications could be

performed. Therefore, laboratory assessment of 222Rn exhalation rates was also

undertaken for twenty corresponding field samples. These measurements were

then cross-referenced with the charcoal cup measurements, in order to verify the

accuracy of the emanometer. All of the reported radon exhalation rates (apart

from two out of the four Oman ambient soil samples) in Chapter 6 are from

values obtained using the emanometer.

44

Flow rate meter

ZnS(Ag) scintillation chamber

PM-tube

Emanometer assembly schematic

8 mm tube

Electronics

Pump

Absolute filter(Cotton wool)

Signal and power lines

Valves

inflow

outflo

w

Sample

Sample chamber

Table

Figure 3.5: (a) Schematic diagram of the emanometer, and (b) The emanometer in its wooden box housing.

This technique for the determination of 222Rn exhalation rate made use of

ZnS(Ag) scintillation chambers, coupled with photo-multiplier tubes. The main

components of the emanometer were: 2 x ZnS(Ag) scintillation chambers,

(a)

(b)

45

2 x photo-multiplier tubes coupled to the scintillation chambers, 2 x DayBreak

digital power supplies and counters, 2 x pumps with flow-rate meters, 2 x cotton

wool absolute filters and a suite of sample chambers. Each sample chamber was

a 3.5 L PVC cylinder with a clear Perspex lid (Figure 3.6 (a)). The cover was

fitted with inlet and outlet valves, and an O-ring, along with six screws and

butterfly nuts were fastened to the cover, in order to obtain an air tight seal

between the chamber and the lid. An 8 mm capillary tube connected the sample

chamber to the rest of the system.

Figure 3.6: (a) Emanometer’s airtight PVC sample chambers with FESTO valves and Perspex cover, and (b) a sludge sample wrapped in perforated textile material, labelled and ready for 222Rn counting.

Sample collection has been described in Section 3.4.1. The samples are kept

as intact as practically possible in order to obtain the same 222Rn exhalation rate

found in the field. The samples analysed by the emanometer included petroleum

scales, treated and untreated sludge from sludge farms, sludge stored in barrels,

sediment soil from a produced water evaporation pond and ambient soil. Each

0.25 kg sample was wrapped in 225 cm2 of perforated textile, tied with a wire

ribbon, labelled and placed on a plastic table inside the sample chamber

(Figure 3.6 (b)). The chamber was then sealed for 24 h to allow for 222Rn gas

(a) (b)

46

accumulation, before being connected to the system. Air was then pumped

through a closed loop, at a rate of 6 L min-1, moving from the sample chamber,

through a wool cotton absolute filter, into the ZnS(Ag) scintillation chamber,

through the pump and the flow rate meter, then back to the sample chamber.

47

Chapter 4 RADIOACTIVITY CONCENTRATION OF SCALE, SLUDGE AND SOIL SEDIMENT, FROM THE OIL FIELDS OF THE SOUTHERN OMAN DIRECTORATE

4.1 Introduction

The presence of technologically enhanced NORMs in petroleum industry

scales and sludge has been reported by many of the world’s oil producing

countries. In this work, we have attempted to characterise and quantify this

radiation, based on samples taken from the oil fields of the Southern Oman

Directorate (SOD). This chapter will report the radioactivity concentrations of

sludge, oil and gas scales, and evaporation pond soil sediment, as well as the use

of radioisotopes to date scales and sludge.

Gamma spectroscopy measurements were performed on various solid waste

samples, including: (1) nascent sludge and sludge in Bahja, Nimr and Marmul

sludge farms; (2) oil and gas scales, and sludge stored in barrels in Bahja

NORM store yard; and (3) sediment in Al-Noor evaporation pond. Sludge

samples were also tracked from its initial point of accumulation at the separator

tanks, to its final destination at the sludge farm or in barrels at the NORM store

yard.

48

4.2 Radioactivity in sludge

Oman generates approximately 7.2 x 104 tonnes of sludge per annum (Al-

Futaisi et al., 2007). As mentioned in Chapter 1, Section 1.3, the word ‘sludge’

refers to a mixture of hydrocarbon, mud, natural radionuclides, sediments,

bacterial growth, corrosion particles and some scale debris. In Oman, the

presence of petroleum industry NORMs was first discovered in a sludge sample

removed by a pig device from the Hasirah of Zauliyah line, in the Bahja cluster

in 1997. Today, PDO conduct an analysis of all sludge removed from its storage

and separator tanks, using a Mini-Instrument 900 series count rate meter, with a

gamma scintillation probe (Model 44A). The nominal background surface count

rate for these meters is 3 CPS and sludge samples with count rates 5 CPS higher

than the nominal background (i.e. 8 CPS) are considered to be NORM

contaminated. According to an unpublished paper, presented at the ‘PDO

Workshop on NORM’ in Muscat (2005), PDO’s total accumulated sludge was

more than 3.3 x 105 tonnes and out of this mass, almost 2.0 x 104 tonnes was

NORM contaminated.

In a national ambient soil radioactivity survey carried out by Goddard

(2002), the average activity concentrations of 226Ra and 232Th in the upper 1 cm

of exposed surface soil or rock were reported as 29.7 ± 8.9 and

15.9 ± 7.6 Bq kg-1, respectively. Assuming equilibrium between 232Th and

228Ra, the latter reported average could be considered the national average for

228Ra activity concentration. A report on 226Ra and 224Ra activity concentrations

in 50 oily sludge samples collected from the northern Oman oilfields was also

published by Salih et al. (2005). The activity concentrations obtained using

49

laboratory HPGe gamma spectroscopy ranged from 0.15-1 kBq kg-1 for 226Ra,

and 0.1-0.6 kBq kg-1 for 224Ra.

4.2.1 Sludge farming

The presence of long lived radioisotopes (e.g. 226Ra, T½: 1602 years; 210Pb,

T½: 22.26 years and 228Ra, T½: 5.75 years) in sludge, at levels higher than those

found in ambient soil, poses a significant radiological hazard to the

environment. Shailubhai (1986) discussed various options for the disposal of

oily sludge, including ocean disposal, incineration and land farming. However,

the presence of toxic chemicals in the oily sludge means that ocean disposal is

not a desirable option, since aquatic organisms may be poisoned by these toxins.

Since global warming has become an issue, incineration is no longer desirable

either, because it is not only energy intensive, but it also contributes

significantly to air pollution. Land farming, on the other hand, is highly cost

effective, provided that the sludge does not contaminate clean soil and seep into

underground water supplies. Many other studies have also investigated options

for sludge farming; however none of them have addressed the issue of

radioactivity (Arora et al., 1982, Couillard et al., 1991, Prado-Jatar et al., 1993,

Brown et al., 1998, Vasudevan and Rajaram, 2001, Mater et al., 2006).

In Oman, oily sludge is currently being disposed of at sludge farms, which

utilise micro-organisms to biodegrade the complex hydrocarbons found in oil,

into naturally occurring by-products, such as carbon dioxide and water. This

process requires minimal machinery and labour inputs, and since it is able to

50

occur above ground, in areas exposed to high levels of solar radiation, it is the

most cost effective method of sludge disposal, particularly in the remote desert

oil fields of Oman.

At the time of the field studies conducted during this research, the total land

area of Bahja, Nimr and Marmul sludge farms was about 30 hectares. These

farms usually consisted of two areas – one where the excavated sludge was

heaped, and the other, a flat plot of machine compacted land dedicated for

spreading the sludge. The sludge was spread in rows, otherwise known as

‘strips’, measuring approximately 6 m in width, 75 m in length and 0.4 m in

height, and were separated by 4-8 m of open space, to allow passage for the

water tanker and other service vehicles. Table 4.1 shows the GPS locations,

estimated volume of material in untreated sludge piles and number of treated

sludge strips at Bahja, Nimr and Marmul sludge farms, at the time of our final

visit to Oman (21 April - 8 May 2007). The total volume of sludge found in the

three farms was estimated to be around 34,000 m3.

Table 4.1: Bahja, Nimr and Marmul sludge farm locations, estimated volume of untreated sludge in piles and number of treated sludge Strips at the time of this study (Jan 2006 – June 2007) Sludge farm Geographical location Sludge pile

volume (m3) Number of Strips Latitude Longitude

Bahja N 19° 52.8’ E 56° 02.0’ 10,000 42

Nimr N 18° 33.3’ E 55° 51.9’ 17,000 147

Marmul N 18° 12.6’ E 55° 17.5’ 7,000 64

Total 34,000 253

51

After the sludge is delivered to the sludge farms, the heaps are transported to

the strip site, where the sludge is spread, before being mixed with clean soil by

an earth moving machine (skid loader). Alternatively, sludge can be mixed with

clean sand before it is transported to the strip site. The strips are then tilled and

watered on daily basis, in order to maintain optimal aeration and moisture

conditions for the biodegradation and evaporation of volatile compounds

(Figure 4.1 (a-f)). Typically, the soil to sludge ratio ranges from 1-5:1,

depending on the 226Ra activity concentration of the sludge. Along with daily

tilling, this not only serves to assist bioremediation, but it also helps to dilute the

radioactivity present in the sludge.

A total of 55 untreated sludge samples were analysed from the three sludge

farms - 25 from Bahja, 14 from Nimr and 16 from Marmul. A further 57 treated

sludge samples were also analysed - 12 from Bahja, 16 from Nimr and 29 from

Marmul (see Chapter 3, Sections 3.2.1 and 3.2.2 for details on sampling and

measurement procedures).

52

Figure 4.1: The sludge farming process: (a) sludge removed from a separation tank, (b) untreated sludge piles, (c) sludge piles after transport to the farming area, (d) a typical sludge strip, (e) watering the sludge strips, and (f) tilling the sludge strips.

b

e

d c

a

f

53

4.2.2 Radioactivity in ambient soil

The radioactivity of ambient soil samples from Bahja, Al-Noor, Nimr and

Marmul are shown in Table 4.2. The mean activity concentrations of 226Ra,

228Ra, 228Th and 40K were 34.2 ± 3.8, 8.1 ± 1.3, 7.1 ± 0.8 and 151 ± 55 Bq kg-1,

respectively. Bahja 40K activity concentration (293 ± 8 Bq kg-1) was three times

higher than the mean activity concentration for the other three sites

(104 ± 6 Bq kg-1). As expected, the mean 228Th:228Ra activity ratio for the four

sites was close to one (0.89 ± 0.09), since 228Th and 228Ra were likely to be in

secular equilibrium with 232Th.

A detailed study of Oman’s ambient terrestrial radioactivity concentrations

was performed by Goddard (2002). The study reported mean activity

concentrations for 15 samples from the Wusta region (the same region where

this research was conducted), being 36.2 ± 8.2, 16.4 ± 5.5 and 166 ± 27 Bq kg-1

for 226Ra, 232Th and 40K, respectively. Taking into account the uncertainties of

these types of measurements, the ambient soil concentrations found in this

research are similar to those found by Goddard in 2002. However, a study

conducted by the United Nations (UNSCEAR, 2000) reported population

weighted natural radionuclide activity concentrations in soils to be 35, 30 and

400 Bq kg-1 for 238U, 232Th and 40K, respectively. In comparison to these

findings, the desert environment of Oman seems to have lower 232Th and 40K

concentrations than those found elsewhere in the world.

54

Table 4.2: Activity concentration (Bq kg-1) for ambient soils of Bahja, Al-Noor, Nimr and Marmul and the world average (UNSCEAR, 2000) (uncertainties represent counting error) Sample ID Geographical location 226Ra 228Ra 228Th 40K 137Cs 228Ra:226Ra 228Th:228Ra

Latitude Longitude

Bahja N 19° 53.047' E 56° 01.753' 38.1 ± 5.4 11.3 ± 1.4 9.1 ± 1.4 293 ± 8 1.02 ± 0.23 0.30 ± 0.08 0.81 ± 0.22

Al Noor N 18° 41.391' E 55° 30.376' 30.4 ± 5.2 7.7 ± 0.8 7.1 ± 0.4 108 ± 5 0.58 ± 0.18 0.25 ± 0.07 0.92 ± 0.15

Nimr N 18° 33.257' E 55° 51.864' 27.2 ± 1.3 5.7 ± 0.4 5.7 ± 0.4 111 ± 2 < 0.12 0.21 ± 0.03 1.00 ± 0.14

Marmul N 18° 12.562' E 55° 17.337' 41.2 ± 4.9 7.8 ± 1.1 6.5 ± 1.5 93 ± 5 < 0.44 0.19 ± 0.05 0.83 ± 0.31

Mean ± SE 34.2 ± 3.8 8.1 ± 1.3 7.1 ± 0.8 151 ± 55 - 0.24 (0.03) 0.89 (0.05)

Standard

Deviation

6.5 2.3 1.5 95

-

0.05 0.09

World Median

35 30# 30# 400

# Assuming secular equilibrium of 228Ra and 228Th with 232Th

55

4.2.3 Radioactivity in the sludge recovered from a separator

tank

PDO cleans its separator tanks approximately every 5 years, depending on

the sludge accumulation rate at the bottom of the tank. After cleaning, NORM

contaminated sludge samples are sent to Sultan Qaboos University (SQU) for

gamma spectroscopy analysis, in order to determine 226Ra activity

concentration, and thus, the fate of the sludge. NORM contaminated sludge,

with a 226Ra activity concentration equal to or higher than 1 kBq kg-1, is

transported to the Bahja NORM store yard for storage. NORM contaminated

sludge with a 226Ra activity concentration less than 1 kBq kg-1 is sent to the

Bahja, Nimr and Marmul sludge farms.

Six sludge samples were collected from a newly cleaned separator tank at

Nimr sludge farm. They were assessed for 226Ra, 228Ra, 228Th and 40K

radioactivity and 228Ra:226Ra and 228Th:228Ra ratios (refer to Chapter 3, Section

3.2.1 and 3.2.2 for details on sampling and measurement procedures), in order

to determine the initial radioactivity and the age of the sludge sediment. From

Table 4.3 it can be seen that the mean 226Ra, 228Ra, 228Th and 40K activity

concentrations were 588 ± 106, 264 ± 53, 296 ± 52 and 109 ± 20 Bq kg-1,

respectively. While the mean 40K activity concentration was similar to the

ambient soil value for Nimr sludge farm, the 226Ra, 228Ra and 228Th activity

concentrations were significantly higher than the Nimr ambient soil

concentrations.

56

Table 4.3: Activity concentration (Bq kg-1) and individual reading error of freshly removed sludge from a Nimr station separator tank Sample ID 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra 228Ra:226Ra

ratio at deposition Age of sludge

(years)

NFS 1 446 ± 17 206 ± 4 221 ± 5 149 ± 8 0.46 ± 0.03 1.08 ± 0.04 0.86 0.96 0.77 5.2 5.7

4.8

NFS 2 391 ± 15 163 ± 3 186 ± 4 89 ± 7 0.42 ± 0.02 1.15 ± 0.05 0.85 0.97 0.76 6.0 6.6

5.5

NFS 3 985 ± 24 446 ± 5 496 ± 7 119 ± 12 0.45 ± 0.02 1.13 ± 0.03 0.89 0.96 0.83 5.8 6.1

5.5

NFS 4 697 ± 24 352 ± 5 346 ± 6 117 ± 13 0.51 ± 0.02 0.99 ± 0.03 0.86 0.93 0.79 4.5 4.8

4.2

NFS 5 363 ± 12 139 ± 3 207 ± 5 32 ± 6 0.38 ± 0.02 1.49 ± 0.07 7.0a 7.4 6.6

NFS 6 648 ± 19 280 ± 5 321 ± 6 151 ± 10 0.43 ± 0.02 1.16 ± 0.04 0.89 1.00 0.80 6.2 6.7

5.7

Maximum 985 446 496 151 0.51 1.49

Minimum 363 139 186 32 0.38 0.99

Median 547 243 271 118 0.44 1.14

Standard Deviation 238 118 117 44 0.04 0.17

Mean ± SE 588 ± 106 264 ± 53 296 ± 52 109 ± 20 0.44 ± 0.02 1.16 ± 0.08 0.87 0.97 0.79 5.8 6.3

5.4

Nimr Ambient Soilb 27.2 ± 1.3 5.7 ± 0.4 5.7 ± 0.4 111 ± 2 0.21 ± 0.03 1.00 ± 0.14

a Based on 228Ra:226Ra activity concentration ratio. Not included in the average b From Table 4.2

57

From Table 4.3 it can also be seen that the average 228Th:228Ra activity ratio

was 1.16 ± 0.17, indicating that the sludge was approximately 5.8 3.64.5 years old,

which is consistent with the company’s separator tank cleaning interval of

5 years. The age of individual samples was then used to determine the

228Ra:226Ra ratio at the time of deposition. Since 226Ra (T½: 1602 years) decay

would be insignificant in 5.8 years, no decay correction was necessary when

calculating the mean 228Ra:226Ra activity ratio at the time of deposition, which

was found to be 0.87 97.079.0 . In addition, the deposition activity ratio corresponded

to a 232Th:238U mass ratio of 2.6. These values were similar to those found by

Al-Masri and Aba (2005), who reported a mean 228Ra:226Ra activity ratio and

232Th:238U mass ratio of 0.76 and 2.3, respectively.

As outlined in Chapter 3, Section 3.2, the initial formation of sludge is

usually free of 228Th. Build up, and ingrowth of 228Th (T½: 1.9 years) takes place

as 228Ra (T½: 5.75 years) decays, until a transient equilibrium is reached. In

contrast, considering the difference in the half lives, 228Ra activity reduces

rapidly relative to 226Ra, which explains the inverse trend that was demonstrated

by the 228Ra:226Ra and 228Th:228Ra activity ratios (Figure 4.2).

58

0.3

0.4

0.5

0.6

0.8 1.0 1.2 1.4 1.6

Figure 4.2: Relation between 228Ra:226Ra and 228Th:228Ra activity ratios.

4.2.4 Radioactivity in untreated piles at sludge farms

A summary of the activity concentrations found in untreated sludge from the

Bahja, Nimr and Marmul sludge farms, is presented in Table 4.4, while the

individual values are displayed in Tables 4.5, 4.6 and 4.7.

Bahja Sludge Farm: A preliminary survey of the untreated sludge piles,

using a portable gamma count rate meter, revealed that about 75% of the total

piles had gamma counts close to ambient soil, and therefore no samples were

collected from those piles. The remaining 25% had enhanced activity

concentrations equal to or greater than the set limit of 1 kBq kg-1 (Petroleum

Development Oman, 2005), which meant that they were not suitable to be used

in the sludge farming process . These high activity sludge piles were segregated

228Th:228Ra activity ratio

228 R

a:22

6 Ra

activ

ity ra

tio

59

from the rest of the piles, in an area measuring approximately 600 m2 and they

will remain there until an alternative disposal or treatment method is approved.

The mean (± standard error) 226Ra, 228Ra, 228Th and 40K activity concentrations

for these high activity piles were found to be 3289 ± 264, 261 ± 19, 338 ± 26

and 427 ± 50 Bq kg-1, respectively. The mean 228Th:228Ra and 228Ra:226Ra

activity ratios were then used to estimate the age of the sludge, which was found

to be 9.0 ± 0.4 and 15 years (assuming the initial 228Ra:226Ra ratio is the same as

Nimr nascent sludge), respectively. As expected, these age estimates for Bahja

farmed sludge were 3-9 years more than the age of sludge freshly removed from

the tank, as determined in Section 4.2.3.

Nimr Sludge Farm: All of the untreated sludge samples had 226Ra activity

concentrations less than 1 kBq kg-1 and the distribution was also less dispersed

between piles. Averages for 226Ra, 228Ra, 228Th and 40K activity concentrations

were 343 ± 35, 129 ± 13, 123 ± 14 and 433 ± 27 Bq kg-1, respectively.

Marmul Sludge Farm: On the basis of a gamma dose rate survey of the

Marmul untreated sludge piles, about 10% were expected to have activity

concentrations exceeding 1 kBq kg-1, and sample collection was biased towards

the higher activity piles. The maximum activity concentrations detected in

Marmul sludge were 3690 ± 60, 6036 ± 20, 5164 ± 20 and 720 ± 13 Bq kg-1 for

226Ra, 228Ra, 228Th and 40K, respectively. When weighted according to the area

of higher and lower concentration piles, the mean (± standard error) activity

concentrations for 226Ra, 228Ra, 228Th and 40K were 326 ± 153, 394 ± 205, 342

± 179 and 360 ± 79 Bq kg-1, respectively.

60

Table 4.4: Sludge activity concentrations (Bq kg-1) and radioisotope ratios from Bahja, Nimr and Marmul untreated sludge piles Location 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra Bahja Median 3164 268 344 272 0.083 1.29

Mean 3289 261 338 427 0.082 1.29 Standard Error 264 19 26 50 0.004 0.01 Standard Deviation 1295 93 125 243 0.015 0.05 Number of Samples 25

Nimr Median 323 123 113 448 0.37 0.95 Mean 343 129 123 433 0.40 0.95 Standard Error 35 13 14 27 0.03 0.02 Standard Deviation 128 46 51 99 0.11 0.07 Number of Samples 14

Marmul * Mean 356 394 342 360 0.60 0.85 * Standard Error 153 205 179 79 0.12 0.04 * Standard Deviation 342 421 428 247 0.29 0.10 Number of Samples 16

* Area weighted values due to distinct low and high radioactivity sections

61

Table 4.5: Activity concentrations (Bq kg-1) of untreated Bahja sludge piles Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra

Easting Northing

BHJP 1 * 398306 2198873 2210 ± 40 191 ± 7 240 ± 10 185 ± 21 0.086 ± 0.004 1.26 ± 0.10 BHJP 2 * 398318 2198877 1310 ± 20 103 ± 5 132 ± 7 252 ± 15 0.079 ± 0.005 1.28 ± 0.13 BHJP 3 * 398316 2198885 2980 ± 30 203 ± 6 255 ± 9 229 ± 18 0.068 ± 0.003 1.25 ± 0.09 BHJP 4 * 398318 2198888 2150 ± 70 143 ± 11 180 ± 19 264 ± 27 0.066 ± 0.007 1.26 ± 0.23 BHJP 5 * 398318 2198898 2240 ± 40 150 ± 5 190 ± 11 254 ± 21 0.067 ± 0.003 1.27 ± 0.11 BHJP 6 * 398328 2198909 4520 ± 50 200 ± 10 376 ± 15 209 ± 23 0.066 ± 0.003 1.25 ± 0.09 BHJP 7 * 398363 2198960 4000 ± 40 253 ± 9 330 ± 16 217 ± 23 0.063 ± 0.003 1.32 ± 0.11 BHJP 8 * 5670 ± 50 373 ± 9 501 ± 13 182 ± 22 0.066 ± 0.002 1.34 ± 0.06 BHJP 9 * 5180 ± 40 345 ± 7 454 ± 3 187 ± 12 0.067 ± 0.002 1.32 ± 0.04 BHJP 10 * 5300 ± 40 345 ± 7 506 ± 6 200 ± 19 0.065 ± 0.002 1.47 ± 0.05 BHJP 11 * 1090 ± 20 92 ± 4 112 ± 5 232 ± 12 0.088 ± 0.005 1.22 ± 0.10 BHJP 12 * 4580 ± 50 309 ± 8 392 ± 12 192 ± 22 0.065 ± 0.002 1.27 ± 0.07 BHJP 13 * 3164 ± 56 253 ± 11 306 ± 20 272 ± 31 0.084 ± 0.005 1.22 ± 0.14 BHJP 14 # 398367 2198958 1955 ± 56 172 ± 14 221 ± 18 524 ± 122 0.088 ± 0.010 -

62

Table 4.5 (Continued): Activity concentrations (Bq kg-1) of untreated Bahja sludge piles Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra

Easting Northing

BHJP 15 # 398360 2198953 2981 ± 69 289 ± 18 372 ± 24 597 ± 150 0.097 ± 0.008 - BHJP 16 # 398355 2198950 3221 ± 71 271 ± 18 349 ± 23 566 ± 153 0.084 ± 0.007 - BHJP 17 # 398351 2198944 3799 ± 77 315 ± 19 405 ± 25 591 ± 165 0.083 ± 0.007 - BHJP 18 # 398342 2198933 3875 ± 79 375 ± 21 484 ± 27 779 ± 171 0.097 ± 0.007 - BHJP 19 # 398340 2198929 4336 ± 82 366 ± 21 470 ± 26 767 ± 178 0.084 ± 0.006 - BHJP 20 # 398333 2198920 4934 ± 89 472 ± 23 607 ± 30 758 ± 190 0.096 ± 0.006 - BHJP 21 # 398330 2198915 2908 ± 68 258 ± 17 332 ± 22 638 ± 147 0.089 ± 0.008 - BHJP 22 # 398320 2198906 3445 ± 73 287 ± 18 369 ± 23 954 ± 162 0.083 ± 0.007 - BHJP 23 # 398310 2198896 2444 ± 63 268 ± 18 344 ± 23 742 ± 140 0.109 ± 0.010 - BHJP 24 # 398309 2198890 2274 ± 60 200 ± 15 257 ± 20 409 ± 129 0.088 ± 0.009 - BHJP 25 # 398300 2198878 1651 ± 53 198 ± 15 255 ± 19 469 ± 115 0.120 ± 0.013 - Maximum 5670 470 607 954 0.120 1.47 Minimum 1090 92 112 182 0.063 1.22 Number of Samples 25

* Analysis by HPGe gamma spectroscopy system

# Analysis by NaI portable gamma spectroscopy system – 228Ra value were determined by using 228Th:228Ra ratio of 1.29

63

Table 4.6: Activity concentrations (Bq kg-1) of untreated Nimr sludge piles (analysed using the HPGe gamma spectroscopy system) Sample ID Location 40QUTM 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra

Easting Northing NMRP 1 383329 2051550 531 ± 13 139 ± 3 151 ± 4 446 ± 10 0.26 ± 0.01 1.08 ± 0.06 NMRP 2 383337 2051562 403 ± 17 138 ± 5 130 ± 5 487 ± 14 0.34 ± 0.03 0.94 ± 0.07 NMRP 3 383356 2051569 291 ± 14 118 ± 3 106 ± 4 405 ± 10 0.41 ± 0.03 0.90 ± 0.06 NMRP 4 383353 2051598 314 ± 10 114 ± 3 109 ± 3 457 ± 9 0.36 ± 0.02 0.96 ± 0.05 NMRP 5 383309 2051584 320 ± 11 123 ± 2 115 ± 3 457 ± 9 0.38 ± 0.02 0.94 ± 0.04 NMRP 6 383296 2051560 285 ± 10 95 ± 3 94 ± 3 439 ± 9 0.33 ± 0.02 0.99 ± 0.07 NMRP 7 383347 2051609 639 ± 17 270 ± 4 281 ± 5 134 ± 10 0.42 ± 0.02 1.04 ± 0.04 NMRP 8 73 ± 7 55 ± 2 48 ± 2 595 ± 10 0.75 ± 0.10 0.87 ± 0.08 NMRP 9 309 ± 19 130 ± 4 119 ± 6 392 ± 14 0.42 ± 0.04 0.91 ± 0.07 NMRP 10 284 ± 14 141 ± 4 109 ± 5 451 ± 14 0.50 ± 0.04 0.77 ± 0.06 NMRP 11 361 ± 17 136 ± 5 125 ± 6 469 ± 16 0.38 ± 0.03 0.91 ± 0.07 NMRP 12 327 ± 18 111 ± 4 109 ± 6 442 ± 14 0.34 ± 0.03 0.98 ± 0.09 NMRP 13 339 ± 14 112 ± 3 110 ± 4 480 ± 11 0.33 ± 0.02 0.98 ± 0.06 NMRP 14 334 ± 17 123 ± 4 117 ± 5 406 ± 14 0.37 ± 0.03 0.96 ± 0.08 Maximum 639 270 281 595 0.75 1.08 Minimum 73 55 48 134 0.26 0.77 Number of Samples 14

64

Table 4.7: Activity concentrations (Bq kg-1) of untreated Marmul sludge piles (using the HPGe gamma spectroscopy system) Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra

Easting Northing

MRLP 1 # 319390 2014275 195 ± 20 83 ± 12 67 ± 10 47 ± 43 0.35 ± 0.09 0.81 ± 0.24 MRLP 2 319393 2014288 36 ± 6 9 ± 1 9 ± 1 119 ± 6 0.26 ± 0.09 0.99 ± 0.30 MRLP 3 319385 2014287 42 ± 4 10 ± 1 9 ± 1 135 ± 5 0.24 ± 0.05 0.91 ± 0.20 MRLP 4 319378 2014292 36 ± 4 7 ± 1 8 ± 1 118 ± 5 0.20 ± 0.05 1.07 ± 0.33 MRLP 5 319370 2014292 38 ± 4 9 ± 1 7 ± 2 103 ± 5 0.24 ± 0.06 0.76 ± 0.27 MRLP 6 319365 2014314 125 ± 9 92 ± 3 84 ± 3 639 ± 11 0.73 ± 0.08 0.90 ± 0.06 MRLP 7 319350 2014320 46 ± 6 23 ± 2 17 ± 2 683 ± 11 0.49 ± 0.10 0.76 ± 0.13 MRLP 8 319385 2014271 918 ± 21 845 ± 7 773 ± 7 548 ± 15 0.92 ± 0.03 0.91 ± 0.02 MRLP 9 319392 2014271 1022 ± 24 1557 ± 7 1380 ± 8 359 ± 16 1.52 ± 0.04 0.89 ± 0.01 MRLP 10 27 ± 3 7 ± 1 5 ± 1 84 ± 5 0.26 ± 0.07 0.77 ± 0.18 MRLP 11 146 ± 9 123 ± 3 93 ± 3 653 ± 11 0.84 ± 0.07 0.76 ± 0.04 MRLP 12 196 ± 13 162 ± 3 120 ± 3 720 ± 13 0.83 ± 0.07 0.74 ± 0.04 MRLP 13 3690 ± 60 6036 ± 20 5164 ± 20 519 ± 20 1.64 ± 0.03 0.86 ± 0.01 MRLP 14 2290 ± 40 2211 ± 10 2023 ± 13 477 ± 19 0.97 ± 0.02 0.91 ± 0.01 MRLP 15 1310 ± 30 1444 ± 10 1236 ± 11 443 ± 17 1.10 ± 0.03 0.86 ± 0.01 MRLP 16 166 ± 9 139 ± 3 103 ± 3 601 ± 11 0.84 ± 0.06 0.74 ± 0.04 Maximum 3690 6036 5164 720 1.64 1.07 Minimum 27 7 5 47 0.20 0.74 Number of Samples 16 # Analysis by NaI portable gamma spectroscopy system

65

Comparison of Activity Concentrations between Sludge Farms: The data

presented in this section shows that the 226Ra and 228Ra activity concentrations

for the Marmul untreated sludge piles varied significantly (by two and three

orders of magnitude, respectively), compared to the activity concentrations for

the Bahja and Nimr untreated sludge piles, which were within one order of

magnitude (Figure 4.3). Figure 4.3 also illustrates that the two radium isotopes

had a direct proportionality, implying that the encountered increase in gamma

count rate was a result of a simultaneous activity concentration increase for both

226Ra and 228Ra.

1

10

100

1000

10000

1 10 100 1000 10000

MarmulBahjaNimr

Figure 4.3: Relation between 226Ra and 228Ra for Marmul, Bahja and Nimr untreated sludge piles.

228 R

a (B

q kg

-1)

226Ra (Bq kg-1)

66

The mean ages of Nimr and Marmul untreated sludge piles, as estimated by

the 228Th:228Ra activity ratio, were 4.2 ± 0.3 and 3.6 ± 0.4 years, respectively,

indicating that Nimr and Marmul untreated sludge piles were relatively new and

similar in age. The Bahja untreated sludge piles, on the other hand, were

estimated to be as old as 15 years, which is not unexpected, since the bulk of

high activity untreated sludge piles have been at the farm for quite some time,

and will remain there until an alternative disposal or treatment method is

approved. As a result of these age differences, an inverse relationship was

observed between 228Ra:226Ra and 228Th:228Ra mean activity ratios for the three

sludge farms (Figure 4.4). On the other hand, as shown in Figures 4.5 (a) and

(b), no correlation was observed between sludge age and the 226Ra and 228Ra

activity concentrations for each individual farm.

0.00

0.25

0.50

0.75

1.00

0.7 0.9 1.1 1.3 1.5

Figure 4.4: 228Ra:226Ra and 228Th:228Ra mean activity ratios for Bahja, Nimr and Marmul sludge farms.

228 R

a:22

6 Ra

activ

ity ra

tio

228Th:228Ra activity ratio

Bahja

Nimr

Marmul

67

10

100

1000

10000

0 2 4 6 8 10

BahjaNimr

Marmul

1

10

100

1000

10000

0 2 4 6 8 10

Bahja

Nimr

Marmul

Figure 4.5: Sludge pile activity concentration versus age (a) 226Ra, and (b) 228Ra.

Sludge pile age (years)

226 R

a ac

tivity

con

cent

ratio

n (B

q kg

-1)

228 R

a ac

tivity

con

cent

ratio

n (B

q kg

-1)

Sludge pile age (years)

a

b

68

4.2.5 Radioactivity in treated sludge strips

Gamma spectroscopy results for Bahja, Nimr and Marmul treated sludge

strips are presented in Tables 4.8, 4.9 and 4.10, respectively. The bulk of the

data was collected in the field by the portable NaI gamma spectroscopy system,

while the rest was obtained from analysing samples in the laboratory, using an

HPGe gamma spectroscopy system. As outlined in Chapter 3, Section 3.2, 228Ra

activity concentration was not directly measured in the field, but was calculated

from the 228Th:228Ra activity ratio obtained from laboratory measurements for

samples collected from same locations.

Bahja Sludge Farm: Overall, Bahja’s sludge strips had the lowest mean

(± standard error) activity concentration for 226Ra, 228Ra and 228Th, being 55 ± 3,

14 ± 1 and 11 ± 1 Bq kg-1, respectively. However, these activity concentrations

were still higher than the corresponding ambient soil activity concentrations of

38 ± 5, 11 ± 1 and 9 ± 1 Bq kg-1, respectively (Table 4.2). No localised spots of

higher activity (hotspots: often 1-2 m2 surface area and about 0.5 m deep) were

detected on the treated sludge strips. This was due to the fact that only the

sludge piles with activity concentrations similar to ambient soil radioactivity

were approved for the sludge farming process (refer to discussion in

Section 4.2.4.).

Nimr and Marmul Sludge Farms: Despite dilution by the farming process,

the mean and maximum 226Ra activity concentrations for Nimr and Marmul’s

treated sludge strips were about three and ten times higher than their respective

ambient soil activity concentrations.

69

Nimr and Marmul’s treated sludge strips were also found to contain

hotspots, even after being mixed with clean soil, which may have been the result

of a non-uniform activity distribution in the sludge. At Nimr sludge farm, a

hotspot was detected on strip 61, with 226Ra, 228Ra and 228Th activity

concentrations of 1340 ± 26, 736 ± 7 and 441 ± 8 Bq kg-1, respectively. At

Marmul sludge farm, two hotspots were detected. The first was on strip 44

(analysed by HPGe), with 226Ra, 228Ra and 228Th activity concentrations of

2080 ± 40, 184 ± 5 and 160 ± 6 Bq kg-1, respectively, and the second on strip 7

(analysed by portable NaI), with 226Ra and 228Th activity concentrations of

1920 ± 54 and 165 ± 16 Bq kg-1, respectively.

It is unlikely that these hotspots will disappear any time soon, since both

228Ra and 226Ra will require about 7 half lives to decay to ambient soil activity

concentrations. Taking into account the 232Th half life, supported 228Ra found in

ambient soil will remain virtually constant while the unsupported sludge 228Ra

will decay to 0.78 % of its original activity over approximately four decades.

Ra-226, however, has a longer half life, and will require at least 10,000 years to

decay to 0.78 % of its original activity.

In addition, Nimr and Marmul’s ambient soil 228Th:228Ra activity ratios were

1.00 ± 0.14 and 0.83 ± 0.31, respectively, indicating that 228Th and 228Ra were in

transient equilibrium with 232Th. However, at the previously identified hotspots

for both farms, 228Th was in disequilibrium with 228Ra. In Nimr, strip 61’s

228Th:228Ra activity ratio of 0.6 was characteristic of new sludge activity ratios,

where 228Th is in an ingrowth phase with 228Ra. A similar argument could be

70

made for strips 7 and 44 in Marmul, indicating that these high activity

concentrations were due to the sludge and not the ambient soil.

When calculating the mean and median activity concentrations for the two

farms, these hotspots were excluded from the analysis, due to their potential to

bias the results. As such, the overall results suggest that, on average, Nimr and

Marmul sludge 226Ra activity concentration had been reduced by factors of 4.6

± 1.6 and 3.1 ± 1.9, respectively, as a result of the mixing and tilling process.

Finally, it should be mentioned that 40K activity concentration did not follow

a specific trend for samples collected from the treated sludge strips. The activity

concentrations of 40K for Bahja, Nimr and Marmul were very similar, with mean

(± standard error) values of 175 ± 10, 146 ± 19 and 167 ± 21 Bq kg-1,

respectively.

71

Table 4.8: Activity concentration (Bq kg-1) of Bahja sludge strips Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K

Easting Northing BHJS 005 * 48 ± 6 20 ± 2 15 ± 2 161 ± 6 BHJS 008 * 52 ± 6 18 ± 2 15 ± 2 258 ± 8 BHJS 018 398766 2198993 51 ± 10 13 ± 10 10 ± 4 159 ± 30 BHJS 030 398675 2198899 55 ± 10 13 ± 10 10 ± 4 175 ± 32 BHJS 039 * 47 ± 6 15 ± 2 12 ± 2 164 ± 7 BHJS 043 398775 2198963 68 ± 10 8 ± 7 6 ± 3 175 ± 32 BHJS 065 398736 2198925 58 ± 10 11 ± 9 9 ± 4 163 ± 31 BHJS 070 398698 2198876 71 ± 11 13 ± 10 10 ± 4 107 ± 29 BHJS 073 398682 2198837 52 ± 10 17 ± 12 13 ± 4 191 ± 33 BHJS 076 398659 2198802 37 ± 9 15 ± 11 12 ± 4 178 ± 30 BHJS 078 398637 2198780 62 ± 10 11 ± 9 9 ± 4 172 ± 32 BHJS 080 398617 2198763 63 ± 10 11 ± 9 9 ± 4 198 ± 33 Maximum 71 20 15 258 Minimum 37 8 6 107 Median 54 13 10 174 Mean ± SE 55 ± 3 14 ± 1 11 ± 1 175 ± 10 Standard Deviation 10 3 3 35 Number of Samples 12

* Analysis by HPGe gamma spectroscopy system

72

Table 4.9: Activity concentrations (Bq kg-1) of Nimr sludge strips Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K

Easting Northing

NMRS 001 * 133 ± 8 54 ± 2 59 ± 3 200 ± 7 NMRS 024 * 108 ± 7 47 ± 2 41 ± 3 128 ± 7 NMRS 026 * 197 ± 10 103 ± 3 96 ± 3 369 ± 10 NMRS 029 383548 2051927 18 ± 9 22 ± 8 21 ± 6 82 ± 24 NMRS 039 383612 2051829 56 ± 11 19 ± 7 18 ± 5 75 ± 27 NMRS 044a 383648 2051790 22 ± 10 26 ± 9 25 ± 6 102 ± 27 NMRS 044b * 260 ± 10 130 ± 3 136 ± 3 150 ± 7 NMRS 045 * 52 ± 5 17 ± 2 14 ± 2 221 ± 7 NMRS 052 383579 2051742 34 ± 10 20 ± 8 19 ± 5 109 ± 27 NMRS 085 383370 2052107 49 ± 10 12 ± 6 12 ± 4 84 ± 26 NMRS 100 383387 2051795 36 ± 8 8 ± 4 7 ± 3 184 ± 30 NMRS 111 383498 2051667 85 ± 13 29 ± 10 28 ± 6 126 ± 34 NMRS 116 383401 2051590 46 ± 9 12 ± 6 12 ± 4 137 ± 29 NMRS 119 0383375 2051622 37 ± 5 10 ± 1 10 ± 2 118 ± 6 NMRS 126 383332 2051726 17 ± 9 23 ± 8 22 ± 6 132 ± 27 NMRS 143 383291 2051664 32 ± 8 12 ± 6 12 ± 4 113 ± 26

73

Table 4.9 (Continued): Activity concentrations (Bq kg-1) of Nimr sludge strips Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K

Easting Northing Maximum 260 130 136 369 Minimum 17 8 7 75 Median 47 21 20 127 Mean ± SE 74 ± 18 34 ± 9 33 ± 9 146 ± 19 Standard Deviation 69 35 36 73 Number of Samples 16

* Analysis by HPGe gamma spectroscopy system

74

Table 4.10: Activity concentrations (Bq kg-1) of Marmul sludge strips Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K

Easting Northing

MRLS 005 319306 2014175 38 ± 10 16 ± 6 18 ± 5 119 ± 28 MRLS 011 319230 2014186 66 ± 12 17 ± 7 19 ± 5 54 ± 27 MRLS 013 319161 2014080 63 ± 11 13 ± 6 15 ± 5 114 ± 30 MRLS 015 * 9.5 ± 0.7 1.9 ± 0.2 2.3 ± 0.2 16 ± 1 MRLS 018 319120 2014219 62 ± 12 25 ± 8 28 ± 6 39 ± 27 MRLS 019 * 414 ± 13 117 ± 3 138 ± 4 188 ± 9 MRLS 020 * 295 ± 19 76 ± 3 96 ± 4 467 ± 13 MRLS 021 319058 2014135 56 ± 11 19 ± 7 21 ± 6 58 ± 27 MRLS 024 319079 2014233 52 ± 12 24 ± 8 27 ± 6 53 ± 27 MRLS 025a 319352 2014140 153 ± 17 28 ± 9 31 ± 7 141 ± 40 MRLS 025b 319352 2014140 181 ± 16 9 ± 5 10 ± 4 240 ± 44 MRLS 027 * 175 ± 9 60 ± 3 65 ± 3 455 ± 11 MRLS 031 * 79 ± 7 23 ± 2 26 ± 2 163 ± 6 MRLS 033 * 205 ± 11 116 ± 3 116 ± 4 313 ± 10 MRLS 046 * 455 ± 15 121 ± 4 109 ± 5 264 ± 9 MRLS 062 * 115 ± 8 61 ± 2 69 ± 2 166 ± 7 MRLS 069 319026 2014173 80 ± 12 15 ± 6 16 ± 5 284 ± 39 MRLS 102a 319037 2014148 67 ± 15 51 ± 14 56 ± 9 214 ± 40

75

Table 4.10 (Continued): Activity concentrations (Bq kg-1) of Marmul sludge strips

Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K

Easting Northing MRLS 102b 319042 2014179 92 ± 20 100 ± 22 111 ± 13 204 ± 47 MRLS 102c 319046 2014183 180 ± 24 130 ± 27 144 ± 15 137 ± 53 MRLS 112 319143 2014210 56 ± 11 21 ± 8 24 ± 6 35 ± 26 MRLS 115 319174 2014200 22 ± 10 23 ± 8 25 ± 6 166 ± 30 MRLS 118 319202 2014078 59 ± 11 15 ± 6 16 ± 5 110 ± 29 MRLS 148 319125 2014094 27 ± 11 31 ± 10 34 ± 7 103 ± 29 MRLS 183 319261 2014182 37 ± 12 31 ± 10 34 ± 7 54 ± 27 MRLS 186 319091 2014110 42 ± 9 12 ± 5 13 ± 4 79 ± 25 MRLS 189 319199 2014192 44 ± 10 15 ± 6 16 ± 5 93 ± 27 MRLS 192 319318 2014095 118 ± 13 5 ± 3 6 ± 3 205 ± 38 MRLS 193 319283 2014108 121 ± 14 11 ± 5 12 ± 4 197 ± 38 Maximum 455 130 144 467 Minimum 9.5 1.9 2.3 16 Median 67 23 26 141 Mean ± SE 116 ± 21 41± 7 45 ± 8 163 ± 22 Standard Deviation 110 39 42 114 Number of Samples 29

* Analysis by HPGe gamma spectroscopy system

76

4.3 Bahja NORM store yard

As mentioned in Section 4.2, any sludge or scale with a 226Ra activity

concentration higher than 1 kBq kg-1 is transported to the Bahja NORM storage

facility, in order to safely segregate the NORM contaminated scales and sludge

until a suitable disposal method is found. The yard is also used to store

decommissioned oil and gas distribution pipes before they are sent for cleaning,

along with a number of submersible electrical pumps, well heads and other

surface equipment.

The Bahja store yard consists of approximately 6 acres of flat compacted

land, surrounded by a boundary fence, with a locked gate. Figure 4.3 (a) shows

some of the sludge barrels, originally stored in Zuliya NORM store yard,

located some 70 km northwest of Bahja (refer to map in Chapter 2, Figure 2.1),

which were relocated to the Bahja NORM store yard in February 2007. At the

Bahja store yard, some of the oily sludge is also stored in one of two storage

pits, lined by a geo-textile material, designed to prevent seepage into the

surrounding earth (Figure 4.3 (b)). These pits are about 1 m deep, 10 m wide

and 100 m long. At the time of our last site visit (21 April – 8 May 2007), there

were approximately 5,000 decommissioned gas pipes (with a length and an

inner diameter of 8 m and 9 cm, respectively) and 20 decommissioned oil pipes

(of varying lengths and shapes, with an inner diameter of 35 cm) in the store

yard (Figure 4.3 (c)).

77

Figure 4.6: NORM store yards: (a) Sludge stored in 120 steel barrels with 226Ra activity content ≥ 1 kBq kg-1 (Zuliya store yard) (b) Oily sludge in one of the two geo-textile lined pits (Bahja store yard), and (c) Oil and gas pipes and other oil processing equipment contaminated with NORM at (Bahja store yard).

(a) (c) (b)

78

4.3.1 Oil industry scales

4.3.1.1 Oil scale formation and removal

The initial extraction of oil from the reservoir is usually water-free,

however, as the well ages and more oil is extracted from the formation rock, the

temperature and pressure in the reservoir decreases and the natural water present

in the reservoir will begin to be co-produced with the oil (Smith, 1987).

Subsequent to extraction, the dissolved radium present in the produced water is

co-precipitated with calcium and barium, in the form of carbonates and

sulphates, resulting in the formation of hard and highly insoluble scale deposits

on the interior walls of both the pipes, and other production equipment (Testa et

al., 1994, Hamlat et al., 2001, Godoy and Petinatti da Cruz, 2003, Paschoa,

2003, Hamlat et al., 2003b, Hamilton et al., 2004, Al-Masri and Aba, 2005,

Bader, 2006). The predominant scale constituents are those of calcium

carbonate (CaCO3) and calcium and barium sulphate (Ca/BaSO4), however,

barium and magnesium carbonate (Ba/MgCO3), along with strontium sulphate

(SrSO4) may also be found to a lesser degree (Hamilton et al., 2004) . Rn-222

(T½: 3.824 days, alpha energy 5.49 MeV) and 220Rn (T½: 55.6 seconds, alpha

energy 6.29 MeV), along with their progeny, may also be exhaled from scales

and/or build up in the crude oil and gas streams (Rood et al., 1998, Worden et

al., 2000). Like barium and strontium, radium is an alkaline earth metal that

belongs to the group II elements in the periodic chart, and as such, it also

displays similar chemical properties.

79

According to Wilson and Scott (1992), the scale formation process occurs in

three stages. Firstly, a scale molecule is formed, when there is a supersaturation

or other chemical disequilibrium in the environment. Secondly, these molecules

come together, forming microcrystalline nuclei that grow and coalesce to form

clusters. When the clusters reach a specific size, precipitation takes place.

Finally, the precipitate adheres to the inner walls of the production equipment,

including the well fluid handling system, down-hole pipes, well heads,

subsurface safety valves, manifolds, separators, oil coolers and produced water

distribution pipes. Although the sticking mechanism is not well understood,

initial surface nucleation seems to be an important factor, before the

precipitation can take place.

In addition to increasing production costs, as a result of the maintenance and

downtime associated with their removal, these scales also reduce efficiency by

clogging valves, restricting flow and damaging equipment (IAEA, 2003,

Hamilton et al., 2004). The removal of scale not only requires time and money,

but also significant expertise, especially since most scales are NORM

contaminated. As such, appropriate radiation protection measures should be

enforced according to the method adopted for scale removal. In Oman, PDO

have approved two main methods of scale removal. Initially, partially clogged

flow line pipes are physically cleared of solid scale deposits while still in

operation, using a process called ‘pigging’. This process utilises a rotating

plastic or rubber plug, termed a ‘pig’, which is launched upstream of the main

flow and then recovered (along with the removed scale) using a ‘pig trap’,

80

located further down-stream (IAEA, 2003). The removed scale/sludge is then

transported to the nearest sludge farm for storage and/or disposal.

When the flow lines become completely clogged and pigging is no longer an

option, the pipes are decommissioned and transported to Bahja NORM store

yard. It is then possible for scale build-up to be removed, through the

application of a mechanically controlled metal rod, used to break up the scale.

This would then be followed by the application of high pressure water jets

(1500-2500 bar), designed to clear the broken scale from the pipes (Al-Masri

and Aba, 2005). However, at the time of writing this thesis, PDO was still in the

process of finding an appropriate contractor to build a decontamination facility

for the removal of these scales, and until such time as this occurs, these pipes

and contaminated equipment are unable to be re-used or melted down and

recycled.

In addition to the number of other scale decontamination and disposal

options discussed by Rood et al. (1998), IAEA (2003) and Hamilton et al.

(2004), IAEA (2003) also discussed the possibility of preventing scale

precipitation, by introducing scale-inhibiting chemicals into the seawater

injection systems, which would act to prevent sulphate and carbonate scale

deposition. The prevention of scale build-up would mean that the radium

isotopes would actually move through production system, and would only be

found in the final produced water by-product. However, this theory was

challenged by several authors (Rajaretnam and Spitz, 2000, Shawky et al.,

81

2001), who argued that scale prevention or remediation processes are not only

difficult, but also very expensive to manage.

4.3.1.2 Radioactivity in oil scales

Seven oil scale samples were collected from decommissioned flow line

pipes (35 cm in diameter), originally used to pump produced water from the

separator tanks (see Chapter 3, Section 3.2.1 and 3.2.2 for details on sampling

and measurement procedures). The radioactivity of these samples was then

analysed using the HPGe system and the full range of gamma spectroscopy

results for the scale samples are presented in Table 4.12.

Radium 226 and 228: The main isotopes found during this analysis were

226Ra (T½: 1602 years; 238U primordial series), and to a lesser degree 228Ra

(T½: 5.75 years; 232Th primordial series), with the highest activity concentrations

for 226Ra and 228Ra being about 500 times higher than the mean ambient soil

activity concentrations found in the south of Oman (Table 4.2).

This finding can be explained by the fact that radium is somewhat soluble in

water, when placed under the high temperatures and pressures found in

petroleum reservoirs, 226Ra tends to leach into co-existing brines or formation

water (Rajaretnam and Spitz, 2000, Shawky et al., 2001). Upon 226Ra decay, the

product, 222Rn, migrates back into the organic liquid and natural gas phases

(Jerez Vegueria et al., 2002b). In contrast, uranium and thorium are part of the

rock matrix core, and since neither are highly soluble in water, only trace

82

amounts of 238U, 232Th, 228Th, 210Pb and 210Po are generally found in reservoir

fluids (White and Rood, 2001).

Overall, 226Ra, 228Ra and 228Th activity concentrations ranged between

3.4-17.3, 1.4-4.3 and 1.4-5.7 kBq kg-1, respectively, and Ra-228 activity

concentrations were less than 226Ra values, with a mean (± standard error) for

228Ra:226Ra activity ratio of 0.307 ± 0.026. A number of other studies also

reported radium activity in scales. For example, in Malaysia, Omar et al. (2004)

reported maximum activity concentrations of 434 and 479 kBq kg-1 for 226Ra

and 228Ra, respectively, while Godoy and Petinatti da Cruz (2003) reported

activity concentrations of 19.1-323 kBq kg-1 for 226Ra and 4.21-235 kBq kg-1 for

228Ra, in Brazil. Al-Masri and Aba (2005) also investigated activity

concentrations for oil scales in the Republic of Syria, and found maximum

(mean) radium isotope activity concentrations of 1520(174), 868(91) and

780(67) kBq kg-1 for 226Ra, 228Ra and 224Ra, respectively.

While the radium activity concentration values reported in this study were

within the typical range reported internationally of 0.1-15,000 kBq kg-1 (Jonkers

et al., 1997, IAEA, 2003), it was found that the activity concentrations observed

in Oman were somewhat lower than the values reported in the above mentioned

studies. This could either be attributed to the difference in Oman geological

formations to those found elsewhere in the world, or an indication that

radioactive scale formation is still in its early stages, since older reservoirs tend

to have higher produced water to crude oil volume ratio, and hence higher

quantity of NORM (Rood et al., 1998). Another possible reason for the

83

discrepancy is that the scale samples collected in this study were taken from the

available downstream pipes, used for handling produced water once it was

separated from the crude oil. This can be explained by other reported findings,

which suggest that radium isotopes activity concentrations are higher in up-

stream scales that form between the reservoir and the well head, compared to

down-stream scales that form in the distribution pipes (Hamilton et al., 2004,

Al-Masri and Aba, 2005).

Actinium 227: In addition to radium isotopes, unsupported 227Ac (T½:

21.77 years; 235U primordial series) was also detected in the oil scale samples.

Whilst no other activity concentrations for 227Ac in oil scales have been reported

to date, its association with naturally occurring brines in the environment has

been reported by a number of authors, such as Dickson (1991), Lieser et al.

(1999), and Martin and Akber (1999). For example, Martin and Akber (1999)

studied 227Ac behaviour in aqueous solutions containing seepage from the

tailings impoundments of a uranium mine, by looking at 227Ac:223Ra ratio, and

came to the conclusion that not all of the 223Ra in solution was supported by

227Ac, due to the absorption of the intermediate radionuclide 227Th

(T½: 18.7 days).

In contrast, results for the solid scales analysed in this research showed

223Ra (T½: 11.4 days) in equilibrium with 227Ac at the time of measurement, and

since our samples were collected from pipes which had been decommissioned

several years prior, any unsupported 223Ra would have been completely

decayed, indicating that the only possible source of the detected 223Ra was the

84

disintegration process of 227Ac. This newly quantified presence of 227Ac in oil

and gas scales is highly important, since it can not only pose a significant

radiological hazard, but it also has a higher committed effective dose coefficient

for inhalation and ingestion compared to the other radionuclides found in scales

and sludge (Table 4.11).

Table 4.11: Committed effective dose coefficients (µSv Bq-1) of selected radionuclides likely to be present in petroleum scales (ICRP68) Mode of Entry ↓

Radioisotope →

227Ac 228Ra 226Ra 210Pb + 210Po

Inhalation for 1 µm AMAD 540 2.6 3.2 1.5

Ingestion 1.1 0.67 0.28 0.92

As such, the presence of 227Ac in oil scales is thought to be a result of its

affinity to the brines found in oil reservoirs, which would then allow it

precipitate with the other minerals on the internal surface of the pipes. The

maximum 227Ac activity concentration, of 123 Bq kg-1, was more than 70 times

higher than the theoretically calculated ambient soil value of 1.7 Bq kg-1.

Lead 210: The oil scale samples analysed in this study were collected from

pipes decommissioned in 1999. Considering that these pipes had been in service

for 10 years prior to decommission, this means that the scales would have been

in the pipes for at least 18 years. As such, assuming zero 210Pb activity at the

time the scales were forming, the expected 210Pb:226Ra activity ratio would be

0.42. Moreover, dating the oil scales using the 228Th:228Ra activity ratio gave an

average age of 15 years, indicating that the expected 210Pb:226Ra activity ratio

was 0.36. However, both of these calculations were significantly lower than the

85

actual mean (± standard error) for 210Pb:226Ra activity ratio measured in this

study, being 0.64 ± 0.08. This discrepancy indicates that there was higher than

expected 210Pb in the oil scale samples collected in this study, possibly brought

to the surface from the reservoir rock by the produced water.

86

Table 4.12: Activity concentrations (Bq kg-1) of oil scale samples Sample ID 226Ra 210Pb 228Ra 228Th 227Ac 40K 228Ra:226Ra 228Th:228Ra 210Pb:226Ra

FHDOS 1 4360 ± 50 3060 ± 610 1550 ± 10 2070 ± 380 40 ± 7 < 28 0.356 ± 0.006 1.34 ± 0.25 0.70 ± 0.15 FHDOS 2 13000 ± 100 4990 ± 1410 3230 ± 20 4800 ± 720 68 ± 14 < 54 0.248 ± 0.003 1.49 ± 0.23 0.38 ± 0.11 FUDOS 3 3380 ± 40 3310 ± 470 1360 ± 10 1930 ± 390 47 ± 7 < 27 0.402 ± 0.008 1.42 ± 0.30 0.98 ± 0.15 FHDOS 4 11800 ± 100 6450 ± 1490 3100 ± 20 4950 ± 740 67 ± 13 < 52 0.263 ± 0.004 1.60 ± 0.25 0.55 ± 0.13 FHDOS 5 6340 ± 60 4330 ± 870 2250 ± 10 2920 ± 420 34 ± 9 < 33 0.355 ± 0.005 1.30 ± 0.19 0.68 ± 0.14 FHDOS 6 17300 ± 100 7590 ± 1640 4310 ± 20 6810 ± 950 123 ± 17 < 65 0.249 ± 0.003 1.58 ± 0.23 0.44 ± 0.10 FHDOS 7 6380 ± 50 4720 ± 570 1740 ± 10 2610 ± 440 71 ± 9 < 32 0.273 ± 0.004 1.50 ± 0.26 0.74 ± 0.10 Maximum 17300 7590 4310 6810 123 0.402 1.60 0.98 Minimum 3380 3060 1360 1930 34 0.248 1.30 0.38 Median 6380 4720 2250 2920 67 0.273 1.49 0.68 Mean ± SE 8940 ± 2110 4920 ± 670 2510 ± 440 3730 ± 750 64 ± 12 0.307 ± 0.026 1.46 ± 0.05 0.64 ± 0.08 Standard Deviation 5160 1630 1080 1830 30 0.063 0.11 0.20 Number of Samples 7

87

4.3.2 Gas industry scales

4.3.2.1 Gas scale formation and removal

Radon is a noble gas, also known to be soluble in organic matter, and its

association with natural gas streams has been known for almost 100 years

(Satterly and McLennan, 1918). From a radiological hazard point of view, 220Rn

(Ra-228 progeny, T½: 55.6 seconds) does not pose as high a risk in oil industry,

as 222Rn (Ra-226 progeny, T½: 3.824 days), the reason being that the former

only has a few short-lived progeny, whereas the latter has two long-lived

progeny, 210Pb and 210Po, with half-lives of 22.3 years and 138.4 days,

respectively.

The precipitation of 210Pb results in a thin radioactive film forming on the

internal walls of gas pipes, pumps and vessels, with a reported specific activity

higher than 1 kBq g-1 (Hamlat et al., 2003a, Al-Masri and Shwiekani, 2008).

Although Pb-210 has a soft gamma of 46.5 keV, which cannot even be detected

by a conventional dose rate meter outside the pipes, it has an extremely high

toxicity value if inhaled or ingested via dust particles, during plant maintenance.

Both stable lead and 210Pb are known to be mobilised from the reservoir rock,

however the mechanism by which this occurs is not yet well understood

(Jonkers et al., 1997, IAEA, 2003). Condensates derived from natural gas have

also shown elevated 210Po relative to its grandparent 210Pb, suggesting a direct

emanation from the reservoir rock (IAEA, 2003).

88

In addition, because 222Rn has a boiling point between ethane and propane,

unsupported 222Rn is also known to migrate with the ethane and propane

fractions of the natural gas stream. As such, high levels of 222Rn daughters

(210Pb, 210Bi and 210Po) are often found in the internal walls of ethane and

propane processing pumps and vessels.

4.3.2.2 Radioactivity in gas scales

The radioactivity of gas scale samples was also analysed using the HPGe

system. Scale samples were collected from 12 pipes, according to the sampling

and measurement procedures outlined in Chapter 3, Section 3.2.1 and 3.2.2. The

main gamma emitting radionuclides identified during the analysis were 210Pb,

227Ac, 40K and 226Ra. The full range of gamma spectroscopy results are shown in

Table 4.13.

Radium isotopes and Lead 210: The 210Pb activity concentration of the

samples ranged from 0.959-66.4 kBq kg-1, with a mean value of

30.56 ± 7.83 kBq kg-1, however the mean 226Ra activity concentration of

75 ± 10 Bq kg-1 was too low to support such activity. This suggests that the high

210Pb radioactivity may be the result of 222Rn decay, which is known to migrate

with the gaseous organic phase. While the activity ratio of 226Ra:210Pb ranged

between 1.2 x 10-3 - 2.3 x 10-2, with a mean of (7.5 ± 2.5) x 10-3, Jonkers et al.

(1997) reported detection of 210Pb activity concentrations higher than 226Ra,

with a 226Ra:210Pb activity ratio of 0.1. As such, the ratios reported in this study

were approximately two orders of magnitude less than Jonkers et al. (1997)

ratio. Mean 228Ra activity concentration was four times lower than mean 226Ra

89

activity concentration. Gas scales mean activity concentrations for both 226Ra

and 228Ra were two times higher than in ambient soils.

Thorium 228 and Potassium 40: Th-228 activity concentrations were

below HPGe system’s MDA, and therefore the data presented in Table 4.16 for

228Th, are the error weighted averages derived from its progeny; 224Ra, 212Pb,

212Bi and 208Tl. This resulted in relatively higher uncertainties in the 228Th:228Ra

activity ratios in the gas scales compared to oil scales. The mean (± standard

error) 228Th:228Ra activity ratio of 1.47 ± 0.12 results in an average gas scale age

of approximately 16 years.

Actinium 227: As mentioned in Section 4.2.2, the mean 226Ra activity

concentration in ambient soils is 34 Bq kg-1. Assuming equilibrium and uranium

mass balance, this would indicate an average 227Ac activity concentration of

1.7 Bq kg-1 in ambient soils. Higher than ambient 227Ac activity concentrations

were detected in 10 out of the 12 gas scale samples, ranging from 4-181 Bq kg-1.

Hence the maximum activity concentration of 227Ac in the gas scales was more

than 100 times higher than the ambient soil value. To date, the only other

numerical value reported for 227Ac activity concentration in gas scale was

2.5 Bq g-1, published by Kolb and Wojcik (1985).

90

Table 4.13: Activity concentrations (Bq kg-1) of gas scale samples Sample ID 226Ra 210Pb 228Ra 228Th 227Ac 40K 228Ra:226Ra 228Th:228Ra 226Ra:210Pb

ZULGS 1 114 ± 25 66405 ± 882 25 ± 2 39 ± 5 69 ± 4 73 ± 7 0.22 ± 0.06 1.58 ± 0.34 (1.7 ± 0.4)x10-3

ZULGS 2 125 ± 13 49013 ± 703 26 ± 2 41 ± 5 71 ± 4 89 ± 7 0.21 ± 0.04 1.58 ± 0.31 (2.6 ± 0.3)x10-3

ZULGS 3 33 ± 6 6241 ± 200 7.8 ± 1.1 7.4 ± 2.1 5 ± 1 347 ± 8 0.24 ± 0.08 0.95 ± 0.40 (5.3 ± 1.1)x10-3

ZULGS 4 51 ± 21 43377 ± 633 5.6 ± 1.0 7.6 ± 2.0 9 ± 4 26 ± 4 0.11 ± 0.07 1.37 ± 0.60 (1.2 ± 0.5)x10-3

ZULGS 5 53 ± 13 < 541 < 6.3 - < 6.9 76 ± 15 - - - ZULGS 6 83 ± 12 5643 ± 353 19 ± 2 41 ± 5 24 ± 3 157 ± 11 0.23 ± 0.06 2.20 ± 0.58 (1.5 ± 0.3)x10-2

ZULGS 7 84 ± 15 5991 ± 538 24 ± 4 42 ± 7 21 ± 4 212 ± 17 0.28 ± 0.10 1.76 ± 0.62 (1.4 ± 0.4)x10-2 ZULGS 8 82 ± 15 9110 ± 457 16 ± 3 13 ± 4 65 ± 4 236 ± 14 0.20 ± 0.07 0.80 ± 0.37 (9.0 ± 2.1)x10-3

ZULGS 9 < 32 54039 ± 710 6.1 ± 0.9 9.9 ± 2.2 < 7.6 30 ± 4 - 1.65 ± 0.59 - ZULGS 10 84 ± 13 52678 ± 658 24 ± 2 35 ± 5 181 ± 4 98 ± 7 0.28 ± 0.07 1.48 ± 0.32 (1.6 ± 0.3)x10-3 ZULGS 11 22 ± 4 959 ± 108 2.3 ± 0.6 2.9 ± 1.3 4 ± 1 61 ± 4 0.11 ± 0.05 1.26 ± 0.90 ( 2.3 ± 0.7)10-2

ZULGS 12 92 ± 24 42716 ± 502 32 ± 3 48 ± 5 11 ± 5 190 ± 10 0.35 ± 0.12 1.51 ± 0.30 (2.2 ± 0.6)x10-3

Maximum 125 66405 32 48 181 347 0.35 2.20 2.3 x 10-2

Minimum 22 959 2.3 2.9 4 26 0.11 0.80 1.2 x 10-3

Median 83 42716 19 35 22 94 0.22 1.51 3.9 x 10-3

Mean ± SE 75 ± 10 30561 ± 7830 17 ± 3 26 ± 6 46 ± 18 133 ± 29 0.22 ± 0.02 1.47 ± 0.12 (7.5 ± 2.5)x10-3

Standard Deviation

32 24759 10 18 55 97 0.07 0.38 7.4 x 10-3

Number of Samples

12

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4.3.3 Comparison between oil and gas scales

Figure 4.7 shows a comparison between average radionuclide activity

concentration for the oil and gas scales analysed in this study. Ac-227 activity

concentrations for both oil and gas scales were within the same order of

magnitude, whilst the 226Ra activity concentration for oil scales was two orders

of magnitude higher than for gas scales. In contrast, the 210Pb activity

concentration for gas scales was one order of magnitude higher than for oil

scales. The 226Ra:210Pb activity ratio of gas scales, being (7.5 ± 2.5) x 10-3,

supports the theory outlined earlier in this chapter, that excess 210Pb was

transported in the natural gas stream as a result of 222Rn decay, as well as being

directly mobilised from the reservoir rock. Similarly, the 210Pb:226Ra activity

ratio for oil scales, being 0.64 ± 0.08, indicates that the ratio is higher than

expected by 210Pb ingrowth from 226Ra decay, given the scale age, suggesting

direct mobilisation of 210Pb from the reservoir rock by the produced water.

92

0.01

0.1

1

10

100

Ac-227 Ra-226 Pb-210 Ra-228 Th-228 K-40

Gas scalesOil scales

Figure 4.7: Average activity concentrations for radionuclides found in oil and gas scales.

Although 228Ra and 228Th activity concentrations for oil scales were two

orders of magnitude higher than for gas scales, the 228Th:228Ra mean (± standard

error) activity ratios of 1.46 ± 0.05 and 1.47 ± 0.12, respectively, indicate a

similarity in their average age. While the mean (± standard error) for gas scales

40K was 133 ± 29 Bq kg-1, oil scale 40K activity concentrations were less than

the HPGe system’s MDA.

4.3.4 Radioactivity in the sludge stored in barrels

According to PDO’s Health, Safety and Environment Specifications (2005),

NORM contaminated sludge, with a 226Ra activity concentration equal to or

higher than 1 kBq kg-1, is separated from the rest of the sludge, packed in steel

Act

ivity

con

cent

ratio

n (B

q g-1

)

Radionuclide

93

barrels and transported to the Bahja NORM store yard as soon as possible. At

the time of our last visit to the site (21 April – 8 May 2007), there were more

than 200 barrels stored at the Bahja NORM store yard, and this number will

continue to increase, until a suitable disposal method is found for this

radioactive oily sludge.

Barrel surface dose rates of up to 40 µSv h-1 were encountered during the

field work, with six sludge samples collected from the barrels and taken back to

the laboratory for further analysis (see Chapter 3, Section 3.2.1 and 3.2.2 for

details on sampling and measurement procedures). Single samples were also

collected from separate barrels containing beads and sand. All samples were

analysed using the HPGe system and the results are shown in Table 4.14.

Radium and Thorium Isotopes: 226Ra radioactivity concentrations for the

barrel stored sludge varied over a wide range of at least two orders of

magnitude. The highest activity concentrations were 223 kBq kg-1 for 226Ra,

34 kBq kg-1 for 228Ra and 45 kBq kg-1 for 228Th, which were similar to values

measured in Brazil, reported by Matta et al. (2002), which were up to

331 kBq kg-1 for 226Ra, 245 kBq kg-1 for 228Ra and 272 kBq kg-1 for 228Th

(Table 4.15). Possible reasons for this difference have already been outlined in

Section 4.3.1.2.

Gazineu et al. (2005) also reviewed some of the existing international data

for 226Ra and 228Ra activity concentrations in sludge samples from various oil

94

exploration operators. Table 4.15 provides a summary of their findings,

alongside the findings from Matta et al. (2002) and the findings from this study.

Table 4.14: Range of sludge 226Ra and 228Ra activity concentrations (kBq kg-1) for oil exploration operations of several countries of the world

Country Material 226Ra 228Ra

Brazil * sludge 0.13 – 331 0.01 – 245

Brazil sludge 0.36 – 367 0.25 – 343

Norway sludge 0.1 – 4.7 0.1 – 4.6

Algeria sludge 0.069 – 0.393 -

Oman ** sludge (in store) 1.7 – 223 1.2 – 34.4

Oman ** sludge (separator tank) 0.36 – 0.99 0.14 – 0.45

Oman ** sludge (piles) 0.027 – 5.670 0.007 – 6.036

* Matta et al. (2002); ** This study; All other data are from Gazinue et al.

Actinium 227: Both the mean and maximum 227Ac activity concentrations,

of 188 Bq kg-1 and 614 Bq kg-1, were two orders of magnitude higher than our

theoretically calculated value of 1.7 Bq kg-1, for Oman’s ambient soils. In

addition the mean is an order of magnitude higher than both oil and gas scale

averages. However, the maximum activity concentration obtained was four

times less than the single value reported by Kolb and Wojcik (1985), for a

CaCO3 gas field scale in North Germany.

Further, the 228Th:228Ra activity ratio indicates that the sludge found in the

barrels was quite new, with a mean age of 4 years (except for sample 6, which

gave an average age of 8.3 years).

95

Table 4.15: Activity concentrations (Bq kg-1) for sludge stored in barrels Sample ID 226Ra 228Ra 228Th 227Ac 40K 228Ra:226Ra 228Th:228Ra

BHJB 1 14000 ± 100 10489 ± 32 8975 ± 37 141 ± 18 336 ± 32 0.74 ± 0.01 0.88 ± 0.01 BHJB 2 1700 ± 30 1212 ± 10 916 ± 9 286 ± 8 1480 ± 30 0.70 ± 0.02 0.80 ± 0.01 BHJB 3 5560 ± 69 3747 ± 18 3320 ± 20 40 ± 20 952 ± 33 0.66 ± 0.01 0.92 ± 0.01 BHJB 4 2000 ± 40 1478 ± 10 1218 ± 11 15 ± 7 953 ± 23 0.73 ± 0.02 0.86 ± 0.01 BHJB 5 6700 ± 70 4639 ± 20 4041 ± 21 31 ± 17 765 ± 25 0.68 ± 0.01 0.91 ± 0.01 BHJB 6 223000 ± 355 34413 ± 47 44639 ± 83 614 ± 66 < 170 0.15 ± 0.00 1.30 ± 0.00 Beads * 38012 ± 131 39491 ± 39 29109 ± 36 247 ± 24 < 80 1.04 ± 0.00 0.74 ± 0.00 Sand * 35386 ± 162 69406 ± 58 53653 ± 516 440 ± 49 324 ± 54 1.96 ± 0.01 0.77 ± 0.01 Maximum 223000 34413 44639 614 1480 0.74 1.30 Minimum 1700 1212 916 15 336 0.15 0.80 Median 6130 4193 3680 91 952 0.69 0.89 Mean ± standard error 42160 ± 39670 9330 ± 5696 10518 ± 7587 188 ± 104 897 ± 206 0.61 ± 0.10 0.94 ± 0.08 Standard deviation 88705 12738 16965 232 412 0.22 0.18 Sample count 8

* Not included in the sum calculations

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4.4 Radioactivity in the sediments of Al-Noor

evaporation ponds

Like many other industrial operations, the petroleum industry makes use of

ponds, trenches or pits to store or evaporate liquid wastes generated during oil

production. Al-Noor evaporation pond (Figure 4.7) is used for the evaporation

of produced water generated by the dehydration of crude oil at nearby Al-Noor

station (refer to map in Chapter 2, Figure 2.1). Al-Noor evaporation pond is

secured by a boundary net with a lockable gate; it has an area of 14,400 m2, a

depth of about 1.5 m and is lined by high density polyethylene thermoplastic.

The pond is comprised of two sections (Section 1 and Section 2),

interconnected by four channels, which allow water to flow from Section 1 to

Section 2. Approximately 10 m3 of produced water is pumped into Section 1 on

a daily basis, and this waste water is contaminated by NORM, heavy metals,

volatile organic compounds, polycyclic aromatic hydrocarbons and other toxic

compounds. Evaporation concentrates the NORM activity content of the

produced water, which then crystallises and eventually leads to scale formation

on the ponds internal walls. Six samples were collected from the pond for

analysis, according to the sampling and measurement procedures described in

Chapter 3, Section 3.2.1 and 3.2.2. The radioactivity of these sediment samples

was analysed using the HPGe system.

97

Figure 4.8: Section 2 of Al-Noor evaporation pond [picture courtesy of Mohammad Al-Masri].

During this study, 227Ac was detected for the first time in oil scales and

sludge, however it was not detected in produced water evaporation pond

sediments, because it is most likely transported as a vector and deposited with

scale before reaching the ponds. The samples were found to contain both 238U

and 232Th progeny, with maximum 226Ra and 228Ra activity concentrations of

5.26 and 0.58 kBq kg-1, respectively (Table 4.16). This activity was localised at

the drainage point of the pond. The median and maximum 226Ra activity

concentrations were also more than 10 and 170 times higher than the Al-Noor

ambient soil radioactivity, respectively.

At the time of our last visit to the site (21 April – 8 May 2007) the pond had

also accumulated a 50 cm thick layer of sediment. At the time of deposition, this

98

sediment was found to have a mean 228Ra:226Ra activity ratio of 0.12. This

corresponded to a 232Th:238U mass ratio of 0.36, which is approximately half the

value obtained for nascent sludge at Nimr (0.73), which may serve to illustrate

the difference in geological formation fingerprints between the two regions.

99

Table 4.16: Activity concentrations (Bq kg-1) of Al-Noor evaporation pond soil sediments Sample ID 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra

NOR 1 5260 ± 48 585 ± 9 188 ± 6 193 ± 20 0.11 0.32 NOR 2 354 ± 13 35 ± 2 17 ± 2 109 ± 8 0.10 0.48 NOR 3 119 ± 7 10 ± 1 5 ± 1 30 ± 5 0.08 0.46 NOR 4 743 ± 16 77 ± 3 31 ± 4 429 ± 13 0.10 0.40 NOR 5 379 ± 12 46 ± 2 8 ± 3 379 ± 9 0.12 0.17 NOR 6 107 ± 6 13 ± 2 5 ± 2 837 ± 12 0.12 0.40 Maximum 5260 583 205 837 0.12 0.48 Minimum 107 10 5 30 0.08 0.17 Median 367 41 13 286 0.11 0.40 Mean ± standard error 1160 ± 904 127 ± 100 46 ± 35 330 ± 131 0.11 ± 0.01 0.37 ± 0.05 Standard deviation 2022 225 79 292 0.01 0.11 Sample number 6

100

4.5 Discussion and conclusions

This chapter outlined the radioactivity concentrations found in oilfields of

the Southern Oman Directorate, where the radioactivity of untreated and treated

sludge, oil and gas scales, barrel stored sludge and evaporation pond sediment,

was assessed using both in-situ and laboratory gamma spectrometers. This is the

first comprehensive survey of radioactivity concentrations to be conducted for

the petroleum industry, in the Sultanate of Oman, and it is also the first ever

study to report the detection of 227Ac in oil sludge and in both oil and gas scales.

Ac-227 half life (21.8 years) is similar to that of 210Pb (22.3 years), but

because it is unsupported it would decay to ambient levels in seven to nine half

lives. On the other hand, the 226Ra supported 210Pb is a longer term radiological

hazard. Because once it reached secular equilibrium with its parent radionuclide

by ingrowth in about 100 years, it would decay at a 226Ra half life of 1602 years.

This study successfully made use of a portable NaI(Tl) gamma spectroscopy

system for measuring 226Ra and 228Ra activity concentration, however the

system did have its advantages and disadvantages. One obvious advantage was

the instantaneous radioactivity information that could be obtained in the field,

without having to transport radioactive samples for hundreds of kilometres back

to the laboratory. Another advantage was the relatively short acquisition time

for readings, due to the high efficiency of the scintillating NaI(Tl) 2¼” crystal

(600 s) compared to the semi-conductor HPGe crystal (60,000 s). However,

including the Pb shield, the system weighed approximately 30 kg, which was a

101

significant disadvantage. In addition, the system had a lower energy resolution

compared to the HPGe detectors.

As a result of the findings presented in this work, it was found that whilst

sludge farming was useful in eliminating harmful hydrocarbons from the sludge,

it still comes up short in the reduction of radioactivity concentrations down to

ambient soil levels. For example, although dilution factors of up to 5 times were

used, in an effort to reduce the activity concentration of 226Ra, the treated strips

still showed activity concentrations up to ten times higher than the ambient

levels found at both Nimr and Marmul sludge farms. Despite the daily tilling of

the treated sludge, some hotspots > 1 kBq kg-1 also remained on the strips.

More specifically, this study found that sludge activity concentrations from

Bahja, Nimr and Marmul sludge farms ranged from 0.03-3.69, 0.01-6.04,

0.01-5.16 and 0.05-0.95 kBq kg-1 for 226Ra, 228Ra, 228Th and 40K, respectively.

In addition, 25% and 10% of the untreated sludge piles had 226Ra activity

concentration exceeding 1 kBq kg-1, at Bahja and Marmul farms, respectively.

Bahja sludge farm had the highest mean 226Ra activity concentration of

3289 Bq kg-1, while Nimr and Marmul sludge farms had similar 226Ra mean

activity concentrations of 343 and 356 Bq kg-1, respectively. Nimr and Marmul

mean activity concentrations were found to be lower than the mean activity

concentrations for Bahja sludge piles, by one order of magnitude. Similarly, the

Bahja sludge was found to be the oldest, with a mean age of 9.0 ± 0.4 years,

while the Nimr and Marmul sludge had similar ages of 4.2 ± 0.3 and

3.6 ± 0.4 years, respectively. The nascent Nimr sludge radioactivity results can

102

now be used as a baseline with which to compare future nascent oil sludge

originating from Nimr station. They can also be used as a reference point when

formulating trends in relation to the different types of new technologies that

may be used in the oil recovery process.

This study also found that the average radium isotope activity concentration

in Oman oil scales fell within the lower end of the range of activity

concentrations reported elsewhere (0.1-15,000 kBq kg-1). The range of 226Ra

and 228Ra were 3.38-17.30 and 1.36-4.31 kBq kg-1, respectively. The mean 226Ra

activity concentration of oil scales (8.940 kBq kg-1) was also found to be higher

than the mean activity concentration of the farm sludge found in Bahja, Nimr

and Marmul, which were 3.289, 0.343 and 0.356 kBq kg-1, respectively.

Both oil and gas scales contained detectable levels of 210Pb, however only

the gas scales were characterised by the presence of high activity concentrations

of 210Pb. In gas scales, the activity concentration of 210Pb ranged from

0.959-66.405 kBq kg-1, while the corresponding range for the oil scales was

3.06-7.59 kBq kg-1. Oil and gas scales, and barrel stored sludge also contained

227Ac, with maximum activity concentrations of 123 ± 17, 181 ± 4 and 641

± 66 Bq kg-1, respectively. As such, care should be taken when clearing these

scales from oil and gas pipes, particularly in terms of exposure pathways that

involve direct irradiation, inhalation and ingestion.

Many petroleum companies in Oman dispose of excess produced water in

evaporation in ponds, or by pumping it into producing reservoirs, and

103

abandoned deep and shallow wells. As outlined above, during this study, 227Ac

was detected for the first time in oil scales and sludge, however it was not

detected in produced water evaporation pond sediments, because it is most

likely transported as a vector and deposited with scale before reaching the

ponds. However, 238U and 232Th progeny along with 40K were detected in the

Al-Noor evaporation pond sediment with activity concentrations for 226Ra,

228Ra, 228Th and 40K ranging from 0.107-3.260, 0.010-0.583, 0.005-0.205 and

0.030-0.837 kBq kg-1, respectively. These activity concentrations are similar to

the activity concentrations of untreated sludge piles found in Bahja, Nimr and

Marmul. Hence, this work established that pond sediment is also contaminated

by enhanced NORM and caution should be exercised during its disposal.

In conclusion, this study provided the first ever information on the

radioactivity concentration of treated and untreated sludge, oil and gas scales,

barrel stored sludge and evaporation pond sediment for oilfields in the Southern

Oman Directorate. The information provided in this chapter may be used as

reference for NORM activity concentration assessments in the Oman oil

industry and it may also be used as a baseline for possible future assessments in

both the onshore and offshore oil rigs of the Northern Oman Directorate.

Finally, this pioneering study is also important since similar data is yet to be

published for neighbouring oil producing Gulf countries.

104

105

Chapter 5 GAMMA DOSE RATES AT SLUDGE FARMS IN OILFIELDS OF THE SOUTHERN OMAN DIRECTORATE

5.1 Introduction

This chapter presents findings in relation to gamma dose rates measured in

the air at and around sludge farms in the southern Oman Directorate. The

chapter begins with a brief outline of the terrestrial and cosmic components of

gamma dose in the air, followed by an overview of gamma dose rates in the

petroleum industry. It then goes on to present gamma dose rates for the sludge

farms, over both untreated sludge piles and treated sludge strips (refer to

Chapter 4, Section 4.2.1 for details of the sludge farming process), as well as the

gamma dose rate correlations with the main radionuclides present in the sludge,

namely 226Ra, 228Ra and 40K.

5.2 Terrestrial and cosmic gamma dose rates

Gamma dose rates in the air, from both cosmic and terrestrial sources, have

been the focus of a large number of international studies over the last four

decades, with many factors found to affect the measured rate. The terrestrial

component of the gamma dose rate is dependent on the depth and lateral

distribution profile of activity concentrations of the primordial series 232Th, 238U

and 235U, and the primordial radionuclide 40K, along with soil type, composition

and moisture content. Measurements performed on Anthropomorphic phantoms,

in order to determine dose to organ, and effective dose, have shown that there is

106

a strong dependence of measured dose on radiation energy and incident angle

(Saito and Jacob, 1995). The cosmic component of the gamma dose rate, on the

other hand, is dependent on the altitude at which the measurements are carried

out. The worldwide outdoor altitude adjusted value for the cosmic component of

gamma dose rate is 460 µSv y-1, reported by UNSCEAR (2000).

A survey on outdoor terrestrial gamma radiation in the Sultanate of Oman,

conducted by Goddard (2002), reported that most of Oman’s surface rock is

limestone, which is low in primordial uranium and thorium. Hence, the average

gamma dose rate, measured at the conventional height of 1 m above the ground,

was lower than the world average of 0.45 mSv y-1, with the mean population

weighted dose rate for Oman found to be 0.30 mSv y-1. Goddard also tested the

validity of measured ambient dose rates, using both the ICRU (1995) and Saito

and Jacob’s (1995) dose coefficient models, on activity concentration of soil or

rock collected from the in situ measurement locations, and developed the

following equation, which gave a strong linear correlation (R2) of 0.84:

Dm = 0.89 Dc - 0.17 (5.1)

where Dm and Dc are the measured and predicted dose rates.

In addition to Goddard (2002), many other authors have also modelled

gamma dose rates based on activity concentrations that arise from natural

sources in the ground (Lovborg et al., 1979, Kocher and Sjoreen, 1985,

Battaglia and Bramati, 1988, Deworm et al., 1988, Chen, 1991, Clark et al.,

1993, Saito and Jacob, 1995, UNSCEAR, 2000, Losana et al., 2001, Ajayi,

2002). Carter and Sonter (2003) reviewed much of this literature and tabulated

107

the conversion coefficients used in each study, to find dose rates from 238U,

232Th and 40K activity concentrations. The conversion coefficients differed for

the three radionuclides and varied by a factor of about two, ranging from

0.28-0.52 µSv h-1 per Bq g-1 for 238U, 0.295-0.73 for 232Th, and 0.029-0.05 for

40K.

Saito and Jacob (1995) also calculated Kerma coefficient factors at 1 m

height per disintegration for each individual radionuclide in the main primordial

series and 40K (see Table 5.1). These conversion factors allowed for the

calculation of air Kerma for individual radionuclides, which showed that 98% of

the 238U series air Kerma was due to only two radionuclides, being 214Pb and

214Bi. Further calculations also showed that 90% of the 232Th series total air

Kerma was due to 228Ac and 208Tl. This finding indicates that, while the series is

in equilibrium, 226Ra and 228Ra, along with their precursors, only make a small

contribution to the total measured gamma dose rate.

108

Table 5.1: Air Kerma rate at 1 m height per disintegration rate (nGy h-1 per Bq kg-1) of the parent nuclide per unit soil weight for natural sources uniformly distributed in the ground (adapted from Saito and Jacob)

238U series

Kerma rate per unit activity

(µGy h-1 per Bq g-1)

232Th series

Kerma rate per unit activity

(µGy h-1 per Bq g-1)

235U series

Kerma rate per unit activity

(µGy h-1 per Bq g-1)

40K Kerma rate per unit activity

(µGy h-1 per Bq g-1) 238U 4.33x10-5 232Th 4.78x10-5 235U 3.06x10-2 40K 4.17x10-2

234Th 9.47x10-4 228Ra 5.45x10-5 231Th 1.80x10-3 234Pam 4.30x10-3 228Ac 2.21x10-1 231Pa 6.89x10-3 234Pa 4.49x10-4 228Th 3.44x10-4 227Ac 3.54x10-5 234U 5.14x10-5 224Ra 2.14x10-3 227Th 2.10x10-2

230Th 6.90x10-5 220Rn 1.73x10-4 223Fr 1.15x10-4 226Ra 1.25x10-3 212Pb 2.77x10-2 223Ra 2.39x10-2 222Rn 8.78x10-5 212Bi 2.72x10-2 219Rn 1.25x10-2 214Pb 5.46x10-2 208Tl 3.26x10-1 215Po 5.11x10-5 214Bi 4.01x10-1 211Pb 1.70x10-2 210Tl 1.51x10-4 211Bi 1.08x10-2 210Pb 2.07x10-4 207Tl 5.67x10-4

Total 4.63x10-1 6.05x10-1 1.25x10-1 4.17x10-2

109

5.2.1 Gamma dose rates in the petroleum industry

In the petroleum industry, it is recommended that external gamma radiation

dose surveys on installations should be conducted both periodically and during

shutdowns. As discussed in Chapters 1 and 4, in addition to 40K and 227Ac (235U

series), it is mainly radium isotopes and 210Pb (one of 226Ra progeny – 238U

series) from the 238U and 232Th series, which are brought to the surface during

the oil extraction process. These radium isotopes and their progeny accumulate

on the internal walls of pipes and wellheads and the external radiation dose at

these sites is mainly from the gamma energies of 226Ra and its short-lived

daughters, primarily 214Pb and 214Bi. Higher energy photons of 208Tl (one of

228Ra progeny – 232Th series) can also be encountered when the scale has

accumulated over a period of several months. Pb-210, on the other hand, has a

soft gamma of 46 keV, which cannot be detected outside the pipes by a

conventional dose rate meter. However this isotope is toxic if inhaled or

ingested in the form of dust particles, during plant maintenance.

Ac-227 was also detected in sludge, oil and gas scales, and this also

contributes to the radio-toxicity of these materials. Ac-227 has nine radioactive

progeny (refer to Chapter 1, Figure 1.2 (c)), with 227Th, 223Ra and 219Rn being

the main gamma emitting radionuclides. Ac-227 and its progeny are responsible

for 69% of 235U series air Kerma (Table 5.1). As outlined in Chapter 4,

Section 4.3.1.2 and Table 4.11 adapted from (ICRP68), the committed effective

dose coefficient from 227Ac, through the inhalation pathway, is 169, 360 and

208 times higher than that of 226Ra, 210Pb and its progeny, and 228Ra,

110

respectively. Although the committed effective dose coefficient from 227Ac

through the ingestion pathway is in the same order of magnitude as those for

226Ra, 210Pb and progeny, and 228Ra, its annual ingestion radioactivity limits are

80 and 10 times lower than those of 238U and 226Ra, respectively (ICRP, 1991).

As outlined in recently published literature (Paranhos Gazineu et al., 2005,

Salih et al., 2005), the type of duties given to temporary contract workers in the

petroleum industry is one area that requires significant attention. These duties

tend to include equipment maintenance, clearing radioactive sludge from

separation tanks, scale scraping and other general cleaning, which often pose

considerable risks from a radiation protection point of view. Whilst dose rates

outside closed vessels and pipes are usually less than 7.5 µSv h-1, dose rates as

high as 80 μSv h-1 have been reported, which can be attributed to radon

daughter deposits on the surface of some equipment (Kolb and Wojcik, 1985).

As reported by Hamlat et al. (2001), during wellhead maintenance and the

clearing of sludge from separation tanks, the dose rates can be much higher. For

example, the annual effective dose rates due to gamma radiation were found to

be 0.48, 0.04 and 0.6 mSv y-1, for normal activities in the oil sector, 1 m away

from the pipes and separation tanks, and in storage tanks and wellheads,

respectively. However, the dose rates inside these structures will be higher

again, by a factor of at least 5, depending on the extent and configuration of the

contamination. The annual effective dose received by such workers can

therefore be greater than 3 mSv y-1 (Smith, 1987).

111

A comprehensive NORM survey of all PDO station equipment, manifolds,

wellheads and sludge farms is conducted by a scintillation count rate meter

(mini-900 meter coupled with an A44 scintillation probe) at least once every

four years (Petroleum Development Oman, 2005). During these surveys, a

NORM contaminated location is indicated by a reading of five CPS above the

background count rate of three CPS, and any facilities and equipment found to

be NORM contaminated are labelled and recorded, in order to alert PDO

personnel and contract workers to the risks (Petroleum Development Oman,

2005).

5.3 Materials and Methods

Gamma dose rates were measured directly from both untreated sludge piles

and treated sludge strips in Bahja, Nimr and Marmul sludge farms.

Measurements were carried out at a height of 1 m, using portable energy

compensated GM-tube, coupled with Mini-Instrument Type 6-80 (refer to

Chapter 3, Section 3.5 for meter calibration and measurement technique details).

The instrument was calibrated before and after the field measurements and the

readings were within a 10% error limit. Calibrations were conducted using a

certified Amersham gamma radiation source of 137Cs.

A total of 77 direct gamma dose rates were recorded, 34 of which were from

untreated sludge piles and 43 of which were from treated sludge strips. The

main radionuclides found in the sludge were 226Ra (T½: 1602 years), 228Ra (T½:

5.75 years) and their progeny, as well as 40K. Gamma dose rates were correlated

112

with 226Ra, 228Ra and 40K activity concentrations, which were obtained by both

portable NaI(Tl) gamma spectroscopy and laboratory HPGe systems (refer to

Chapter 3, Sections 3.3 and 3.4). In-situ gamma spectroscopy measurements

were performed at the same locations as the gamma dose rate measurements,

and additional gamma dose rates were also derived from activity concentrations

using an empirical relation (equation 5.2 below).

5.4 Results and discussion

5.4.1 Correlation between measured and predicted gamma dose rate

The cosmic ray component of 0.06 ± 0.01 µSv h-1 (equivalent to 530

± 90 µSv y-1) derived by this study was reasonably close to the worldwide

outdoor, altitude adjusted value of 460 µSv y-1 (UNSCEAR, 2000). A study

conducted by Bouville and Lowder (1988) on global cosmic radiation,

calculated the collective dose equivalent for the world’s population as a function

of altitude and geographic latitude, in various large cities around the world.

Pakistan was the closest country to Oman that was included in the above study,

and it had an annual dose equivalent of 430 µSv, which is also close to the

empirically derived value obtained in this study (530 ± 90 µSv y-1).

Measured and predicted gamma dose rates using conversion factors

published by UNSCEAR (2000) were also compared, as illustrated in

Figure 5.1. On average, the gamma dose rates were overestimated by 64% using

UNSCEAR (2000) formula, however, there was a strong linear correlation

coefficient (R2) of 0.95 between measured and predicted gamma dose rates.

113

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure 5.1: Relation between measured and predicted gamma dose rates using UNSCEAR (2000) dose conversion factors.

5.4.2 Development of a new gamma dose rate empirical model

In order to find more accurate dose coefficients, a new correlation between

226Ra, 228Ra and 40K activity concentrations and gamma dose rate was defined

by multiple regressions using the SigmaPlot (Version 10.0) program. The

program utilises the Marquardt-Levenberg Algorithm to find the coefficients of

the independent variables that give the best fit between the equation and the

data. This resulted in the development of the following empirical relation, which

includes uncertainties and gave a correlation coefficient (R2) of 0.95. This

equation was used to determine the effective gamma dose rate for petroleum

treated and untreated sludge, as a result of 226Ra, 228Ra and 40K activity

concentrations:

Mea

sure

d ga

mm

a do

se ra

te (µ

Sv h

-1)

Predicted gamma dose rate (µSv h-1)

114

H• = (0.178 ± 0.007) CRa-226 + (0.230 ± 0.038) CRa-228 + (0.016 ± 0.046) CK-40 + HCosmic

(5.2)

where H• is the effective dose rate in µSv h-1, CRa-226, CRa-228 and CK-40 are the

activity concentrations of 226Ra, 228Ra and 40K in Bq g-1, respectively, and

HCosmic is the cosmic component of gamma dose rate, which was equal to 0.06

± 0.01 µSv h-1.

The corresponding UNSCEAR (2000) relationship used to generate

Figure 5.1 was:

H• = (0.462) CU-238 + (0.604) CTh-232 + (0.0417) CK-40 (5.3)

where H• is the effective dose rate in µSv h-1, and CU-238, CTh-232 and CK-40 are

the activity concentrations of 226Ra, 228Ra and 40K in Bq g-1 respectively.

The empirical relationship developed in this study generated approximately

30% variation between 226Ra and 228Ra dose conversion coefficients, which

compared well to the UNSCEAR (2000) 238U:232Th coefficient ratio, as well as

the total air Kerma ratio of 238U and 232Th presented in Table 5.1. However, the

calculated dose conversion coefficients were lower than the range found in

literature and 60% lower than dose coefficients reported by UNSCEAR (2000),

which may be due to one or a combination of the following reasons:

• The ambient soil radionuclides exist in unperturbed soil, whereas the

sludge piles are in a perturbed form, where the newly dumped, less

compacted sludge soil density may be lower than the ambient soil

density;

115

• The presence of heavy metal sediments and corrosive particles in the

petroleum sludge (APPEA, 2002, Omar et al., 2004) may lead to a

greater gamma attenuation coefficient relative to ambient soil;

• The ambient soil measurements were usually taken from flat extended

land, whereas untreated sludge piles are pyramidal, with a height of

1.75 ± 0.25 m and a base area of 4.0 ± 0.5 m2, and the measurements

were taken from the top of the piles. Similarly, the treated sludge strips

were laid over compacted land and had an average height of 0.4 ± 0.1 m.

The strips are 6 ± 2 m in width and 75 ± 10 m in length, separated by

4-8 m of open space, and measurements were usually taken near the

centre of the strips;

• Ambient soil radionuclides are usually found with the complete

primordial series existing in secular equilibrium (ignoring radon isotopes

exhalation) with the parent radionuclide, whereas the petroleum sludge

natural series starts from radium isotopes and not all radionuclides may

have reached equilibrium at the time of measurement; and

• There is a transient rather than secular equilibrium in the 232Th decay

chain between 228Ra and 228Th.

The empirical relation also resulted in a strong correlation between

measured and predicted gamma dose rates, as illustrated in Figure 5.2, with a

linear correlation coefficient (R2) of 0.96 and a correlation equation of:

H•

Measured = (1 ± 0.025) H•Predicted + (0 ± 0.009) (5.4)

where H•

Measured is the measured effective gamma dose rate, and H•Predicted is the

predicted or synthesised effective gamma dose rate.

116

0

0.2

0.4

0.6

0.8

1

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Figure 5.2: Relation between measured and empirically determined gamma dose rates.

Dose coefficients for the ambient soil measurements conducted during this

study were higher than those found for untreated and treated sludge

(equation 5.2 above). In fact, the calculated dose coefficients were close to the

coefficients reported by UNSCEAR (2000). Using multiple regression, the

below equation 5.5 was derived for the ambient soil samples taken from the

Southern Oman Directorate, which gave a correlation coefficient (R2) of 0.66.

However, the coefficient uncertainties were relatively high, which is likely a

result of the small number of available readings:

H• = (0.455) CRa-226 + (0.595) CRa-228 + (0.041) CK-40 + HCosmic (5.5)

where the symbols carry the same meaning as those in equation 5.2.

Predicted gamma dose rate (µSv h-1)

Mea

sure

d ga

mm

a do

se ra

te (µ

Sv h

-1)

117

Separating the untreated from the treated sludge farm data produced

somewhat different dose coefficients (see below), where the treated sludge dose

coefficients were about 20% higher than the untreated sludge values. However,

the values did overlap within the uncertainties:

H•

Untreated = (0.178 ± 0.011) CRa-226 + (0.232 ± 0.057) CRa-228 + (0.016 ± 0.076) CK-40 + HCosmic (5.6) H•

Treated = (0.217 ± 0.031) CRa-226 + (0.283 ± 0.048) CRa-228 + (0.020 ± 0.019) CK-40 + HCosmic (5.7)

This finding supports the five prior assumptions about untreated pile

geometry, soil constituent and density differences. Since the treated sludge

strips were a mixture of petroleum sludge and ambient soil, and were laid on flat

land, their derived dose coefficients were higher than untreated sludge values.

This is important when explaining the low dose coefficients that were obtained

for untreated sludge, and demonstrates that reported literature values for soil

radioactivity and gamma radiation dose do not strictly apply to petroleum

industry sludge piles.

5.4.3 Gamma dose rate measurements

During this field work, the highest measured gamma dose rate was recorded

at Bahja NORM store yard, from the sludge stored in barrels, where the

maximum barrel surface reading was 40 µSv h-1. These barrels contained

petroleum sludge with 226Ra activity concentrations as high as 223 kBq kg-1

(refer to Chapter 4, Section 4.3.4 for radioactivity in the sludge stored in

118

barrels). As expected, dose rates increased with radionuclide activity

concentrations of 226Ra and 228Ra and their progeny.

Goddard (2002) surveyed natural radioactivity in eight governorates of the

Sultanate of Oman, namely Muscat, Sharqiyah, Batinah, Dakhliyah, Wusta

(where this work was conducted), Dhahirah, Dhofar and Musandam. The mean

national average gamma dose rate from terrestrial gamma sources was

33.2 nGy h-1 and using a conversion factor between absorbed dose in air and

effective dose of 0.86 Sv Gy-1 (Clark et al., 1993), the national average effective

gamma dose rate becomes 28.6 nSv h-1. Specific to the Al-Wusta region, where

Al-Noor, Bahja, Nimr and Marmul were situated, the gamma dose rate due to

terrestrial sources was 33.6 ± 5.0 nGy h-1 (n = 72), within a range of

19.4-102.4 nGy h-1 (Goddard, 2002). Using the same conversion factor as

above, this translates to an effective gamma dose rate of 28.9 ± 4.3 nSv h-1. Our

measured Al-Noor, Bahja, Nimr and Marmul average ambient soil effective

gamma dose rate value, excluding the cosmic component of 60 ± 11 nSv h-1,

was 25.5 ± 13.8 nSv h-1, which is close to Goddard’s average value for the

region.

A total of 77 measured gamma dose rates from untreated sludge piles and

treated sludge strips are presented in Tables 5.2 and 5.3, respectively, along with

their sample identification, geographical location, gamma dose rate and the

corresponding 226Ra, 228Ra and 40K activity concentrations. A summary of the

gamma dose rates for both untreated and treated sludge at the three farms

(excluding hotspots in Marmul’s treated sludge strip numbers 7 and 44) is also

given in Table 5.4. The maximum gamma dose rates for the untreated sludge

119

piles at Bahja, Nimr and Marmul were 1.116, 0.238 and 0.613 µSv h-1,

respectively. The mean values for the three locations were 0.667 ± 0.209, 0.169

± 0.033 and 0.101 ± 0.207 µSv h-1, respectively. The values were all above the

natural background value of 0.086 ± 0.014 µSv h-1 and Bahja had the highest

gamma dose rates, due to the relatively higher 226Ra and 228Ra activity

concentrations, with ranges of 1-5 and 0.1-0.5 kBq kg-1, respectively.

120

Table 5.2: Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul untreated sludge piles Sample ID Location 40QUTM Gamma dose rate

(µSv h-1) 226Ra

(Bq kg-1) 228Ra

(Bq kg-1) 40K

(Bq kg-1) Easting Northing Bahja G01 0398306 2198873 0.496 ± 0.007 2210 ± 40 191 ± 7 185 ± 21 G02 0398318 2198877 0.464 ± 0.007 1310 ± 20 103 ± 5 252 ± 15 G03 0398316 2198885 0.591 ± 0.008 2980 ± 30 203 ± 6 229 ± 18 G04 0398318 2198888 0.530 ± 0.007 2150 ± 70 143 ± 11 264 ± 27 G05 0398318 2198898 0.596 ± 0.008 2240 ± 40 150 ± 5 254 ± 21 G06 0398328 2198909 1.116 ± 0.010 4520 ± 50 200 ± 10 209 ± 23 G07 0398363 2198960 0.550 ± 0.007 4000 ± 40 253 ± 9 217 ± 23 G14 0398367 2198958 0.440 ± 0.007 1955 ± 56 172 ± 14 524 ± 122 G15 0398360 2198953 0.637 ± 0.008 2981 ± 69 289 ± 18 597 ± 150 G16 0398355 2198950 0.686 ± 0.008 3221 ± 71 271 ± 18 566 ± 153 G17 0398351 2198944 0.740 ± 0.008 3799 ± 77 315 ± 19 591 ± 165 G18 0398342 2198933 0.851 ± 0.009 3875 ± 79 375 ± 21 779 ± 171 G19 0398340 2198929 0.949 ± 0.010 4336 ± 82 366 ± 21 767 ± 178 G20 0398333 2198920 1.098 ± 0.010 4934 ± 89 472 ± 23 758 ± 190 G21 0398330 2198915 0.693 ± 0.008 2908 ± 68 258 ± 17 638 ± 147 G22 0398320 2198906 0.744 ± 0.009 3445 ± 73 287 ± 18 954 ± 162 G23 0398310 2198896 0.591 ± 0.008 2444 ± 63 268 ± 18 742 ± 140 G24 0398309 2198890 0.529 ± 0.007 2274 ± 60 200 ± 15 409 ± 129 G25 0398300 2198878 0.378 ± 0.006 1651 ± 53 198 ± 15 469 ± 115

121

Table 5.2 (Continued): Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul untreated sludge piles Sample ID Location 40QUTM Gamma dose rate

(µSv h-1) 226Ra

(Bq kg-1)228Ra

(Bq kg-1)40K

(Bq kg-1) Easting Northing Nimr N01 0383329 2051550 0.156 ± 0.004 531 ± 13 139 ± 3 446 ± 10 N02 0383337 2051562 0.151 ± 0.004 403 ± 17 138 ± 5 487 ± 14 N03 0383356 2051569 0.159 ± 0.004 291 ± 14 118 ± 3 405 ± 10 N04 0383353 2051598 0.177 ± 0.004 314 ± 10 114 ± 3 457 ± 9 N05 0383309 2051584 0.166 ± 0.004 320 ± 11 123 ± 2 457 ± 9 N06 0383296 2051560 0.134 ± 0.004 285 ± 10 95 ± 3 439 ± 9 N07 0383347 2051609 0.238 ± 0.005 639 ± 17 270 ± 4 134 ± 10 Marmul M01 0319390 2014275 0.150 ± 0.004 195 ± 20 83 ± 12 47 ± 43 M02 0319393 2014288 0.076 ± 0.003 36 ± 6 9 ± 1 119 ± 6 M03 0319385 2014287 0.079 ± 0.003 42 ± 4 10 ± 1 135 ± 5 M04 0319378 2014292 0.076 ± 0.003 36 ± 4 7 ± 1 118 ± 5 M05 0319370 2014292 0.077 ± 0.003 38 ± 4 9 ± 1 103 ± 5 M06 0319365 2014314 0.123 ± 0.003 125 ± 9 92 ± 3 639 ± 11 M08 0319385 2014271 0.613 ± 0.008 918 ± 21 845 ± 7 548 ± 15 M09 0319392 2014271 0.448 ± 0.007 1022 ± 24 1557 ± 7 359 ± 16 Maximum 1.116 4934 1557 954 Minimum 0.076 36 7 47 Median 0.480 1803 195 443 Mean (standard deviation) 0.456 (0.303) 1836 (1557) 245 (281) 421 (234) Number 34

122

Table 5.3: Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul treated sludge strips Sample ID Location 40QUTM Gamma dose rate

(µSv h-1) 226Ra

(Bq kg-1)228Ra

(Bq kg-1)40K

(Bq kg-1) Easting Northing Bahja Strip # 018 0398766 2198993 0.081 ± 0.003 51 ± 10 13 ± 10 159 ± 30 Strip # 030 0398675 2198899 0.083 ± 0.003 55 ± 10 13 ± 10 175 ± 32 Strip # 043 0398775 2198963 0.085 ± 0.003 68 ± 10 8 ± 7 175 ± 32 Strip # 065 0398736 2198925 0.083 ± 0.003 58 ± 10 11 ± 9 163 ± 31 Strip # 070 0398698 2198876 0.084 ± 0.003 71 ± 11 13 ± 10 107 ± 29 Strip # 073 0398682 2198837 0.085 ± 0.003 52 ± 10 17 ± 12 191 ± 33 Strip # 076 0398659 2198802 0.076 ± 0.003 37 ± 9 15 ± 11 178 ± 30 Strip # 078 0398637 2198780 0.082 ± 0.003 62 ± 10 11 ± 9 172 ± 32 Strip # 080 0398617 2198763 0.086 ± 0.003 63 ± 10 11 ± 9 198 ± 33 Nimr Strip # 029 0383548 2051927 0.072 ± 0.003 18 ± 9 22 ± 8 82 ± 24 Strip # 039 0383612 2051829 0.074 ± 0.003 56 ± 11 19 ± 7 75 ± 27 Strip # 044 0383648 2051790 0.077 ± 0.003 22 ± 10 26 ± 9 102 ± 27 Strip # 052 0383579 2051742 0.080 ± 0.003 34 ± 10 20 ± 8 109 ± 27 Strip # 085 0383370 2052107 0.077 ± 0.003 49 ± 10 12 ± 6 84 ± 26 Strip # 100 0383387 2051795 0.078 ± 0.003 36 ± 8 8 ± 4 184 ± 30 Strip # 111 0383498 2051667 0.094 ± 0.003 85 ± 13 29 ± 10 126 ± 34 Strip # 116 0383401 2051590 0.083 ± 0.003 46 ± 9 12 ± 6 137 ± 29 Strip # 119 0383375 2051622 0.076 ± 0.003 37 ± 5 10 ± 1 118 ± 6 Strip # 126 0383332 2051726 0.075 ± 0.003 17 ± 9 23 ± 8 132 ± 27 Strip # 143 0383291 2051664 0.078 ± 0.003 32 ± 8 12 ± 6 113 ± 26

123

Table 5.3 (Continued): Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul treated sludge strips Sample ID Location 40QUTM Gamma dose rate

(µSv h-1) 226Ra

(Bq kg-1)228Ra

(Bq kg-1)40K

(Bq kg-1) Easting Northing Marmul Strip # 005 0319306 2014175 0.078 ± 0.003 38 ± 10 16 ± 6 119 ± 28 Strip # 007 h 0319248 2014072 0.327 ± 0.006 1920 ± 54 190 ± 30 300 ± 116 Strip # 011 0319230 2014186 0.076 ± 0.003 66 ± 12 17 ± 7 54 ± 27 Strip # 013 0319161 2014080 0.078 ± 0.003 63 ± 11 13 ± 6 114 ± 30 Strip # 018 0319120 2014219 0.086 ± 0.003 62 ± 12 25 ± 8 39 ± 27 Strip # 021 0319058 2014135 0.081 ± 0.003 56 ± 11 19 ± 7 58 ± 27 Strip # 024 0319079 2014233 0.081 ± 0.003 52 ± 12 24 ± 8 53 ± 27 Strip # 025a 0319352 2014140 0.101 ± 0.003 153 ± 17 28 ± 9 141 ± 40 Strip # 025b 0319352 2014140 0.108 ± 0.003 181 ± 16 9 ± 5 240 ± 44 Strip # 044 h - - 0.362 ± 0.006 2080 ± 40 184 ± 5 136 ± 15 Strip # 069 0319026 2014173 0.091 ± 0.003 80 ± 12 15 ± 6 284 ± 39 Strip # 102a 0319037 2014148 0.095 ± 0.003 67 ± 15 51 ± 14 214 ± 40 Strip # 102b 0319042 2014179 0.105 ± 0.003 92 ± 20 100 ± 22 204 ± 47 Strip # 102c 0319046 2014183 0.114 ± 0.003 180 ± 24 130 ± 27 137 ± 53 Strip # 112 0319143 2014210 0.077 ± 0.003 56 ± 11 21 ± 8 35 ± 26 Strip # 115 0319174 2014200 0.079 ± 0.003 22 ± 10 23 ± 8 166 ± 30 Strip # 118 0319202 2014078 0.084 ± 0.003 59 ± 11 15 ± 6 110 ± 29 Strip # 148 0319125 2014094 0.079 ± 0.003 27 ± 11 31 ± 10 103 ± 29 Strip # 183 0319261 2014182 0.081 ± 0.003 37 ± 12 31 ± 10 54 ± 27 Strip # 186 0319091 2014110 0.078 ± 0.003 42 ± 9 12 ± 5 79 ± 25 Strip # 189 0319199 2014192 0.079 ± 0.003 44 ± 10 15 ± 6 93 ± 27 Strip # 192 0319318 2014095 0.106 ± 0.003 118 ± 13 5 ± 3 205 ± 38 Strip # 193 0319283 2014108 0.099 ± 0.003 121 ± 14 11 ± 5 197 ± 38

124

Table 5.3 (Continued): Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul treated sludge strips Sample ID Location 40QUTM Gamma dose rate

(µSv h-1) 226Ra

(Bq kg-1) 228Ra

(Bq kg-1) 40K

(Bq kg-1) Easting Northing

Maximum 0.114 181 130 284 Minimum 0.072 17 5 35

Median 0.081 56 15 126

Mean (standard deviation) 0.085 (0.010) 63 (39) 22 (23) 134 (58)

Number 43 h: hotspots measured on treated sludge strips, not included in the final statistical calculations

125

Table 5.4: Summary of field measured gamma dose rates (µSv h-1) at 1 m height for Bahja, Nimr and Marmul petroleum sludge treatment farms’

Bahja Nimr Marmul

Untreated

piles

Treated

strips

Untreated

piles

Treated

strips

Untreated

piles

Treated

strips

Maximum 1.116 0.086 0.238 0.094 0.613 0.114

Minimum 0.378 0.076 0.134 0.072 0.076 0.076

Median 0.596 0.083 0.159 0.077 0.205 0.081

Mean 0.667 0.083 0.169 0.079 0.101 0.089

S.D. * 0.209 0.003 0.033 0.006 0.207 0.011

n # 19 9 7 11 8 19

* is the standard deviation # is the number of readings

The maximum recorded gamma dose rates for treated sludge strips at Bahja,

Nimr and Marmul (excluding Marmul’s hotspots on treated sludge strips 7 and

44) were 0.086, 0.094 and 0.114 µSv h-1, respectively, while the mean gamma

dose rates were 0.083 ± 0.003, 0.079 ± 0.006 and 0.089 ± 0.011 µSv h-1,

respectively. The average values for the treated sludge strips were within the

average range of measured ambient soil gamma dose rates, being 0.086

± 0.014 µSv h-1. The treated sludge hotspot readings for strips 7 and 44 at

Marmul were 0.327 ± 0.006 and 0.362 ± 0.006 µSv h-1, respectively. The direct

correlation between gamma dose rate and both 226Ra and 228Ra activity

concentrations are shown in Figure 5.3.

126

0.0

0.2

0.4

0.6

0.8

1.0

1.2

10002000

30004000

50000

100

200

300

400

500

Gam

ma

dose

rate

(µSv

h-1

)

226Ra (Bq kg-1)22

8 Ra

(Bq

kg-1 )

Bahja untreated pilesBahja treated stripsNimr untreated pilesNimr treated stripsMarmul untreated pilesMarmul treated strips

Figure 5.3: 3D graph of gamma dose rate relation with both 226Ra and 228Ra activity concentrations.

127

Although a strong correlation was observed between the measured gamma

dose rate and 226Ra and 228Ra, the correlation with 40K was only moderate. The

average measured gamma dose rate for untreated sludge in the Southern Oman

Directorate sludge farms was 0.456 ± 0.303 µSv h-1, while the average

measured gamma dose rate for treated sludge (excluding hotspots) was 0.085

± 0.010 µSv h-1. This difference in values was due to the sludge farming process

used at the farms, and that only sludge with activity concentrations < 1 kBq kg-1

was approved for the farming process. The average gamma dose rate for the

treated sludge was within the average range of measured ambient soil dose rate,

being 0.086 ± 0.014 µSv h-1.

5.4.4 Combining synthesised and measured gamma dose rates

Using the mathematical model developed in this study, it was possible to

derive gamma dose rates for locations which only had activity concentration

data recorded. This produced additional 36 data points for gamma dose rates in

the oilfields of the Southern Oman Directorate. A box plot of the entire gamma

dose rate data is shown in Figure 5.4, where the total number of samples for

each location is reported beneath each box. The box plot shows that Bahja,

Nimr and Marmul untreated sludge piles have different gamma dose rate

profiles, however the gamma dose rates of the three treated sludge farm strips

are not significantly different. The location, number of samples, mean, median

and dose rate ranges are also summarised in Table 5.5.

128

Location

Bahj

a pi

les

Bah

ja s

trips

Nim

r pile

s

Nim

r stri

ps

Mar

mul

pile

s

Mar

mul

stri

ps

Gam

ma

dose

rate

(µS

v h-

1 )

0.0

0.3

0.6

0.9

1.2

1.5

1.8

Figure 5.4: Measured and predicted gamma dose rate profiles at a 1 m height for untreated sludge piles and treated strips at Bahja, Nimr and Marmul sludge farms (where n denotes the total number of samples at each location).

Above 90th percentile 90th percentile

Median

75th percentile

25th percentile 10th percentile Below 10th percentile

n = 12n = 23 n = 14 n = 17 n = 16 n = 31

Mean

129

Table 5.5: Number of samples, mean, median and range of the dose rates for untreated and treated sludge at Bahja, Nimr and Marmul obtained by both direct measurement and empirical relation

Location Number of

samples

Mean dose rate

(µSv h-1)

Median dose rate

(µSv h-1)

Dose rate range

(µSv h-1) Bahja piles 23 0.702 ± 0.250 0.637 0.281 - 1.116 Nimr piles 14 0.166 ± 0.026 0.166 0.126 – 0.238 Marmul piles 16 0.345 ± 0.454 0.146 0.072 – 1.781 Bahja strips 12 0.084 ± 0.004 0.084 0.076 – 0.091 Nimr strips 17 0.109 ± 0.084 0.080 0.072 – 0.426 Marmul strips 31 0.115 ± 0.068 0.091 0.063 – 0.362 Total 113

Since no other published data was found on gamma dose rates in petroleum

sludge treatment farms, we could not make any comparisons with our

measurements and our empirically derived equations for dose coefficients. The

only related scientific literature found was by Smith et al. (1998), who

developed a theoretical model to find absorbed dose rates over a period of time

(up to 1000 years), using RESRAD computer code version 5.782, in order to

assess different scenarios of sludge farm remediated land, including residential,

agricultural, industrial and recreational use. For a 226Ra activity concentration of

185 Bq kg-1 above background, the expected dose was 0.6 mSv y-1, while the

empirical relationship for rehabilitated land derived through the experimental

work of this study (equation 5.7) gives a dose of 0.4 mSv y-1 for the same

activity concentration of 226Ra.

130

5.5 Conclusions

This work derived a new empirical relation between petroleum sludge

activity concentration and gamma dose rates at a height of 1 m above ground.

The derived conversion coefficients were at the lower end of the range found in

literature, for normal soils, which may have been caused by one or a

combination of the following factors: (1) the presence of heavy metal sediments

and corrosive particles in the petroleum sludge, leading to a greater attenuation

coefficient; (2) the radionuclides may not have reached equilibrium at the time

of measurement; (3) the ambient soil measurements are usually conducted on

flat extended land, whereas untreated sludge piles are in small heaps; and (4) the

greater soil density of ambient soil compared to less compacted untreated and

treated sludge. This resulted in a new empirical relation being developed for

petroleum sludge, in order to determine effective gamma dose rate from

radionuclide activity concentrations.

A strong correlation was observed between the measured gamma dose rate

and 226Ra and 228Ra, however the correlation with 40K was only moderate. The

average measured gamma dose rate for untreated sludge in the Southern Oman

Directorate sludge farms was 0.456 ± 0.303 µSv h-1, while the average

measured gamma dose rate for treated sludge (excluding hotspots) was 0.085

± 0.010 µSv h-1. This difference in values was due to the sludge farming process

used at the farms, in that only sludge with activity concentrations < 1 kBq kg-1

was approved for the farming process. The average dose rate for the treated

sludge was within the average range of measured ambient soil gamma dose rate,

being 0.086 ± 0.014 µSv h-1.

131

The effective reduction of petroleum sludge gamma dose rates down to

ambient soil levels, using the sludge farming process, was clearly evidenced by

the results obtained in this study. However, hotspots were still present, which

still raise concern, considering that the desert terrain of mainly limestone

surface rock naturally contains only low concentrations of radionuclides. As

such, this study contributes important and significant knowledge on gamma

dose rates and the derivation of dose coefficients in the petroleum industry,

since no similar work has been reported to date.

132

133

Chapter 6 RADON-222 EXHALATION FROM PETROLEUM INDUSTRY SCALE, SLUDGE AND SEDIMENT

6.1 Introduction

Radon is a naturally occurring, highly mobile, chemically inert radioactive

gas. Its isotope 222Rn is part of the 238U series, produced by the radioactive

decay of 226Ra. Because radium is widely distributed in the earth’s crust, radon

is widely distributed too, with reports of radon detection in dwellings

throughout the world (UNSCEAR, 2001). Once formed by the radioactive decay

of 226Ra, it migrates freely as a gas or is dissolved in water, without being

trapped or removed by chemical reactions.

Whilst radon is inhaled on a regular basis by people all over the world, its

inert characteristics mean that it doesn’t interact with the respiratory system

lining, and a large proportion of the inhaled radon is usually exhaled. However,

the remaining inhaled portion, along with any ingested radon, does pose a

radiological risk from internal exposure of organs, because once inhaled or

ingested, radon and its short-lived progeny will deposit their entire alpha

particle energy in an immediate localised area. From the radiological hazard

point of view, 220Rn (224Ra progeny) does not pose as high a risk as that posed

by 222Rn, since 220Rn only has a few short-lived progeny, whereas 222Rn has two

long-lived progeny, 210Pb and 210Po, with half-lives of 22.3 years and

138.4 days, respectively (refer to Figure 1.2 in Chapter 1, Section 1.3 for 238U

and 232Th primordial radioactive decay series).

134

Radon has long been known as one of the causes of lung cancer and the

radiological dangers of 222Rn reside in its four short-lived radioisotope progeny,

which can attach to ambient aerosols and be inhaled into the bronchial system.

Two of the 222Rn progeny (218Po and 214Po) are alpha emitters with energies of

6.00 MeV and 7.69 MeV respectively. The other two (214Pb and 214Bi) are beta-

gamma emitters, which also contribute to the radiation dose (Wilson and Scott,

1993). On average, 222Rn is the highest single contributor to the ambient

radiation dose to the global human population, with inhalation exposure dose

from radon and its progeny estimated to be 1.2 mSv y-1 of the total of

2.4 mSv y-1 (UNSCEAR, 2000). The other contributors are external terrestrial

radiation, cosmic and cosmogenic, and ingestion exposure.

Due to its gaseous nature, radon can also be exhaled from the ground, as

well as from various building materials, and being chemically inert, it does not

react with other elements, as it finds its way to the atmosphere. The mechanism

of radon generation and transport, along with factors contributing to its

emanation from mineral grains, have been studied by many authors, e.g. Tanner

(1964, , 1980) Semkow (1990), Morawska and Phillips (1992), Greeman and

Rose (1996), Gomez Escobar et al. (1999), and Holdsworth and Akber (2004),

and generally involves processes such as emanation, diffusion, convection,

absorption (in the liquid phase) and adsorption (onto solid surfaces)

(UNSCEAR, 2000).

135

The rate at which radon exhales from a material depends (along with other

factors) on the radon emanation fraction, which is the fraction of radon atoms

formed in a solid that escape and are free to migrate. This fraction varies

depending on several factors, but is largely determined by the physical

properties of the material bearing the radium nuclide. These include (i) radium

distribution within the material, (ii) porosity, (iii) granule surface area to volume

ratio, and (iv) the effective radon diffusion coefficient in the material. These

factors often interact in complex and counter-intuitive ways, such that the

process of radon emanation (and its consequent exhalation) tend to follow a

log-normal distribution (Rood et al., 1998, White and Rood, 2001).

Radon exhalation into the atmosphere is a two stage process – the first stage

involves emanation from the material into the porous space of the ground (as

described above), while in the second stage, the radon diffuses, either by a

process of molecular diffusion or by advection and/or convection for up to

several metres in the interstitial space, before emerging into the atmosphere.

The emergence of radon from the interstitial space to the atmosphere is termed

‘exhalation’.

As outlined in Chapter 4, Section 4.3.1.1, as oil is extracted from the

formation rock and brought to the surface, its temperature and/or pressure

decrease, which allows the solutes contained within the produced water to

precipitate. Consequently, during the extraction process, radium is co-

precipitated with barium, strontium and calcium, in the form of sulphates and

carbonates (Kolb and Wojcik, 1985, Jerez Vegueria et al., 2002a, Hamilton et

136

al., 2004, Al-Masri and Aba, 2005). This precipitation results in the formation

of hard and highly insoluble scale deposits on the interior walls of the tubular

and other production equipment. The 222Rn and 220Rn along with their progeny,

which exhaled from the scales, may also be present or build up in the crude oil

and gas streams (Rood et al., 1998, Worden et al., 2000). In addition to scale

formation, once the crude oil is transported from oil wells to processing stations,

oily sludge can form in separation tanks, as a result of the oil, water and gas

separation process. The precipitated oily sludge also contains enhanced

concentrations of radium isotopes and as a result, the exhaled 222Rn and 220Rn,

along with their progeny, may also be present and build up in the separation

tanks. However the separation tanks are ventilated prior to sludge clearing, in

order to remove H2S gas, which suggests that any radon accumulated in the

separation tank would also escape. Radon is also known to be soluble in both

organic matter and water, and therefore, the separated natural gas and produced

water streams would continue transporting radon isotopes even after the oil has

been extracted. The 222Rn long-lived decay product (210Pb) also forms a thin

radioactive film on the internal walls of gas pipes and vessels, with a reported

specific activity greater than 1 kBq g-1 (Hamlat et al., 2003a).

The earliest account of radon gas found in the hydrocarbon industry noted

its presence in Canadian natural gas, as reported by McLennan in 1904 (Satterly

and McLennan, 1918). Many authors have since investigated and published

reports on the presence of 222Rn in the natural gas industry. Whilst such

investigations are still ongoing, with the most recent work by authors including

Al-Masri and Shwiekani (2008) and Hamlat et al. (2003a), who evaluated

137

activity flux and distribution of 222Rn and radiation exposure rates in the gas

industry, few have measured emanation factors of 222Rn from petroleum scales

e.g. White and Rood (2001) and Rood et al. (1998). In addition, this study is the

first ever to evaluate 222Rn exhalation rates from barrel stored oily sludge,

treated and untreated petroleum sludge and from sediments of a produced water

evaporation pond. This study also measured 222Rn exhalation rates from

petroleum scales and ambient soils, in order to make a preliminary assessment

of 222Rn exhalation from petroleum sludge samples, and then to compare

differences in both radon exhalation rates and radon exhalation to radium

concentration ratios for the different sample types.

6.2 Materials and methods

Overall, radon emanation rates were determined for 72 petroleum industry

samples and four ambient soil samples. A further six ambient soil locations

were assessed in Australia, for comparison. Field, as well as laboratory

measurements, were performed for six different sample types, including

ambient soils, petroleum scales, barrel stored oily sludge, treated sludge,

untreated sludge and evaporation pond sediments. The ambient soils were

collected from Al-Noor, Bahja, Nimr and Marmul (refer to map in Chapter 2,

Figure 2.1), while the petroleum scale and barrel stored oily sludge samples

were collected from Bahja NORM store yard. Treated and untreated sludge

samples were collected from Bahja, Nimr and Marmul sludge farms, and the

sediment samples were collected from the Al-Noor station produced water

evaporation pond. Grab sediment samples were collected using a metal scooper,

138

to a depth of about 5 cm, and placed in plastic bottles, along with a single bead

(up to 1 cm in diameter) and a single sand sample collected from barrels of an

unknown origin, that were stored in the Bahja NORM store yard. The reason for

collecting these two additional samples was because of their high gamma count

rate. The quantity of material collected for the entire set of samples varied from

100-300 g and all samples were then transported to the laboratory in Muscat for

radon emanation assessment and spectroscopy analysis. In order to preserve

their pore and grain structure, as well as the moisture and organic content, the

samples were not subjected to any vigorous preparation procedure before being

placed in the emanometer. A detailed outline of the materials and methods used

in this study can be found in Chapter 3, Section 3.6, which includes information

on the instruments used, as well as techniques applied for measuring 222Rn

exhalation rate.

6.3 Results and discussion

6.3.1 Rn-222 exhalation rates

Tables 6.1(a) and 6.1(b) present the 222Rn exhalation rate, 226Ra activity

concentration and 222Rn exhalation to 226Ra activity ratio for untreated sludge

piles, treated sludge strips, oil scales, sediments and barrel stored oily sludge.

Only emanometer 222Rn exhalation rates were used to determine the 222Rn to

226Ra ratio for samples with both charcoal cups and emanometer values.

Table 6.1(a) displays 222Rn exhalation rates determined by both the charcoal

cups and the emanometer, which were generally similar and within

measurement uncertainties. A good linear correlation (R2 = 0.92) was also found

139

between the emanometer and the charcoal cup readings (Figure 6.1), according

to the following correlation equation:

XEmanometer = (0.95 ± 0.05) XCharcoal Cup (6.1)

0

150

300

450

600

750

900

0 150 300 450 600 750 900

Figure 6.1: Emanometer to charcoal cup readings correlation.

Table 6.1(b) displays 222Rn exhalation rates determined by only the

emanometer. Table 6.1(b) also includes the results from the additional bead and

sand samples, however these values were not included in the averages.

Eman

omet

er 22

2 Rn

exha

latio

n ra

te

(mB

q m

-2 s-1

)

Charcoal cup 222Rn exhalation rate (mBq m-2 s-1)

140

Table 6.1(a): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for various petroleum industry radioactive wastes

Sample ID

222Rn (mBq m-2 s-1)c

222Rn (mBq m-2 s-1)e

226Ra (Bq kg-1)

222Rn:226Ra ratio (mBq m-2 s-1/Bq kg-1)

Bahja untreated sludge piles

G01 579 ± 99 598 ± 79 2210 ± 40 0.27 ± 0.04

G02 316 ± 55 218 ± 35 1310 ± 20 0.17 ± 0.03

G03 303 ± 53 360 ± 47 2980 ± 30 0.12 ± 0.02

G04 249 ± 44 300 ± 40 2150 ± 70 0.14 ± 0.02

G05 301 ± 53 189 ± 24 2240 ± 40 0.08 ± 0.01

G06 691 ± 117 579 ± 69 4520 ± 50 0.13 ± 0.02

G07 325 ± 57 447 ± 49 4000 ± 40 0.11 ± 0.01

Bahja NORM store yard pit sludge

Gp1 45 ± 12 - 1245 ± 36 0.04 ± 0.01

Gp2 33 ± 9 - 1385 ± 36 0.02 ± 0.01

Nimr untreated sludge piles

N01 7.7 ± 3.0 39 ± 9 531 ± 13 0.07 ± 0.02

N02 14 ± 4 37 ± 7 403 ± 17 0.09 ± 0.02

N03 8.6 ± 3.4 26 ± 5 291 ± 14 0.09 ± 0.02

N04 16 ± 5 23 ± 5 314 ± 10 0.07 ± 0.02

N05 6.3 ± 2.8 19 ± 4 320 ± 11 0.06 ± 0.02

N06 52 ± 11 23 ± 4 285 ± 10 0.08 ± 0.02

N07 2.8 ± 2.1 15 ± 3 639 ± 17 0.024 ± 0.005

N08 - 16 ± 4 73 ± 7 0.22 ± 0.08

N09 - 76 ± 21 309 ± 19 0.25 ± 0.08

141

Table 6.1(a) (Continued): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for various petroleum industry radioactive wastes

Sample ID

222Rn (mBq m-2

s-1)c

222Rn (mBq m-2

s-1)e

226Ra (Bq kg-1)

222Rn:226Ra ratio (mBq m-2 s-1/Bq kg-1)

N10 - 37 ± 9 284 ± 14 0.13 ± 0.04

N11 - 42 ± 10 361 ± 17 0.12 ± 0.03

N12 - 13 ± 3 327 ± 18 0.04 ± 0.01

N13 - 31 ± 8 339 ± 14 0.09 ± 0.03

N14 - 18 ± 4 334 ± 17 0.05 ± 0.02

Marmul untreated sludge piles

M02 4.3 ± 2.4 18 ± 11 36 ± 6 0.49 ± 0.38

M03 - 12 ± 7 42 ± 4 0.29 ± 0.20

M04 4.3 ± 2.4 12 ± 6 36 ± 4 0.33 ± 0.21

M05 - 17 ± 10 38 ± 4 0.44 ± 0.31

M06 13 ± 4 54 ± 17 125 ± 9 0.43 ± 0.17

M07 5.5 ± 2.6 14 ± 7 46 ± 6 0.30 ± 0.19

M08 56 ± 12 63 ± 16 918 ± 21 0.07 ± 0.02

M09 10 ± 4 27 ± 11 1022 ± 24 0.03 ± 0.01

M10 - 13 ± 8 27 ± 3 0.48 ± 0.37

M11 - 66 ± 18 146 ± 9 0.45 ± 0.15

M12 - 38 ± 9 196 ± 13 0.19 ± 0.06

M13 - 82 ± 20 3690 ± 60 0.02 ± 0.01

M14 - 155 ± 30 2290 ± 40 0.07 ± 0.01 c Determined by charcoal cups e Determined by the emanometer

142

Table 6.1(b): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for petroleum industry’s various radioactive wastes (emanometer measurements only)

Sample ID

222Rn (mBq m-2 s-1)e

226Ra (Bq kg-1)

222Rn:226Ra ratio (mBq m-2 s-1/Bq kg-1)

Bahja treated sludge strips

Strip # 005 23 ± 12 48 ± 6 0.48 ± 0.30

Strip # 008 10 ± 5 52 ± 6 0.19 ± 0.12

Strip # 039 20 ± 12 47 ± 6 0.42 ± 0.32

Nimr treated sludge strips

Strip # 001 37 ± 11 133 ± 8 0.28 ± 0.10

Strip # 024 10 ± 3 108 ± 7 0.09 ± 0.03

Strip # 026 14 ± 4 197 ± 10 0.07 ± 0.02

Strip # 044 35 ± 10 260 ± 10 0.14 ± 0.04

Strip # 045 19 ± 9 52 ± 5 0.36 ± 0.20

Strip # 061 22 ± 6 1340 ± 26 0.016 ± 0.004

Strip # 119 6 ± 3 37 ± 5 0.15 ± 0.09

Marmul treated sludge strips

Strip # 015 12 ± 2 10 ± 1 1.23 ± 0.32

Strip # 018 34 ± 6 414 ± 13 0.08 ± 0.02

Strip # 020 12 ± 3 295 ± 19 0.04 ± 0.01

Strip # 027 13 ± 5 175 ± 9 0.08 ± 0.03

Strip # 031 7 ± 3 79 ± 7 0.09 ± 0.04

Strip # 033 24 ± 5 205 ± 11 0.12 ± 0.03

Strip # 044 9 ± 3 2080 ± 40 0.005 ± 0.002

Strip # 046 11 ± 4 455 ± 15 0.02 ± 0.01

Strip # 062 16 ± 4 115 ± 8 0.14 ± 0.05

143

Table 6.1(b) (Continued): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for petroleum industry’s various radioactive wastes (emanometer measurements only)

Sample ID

222Rn (mBq m-2 s-1)e

226Ra (Bq kg-1)

222Rn:226Ra ratio (mBq m-2 s-1/Bq kg-1)

Scale samples

FHDOS 1 583 ± 66 4360 ± 50 0.13 ± 0.02

FHDOS 2 2760 ± 242 13000 ± 100 0.21 ± 0.02

FUDOS 3 274 ± 39 3380 ± 40 0.08 ± 0.01

FHDOS 4 1509 ± 156 11800 ± 100 0.13 ± 0.01

FHDOS 5 3209 ± 260 6340 ± 60 0.51 ± 0.05

FHDOS 6 5107 ± 415 17300 ± 100 0.30 ± 0.03

FHDOS 7 2779 ± 231 6380 ± 50 0.44 ± 0.04

Al-Noor evaporation pond sediments

NOR 1 102 ± 20 5260 ± 48 0.019 ± 0.004

NOR 2 108 ± 35 354 ± 13 0.31 ± 0.11

NOR 3 84 ± 35 119 ± 7 0.71 ± 0.34

NOR 4 120 ± 40 743 ± 16 0.16 ± 0.06

NOR 5 77 ± 31 379 ± 12 0.20 ± 0.09

NOR 6 31 ± 15 107 ± 6 0.29 ± 0.15

Barrel stored oily sludge, beads and sand

BHJB 1 152 ± 20 14000 ± 100 0.011 ± 0.001

BHJB 2 40 ± 7 1700 ± 30 0.023 ± 0.005

BHJB 3 79 ± 12 5560 ± 69 0.014 ± 0.002

BHJB 4 26 ± 5 2000 ± 40 0.013 ± 0.003

Beads 18 ± 3 38012 ± 131 0.0005 ± 0.0001

Sand 712 ± 87 35386 ± 162 0.020 ± 0.003 e Determined by the emanometer

144

A general upward trend was observed for 222Rn exhalation rate versus 226Ra

activity concentration (Figure 6.2). The overall slope and correlation coefficient

(R2) for 222Rn exhalation rate to 226Ra activity concentration were 0.095

± 0.014 mBq m-2 s-1/Bq kg-1 and 0.37, respectively. These results confirm a

general consensus between 222Rn exhalation and 226Ra activity concentration.

0.1

1.0

10.0

100.0

1,000.0

10,000.0

1 10 100 1,000 10,000 100,000

Bahja Strips

Bahja Piles

Nimr Strips

Nimr Piles

Marmul Strips

Marmul Piles

Al-Noor Sediments

Barrel Sludge

Scales

Figure 6.2: 222Rn exhalation rate versus 226Ra activity concentration.

Because radon emanation and its consequent exhalation is a random process,

depending upon multiple variables, it tends to follow a log-normal distribution

(refer to Section 6.1). Therefore, a summary of the exhalation data would be

better presented as a geometric mean rather than an arithmetic mean. However,

in this study; both arithmetic and geometric means have been presented.

Table 6.2 presents a summary of all of the measurements carried out in oilfields

of the Southern Oman Directorate. It includes 222Rn exhalation rate arithmetic

226Ra activity concentration (Bq kg-1)

222 R

n ex

hala

tion

rate

(mB

q m

-2 s-1

)

145

and geometric mean, along with the maximum and minimum measured values

for scales, evaporation pond sediments, barrel stored oily sludge, sludge piles,

sludge strips and ambient soil. It also contains the calculated arithmetic and

geometric mean 222Rn exhalation rate to 226Ra activity concentration ratio for

the samples.

Radon Exhalation Rate: Ambient soil radon exhalation rate measurements

were performed in Bahja, Al-Noor, Nimr and Marmul, and the arithmetic and

geometric mean values for the above locations were 3.7 ± 2.1 and

7.90.11.3 mBq m-2 s-1, respectively. For comparison purposes, six environmental

measurements of radon exhalation rates were also carried out in Jimboomba,

Queensland, Australia using activated charcoal cups. The calculated arithmetic

and geometric mean radon exhalation rates were 4.5 ± 0.7 and

9.51.33.4 mBq m-2 s-1, respectively. Given measurement uncertainties, the two

ambient soil 222Rn exhalation rates were similar.

The range, median and both arithmetic and geometric means of the 222Rn

exhalation rate for the six different sample types are illustrated in Figure 6.3.

Overall, the radon exhalation rates for the treated sludge strips were higher than

the rates for ambient soil, which can be attributed to the presence of 226Ra in the

sludge strips, introduced by mixing of sludge with clean soil. However, the

treated sludge strips exhalation rates ( 26915 mBq m-2 s-1) were lower than for the

untreated sludge piles ( 1541547 mBq m-2 s-1). The 222Rn exhalation rate for the

scales was three orders of magnitude higher than the ambient soil, and barrel

146

stored oily sludge had comparable radon exhalations to the Al-Noor water pond

sediments.

Rn-222 Exhalation Rate to 226Ra Activity Concentration Ratio:

Figure 6.4 presents the range, median and both arithmetic and geometric means

of the 222Rn exhalation rate to 226Ra activity concentration ratio for the

individual sample types. Because of the potential affects of the log-normal

distribution of data and the modest sample size on F-statistics: the Kruskal-

Wallis non-parametric test, followed by Mann-Whitney post-hoc analysis was

used to compare sample median scores across the groups. The analysis showed

that the difference between the averages for the 222Rn exhalation rate to 226Ra

activity concentration ratio for ambient soil, treated and untreated sludge, scales

and pond sediments was not statistically significant. The geometric mean 222Rn

exhalation rate to 226Ra activity concentration ratio for treated sludge strips and

untreated sludge piles were comparable ( 37.003.010.0 and

29.005.012.0 mBq m-2 s-1/Bq kg-1, respectively), however the Bahja barrel stored

oily sludge geometric mean ( 020.0011.0015.0 mBq m-2 s-1/Bq kg-1) was one order of

magnitude lower than the means for the rest of the samples and this difference

was found to be statistically significant (p < 0.05). As previously outlined in

Chapter 3, the collected sludge was either dry (if exposed to open sun for an

extended period of time) or oily (if recently removed from separation tanks or

stored in barrels). As such, the significantly lower 222Rn exhalation rate to 226Ra

activity concentration ratio for the Bahja barrel stored oily sludge might be a

result of its higher oily hydrocarbon content.

147

The radon exhalation to radium concentration ratios for both pond sediments

and scales ( 62.006.019.0 and 42.0

11.021.0 mBq m-2 s-1/Bq kg-1, respectively) were about

twice those obtained for ambient soil, treated sludge and untreated sludge. In

pond sediments, the 226Ra (originating from produced water) is thought to be

present as a surface coating on the sediment grains, resulting in a non-uniform

distribution, which may explain the greater fraction of 222Rn emanation per

226Ra activity concentration for the pond sediment samples. In contrast, a

uniform distribution of 226Ra is expected in the scale grains throughout the bulk

volume of the scale where it is contained within lattice of the barite or sulphite

(White and Rood, 2001), but because the scale grains are finer relative to sludge

grains, hence higher grain surface to volume ratio which may also explain the

greater fraction of 222Rn emanation per 226Ra activity concentration of the scale

samples.

148

Table 6.2: Arithmetic mean, geometric mean maximum and minimum 222Rn exhalation rates, and arithmetic and geometric means of 222Rn:226Ra ratio for the various samples

Number of

samples

222Rn 222Rn:226Ra ratio

Sample type arithmetic mean a geometric mean b maximum minimum arithmetic meana geometric meanb

(mBq m-2 s-1) (mBq m-2 s-1/Bq kg-1)

Scales 7 2317 ± 684 46355721628 5107 274 0.26 ± 0.07 42.0

11.021.0

Al-Noor pond sediment 6 87 ± 14 1324980 120 31 0.28 ± 0.10 62.0

06.019.0

Barrel stored sludge 4 74 ± 33 1292759 152 26 0.015 ± 0.003 020.0

011.0015.0

Bahja piles 9 309 ± 74 62075216 598 33 0.12 ± 0.03 21.0

05.010.0

Nimr piles 13 30 ±5 431627 76 13 0.10 ± 0.02 16.0

05.008.0

Marmul piles 14 44 ± 12 731331 155 12 0.28 ± 0.05 57.0

06.019.0

All pile samples 36 104 ± 27 1541547 598 12 0.17 ± 0.00 29.0

05.012.0

Bahja strips 3 18 ± 5 261117 23 10 0.36 ± 0.11 56.0

21.034.0

Nimr strips 9 20 ± 5 29817 37 6 0.16 ± 0.05 32.0

04.011.0

Marmul strips 7 15 ± 3 22914 34 7 0.20 ± 0.14 32.0

02.007.0

All strip samples 19 18 ± 2 26915 37 6 0.20 ± 0.02 37.0

03.010.0

149

Table 6.2 (Continued): Arithmetic mean, geometric mean maximum and minimum 222Rn exhalation rates, and arithmetic and geometric means of 222Rn:226Ra ratio for the various samples

Number of

samples

222Rn 222Rn:226Ra ratio

Sample type arithmetic mean a geometric mean b Maximum Minimum arithmetic meana geometric meanb

(mBq m-2 s-1) (mBq m-2 s-1/Bq kg-1)

Ambient soil 4 3.7 ± 2.1 7.90.11.3 8.2 0.7 0.13 ± 0.06 26.0

03.009.0

Ambient soilc 6 4.5 ± 0.7 9.51.33.4 6.3 2.9 - -

Beads 1 18 ± 3 d - - - 0.0005 ± 0.0001 d -

Sand 1 712 ± 87 d - - - 0.020 ± 0.003 d - a The errors reported are the arithmetic standard error b The upper and lower values geometric standard deviation from the geometric mean c Measurements carried out in Jimboomba, SE Queensland, Australia, included for comparison d Uncertainties represent counting error

150

Sample type

Am

bien

t soi

l

Trea

ted

slud

ge s

trip

Unt

reat

ed s

ludg

e pi

le

Bar

rel s

tore

d sl

udge

Pon

d se

dim

ent

Sca

le

222 R

n ex

hala

tion

rate

(mB

q m

-2 s

-1)

1

10

100

1000

10000

Figure 6.3: 222Rn exhalation rate range and averages for the various sample types.

95th percentile 90th percentile

Median

75th percentile

25th percentile 10th percentile

5th percentile

n = 19n = 4 n = 36 n = 6 n = 6 n = 7

Mean

Geomean

151

Sample type

Am

bien

t soi

l

Barre

l sto

red

slud

ge

Unt

reat

ed s

ludg

e pi

le

Trea

ted

slud

ge s

trip

Pond

sed

imen

t

Sca

le

222 R

n ex

hala

tion

rate

(mB

q m

2 s-1

) / 22

6 Ra

activ

ity c

once

ntra

tion

(Bq

kg-1

)

0.01

0.1

1

Figure 6.4: Ratio of 222Rn exhalation rate to 226Ra activity concentration range and averages for the various sample types.

90th percentile

5th percentile

75th percentile

95th percentile

10th percentile

Median

25th percentile

n = 6 n = 7n = 6 n = 36 n = 19n = 4

Mean

Geomean

152

Many authors have studied the effect of moisture content on radon

exhalation rate (Stranden et al., 1984, Hart and Levins, 1986, King and

Minissale, 1994, Shweikani et al., 1995, Sun and Furbish, 1995, Menetrez et al.,

1996, Nielson et al., 1996, Jha et al., 2000, Barillon et al., 2005, Faheem and

Matiullah, 2008), and they all agreed that as moisture content is increased,

radon exhalation rate increases to a maximum point, followed by a subsequent

decrease. This is because the presence of a small amount of water increases the

emanation rate by stopping the recoiled radon in the interstitial space, where

some of the additional stopped radon atoms would escape from water and

diffuse to the surface. However as the amount of water in the sample increases,

it leads to more radon getting trapped, and hence a decrease in the exhalation

rate.

It has been known since the early 1900s that radon is more soluble in

organic liquids than it is in water (Tanner, 1980), however there is no published

data available on the effect of hydrocarbon content on radon exhalation rate for

petroleum industry sludge. The results of this study suggest that samples with an

appreciable amount of oily hydrocarbon content tended to exhale less 222Rn,

which could be due to the fact that the higher the organic liquid content in the

sludge sample, the greater the proportion of recoiled radon that is absorbed and

retained.

153

6.4 Conclusions

Results of this study showed a direct relationship between 222Rn exhalation

and 226Ra activity concentration, along with a variation in 222Rn exhalation rates

up to three orders of magnitude for the various types of samples investigated.

The geometric mean of 222Rn exhalation rate for the surveyed samples, was

greatest for scales, followed by soil sediments, barrel stored oily sludge,

untreated sludge, treated sludge and ambient soil. The observed lower 222Rn

exhalation rates in treated sludge ( 26915 mBq m-2 s-1), when compared to

untreated sludge ( 1541547 mBq m-2 s-1), can be attributed to the lower 226Ra

activity concentration in the treated sludge, as a result of mixing the sludge with

clean soil at the sludge farms.

An investigation of 222Rn exhalation rate and 226Ra activity concentration

ratios showed that apart from barrel stored oily sludge, there was no statistically

significant difference between the ratios for ambient soil and the rest of sample

types. The significant difference observed for barrel stored oily sludge is most

likely due to the absorption of radon in the liquid hydrocarbon organic content

of the barrel stored oily sludge.

No published data is currently available on the effect of hydrocarbon content

on radon exhalation rate for petroleum industry sludge and when coupled with

the complexity of radon exhalation, which is a function of multiple parameters,

it is evident that further experimental work is required to verify the relation

154

between radon exhalation rate and liquid hydrocarbon organic content of the

samples.

155

Chapter 7 SUMMARY AND CONCLUSIONS

7.1 Summary

This study is the first comprehensive assessment and evaluation of the

activity concentration, gamma dose rate and radon exhalation of large-scale

onshore petroleum operations in the Sultanate of Oman. It was carried out in the

arid desert terrain of an operational oil exploration and production region of

Oman, namely the Southern Oman Directorate (SOD), with the main locations

visited during the study being Al-Noor, Bahja, Nimr and Marmul.

This study focused on the radioactive waste products that are generated

during oil exploration and production. The assessment covered:

(i) Concentration of naturally occurring radionuclides;

(ii) External radiation dose rates in areas where waste products are

accumulated and disposed; and

(iii) Radon (222Rn) exhalation rates.

The types of activities covered included:

• Sludge recovery from separation tanks

• Sludge farming

• NORM storage

• Scaling in oil tubulars

• Scaling in gas production

• Sedimentation in produced water evaporation ponds

156

Crude oil is usually co-produced with high salinity produced water, which

coexists with the crude oil in oil reservoirs. Because radium is soluble in water,

it is transported through oil production and processing installations as dissolved

radium in the produced water. It is then co-precipitated with calcium and

barium, in the form of carbonates and sulphates, as hard and highly insoluble

scale deposits on the interior walls of the pipes, and as sludge at the bottom of

separation tanks. In addition to increasing production costs, as a result of the

maintenance and downtime associated with scaled equipment replacement and

sludge removal, the scales also reduce efficiency by clogging valves, restricting

flow and damaging equipment.

After extraction, one of the disposal methods of the excess produced water

is pumping into evaporation ponds, and this waste water is contaminated by

NORM, heavy metals, volatile organic compounds, polycyclic aromatic

hydrocarbons and other toxic compounds. Evaporation concentrates the NORM

activity content of the produced water, which then crystallises and eventually

leads to scale formation on the ponds internal walls. Therefore, radium isotopes

were expected to be the major contributor to the activity in scale, sludge and

produced water evaporation pond sediments. Pb-210 was also expected to be

present in the older oil scales and sludge, as a result of ingrowth with the decay

of 226Ra.

For ease of interpretation, the results are provided in three separate tables.

Tables 7.1 (a-c) summarise findings for the identified radionuclide activity

concentrations (measured by both HPGe and portable NaI(Tl) gamma

157

spectrometers), gamma dose rates (measured by an energy compensated GM-

tube) and radon exhalation rates (measured by both charcoal cups and an

emanometer), respectively.

Table 7.1 (a) presents the range, median and mean radionuclide activity

concentrations for the various sample types. From the table it can be seen that

the gamma spectroscopy analysis of sludge, oil scale, gas scale and pond

sediment showed a large spectrum of radionuclides. These radionuclides are

progeny of the naturally occurring primordial series 238U, 235U and 232Th, along

with 40K. All activity concentrations were higher than the ambient soil level and

varied over several orders of magnitude.

The results also show excess activity of 210Pb in the oil scales, suggesting

that as well as being supported by 226Ra, mobilisation from oil reservoirs by the

produced water also took place. However the excess activity of 210Pb in the gas

scales is not supported by 226Ra, but is a result of 222Rn decay as it migrates in

the organic gaseous stream, and some authors have also suggested direct

mobilisation of 210Pb from the gas reservoir rock, in a process that is not yet

understood.

An interesting feature of our findings is the detection 227Ac, which was not

supported by 235U. During this study, Ac-227 was detected for the first time in

oil scales and sludge, however it was not detected in produced water

evaporation pond sediments, because it is most likely transported as a vector

and deposited with scale before reaching the ponds. Ac-227 half life

158

(21.8 years) is similar to that of 210Pb (22.3 years), but because it is unsupported

it would decay to ambient levels in seven to nine half lives. On the other hand,

the 226Ra supported 210Pb is a long term radiological hazard. Its activity

concentration will increase by ingrowth with 226Ra decay, reaching secular

equilibrium with 226Ra in about 100 years, and consequently decaying at 226Ra

half life of 1602 years.

The average oil scales 226Ra activity concentration (8.9 kBq kg-1) in Oman

fell within the lower end of the world wide range reported by other studies (0.1-

15,000 kBq kg-1). The stored sludge activity concentrations were also on par

with the world wide reported activities, and the 226Ra activity concentrations

were found to be similar to those reported for Australian uranium mining

activities.

The mean 228Ra:226Ra in sludge at time of deposition in a Nimr separation

tank was found to be 0.87 97.079.0 . The freshly removed sludge age calculated by

228Th:228Ra activity ratio was 5.8 years, which was consistent with the industry’s

separation tank clearance frequency of 5 years. No correlation was observed

between sludge age and the 226Ra and 228Ra activity concentrations for each

individual farm, indicating the radioactivity levels had been consistent over the

years. Dating the oil and gas scales using the 228Th:228Ra activity ratio gave an

average age of 15 and 16 years, respectively.

159

Table 7.1 (a): Range median and mean activity concentrations of 226Ra, 210Pb, 228Ra, 228Th, 227Ac and 40K in Bq kg-1, for the various sample types analysed in this study

Activity Concentrations Number

Sample type Median (Range) Mean of

226Ra 210Pb 228Ra 228Th 227Ac 40K samples

Sludge

Stored in barrels 6130(1700-223000)42160 - 4193(1212-34413)9330 3680(916-44639)10518 91(15-614)188 952(336-1480)897 6

Freshly removed from separation tank

547(363-985)588 - 243(139-446)264 271(186-496)296 - 118(32-151)109 6

Untreated piles

Bahja 3164(1090-5670)3289 - 286(92-470)261 344(112-607)338 - 272(182-954)427 25

Nimr 323(73-639)343 - 123(55-270)129 113(48-281)123 - 448(134-595)433 14

Marmul a (27-3690)356 - (7-6036)394 (5-5164)342 - (47-720)360 16

Treated strips

Bahja 54(37-71)55 - 13(8-20)14 10(6-15)11 - 174(107-258)175 12

Nimr b 47(16-260)74 - 21(8-130)34 20(7-136)33 - 127(75-369)146 16

Marmul strips b 67(9.5-455)116 - 23(1.9-130)41 26(2.3-144)45 - 141(16-467)163 29

160

Table 7.1 (a) (Continued): Range median and mean activity concentrations of 226Ra, 210Pb, 228Ra, 228Th, 227Ac and 40K in Bq kg-1, for the various sample types analysed in this study

Activity concentrations Number

Sample type Median (Range) Mean of

226Ra 210Pb 228Ra 228Th 227Ac 40K samples

Oil tubular scales

6380(3380-17300)8940 4720(3060-7590)4920 2250(1360-4310)2510 2920(1930-6810)3730 67(34-123)64 - 7

Gas tubular scales

83(22-125)75 42716(959-66405)30561 19(2.3-32)17 35(2.9-48)26 22(4-181)46 94(26-347)133 12

Produced water evaporation sediments

367(107-5260)1160 - 41(10-583)127 13(5-205)46 - 286(30-837)330 6

Ambient soil 34(27–41)34 - 8(6-11)8 7(6-9)7 - 110(93-293)151 4 a area weighted mean values are reported due to distinct low and high radioactivity sections b excluding hotspots

161

Field surveys of gamma radiation dose rates were carried out in open spaces

where ‘sludge farming’ occurred. Sludge farming is the name given to the

biodegradation process which utilises naturally occurring micro-organisms to

reduce the complex hydrocarbon components of sludge into carbon dioxide and

water. This process occurs after the sludge has been mixed with clean soil and

has undergone frequent tilling and watering. The difference in radioactivity

concentrations between untreated and treated sludge is evident from the values

presented in Table 7.1 (b). Despite obtaining a 226Ra mean activity

concentration and 222Rn exhalation rate at least two times and five times higher

than the ambient soils, respectively (Table 7.1 (a and c)), the treated sludge strip

gamma dose rate averages were close to the ambient soil levels, although some

‘spots’ were detected at Nimr and Marmul sludge farms, which gave higher

gamma dose rates. The reason for this discrepancy in activity concentration and

radon exhalation rate to gamma dose rate averages might be due to the finite

thickness (40 ± 10 cm) of the treated layer of sludge. Therefore, the gamma

dose rate readings cannot be used as a reliable indicator in the assessment of the

radiological contamination of treated sludge.

162

Table 7.1 (b): Mean (± standard deviation), median and range of gamma dose rates in µSv h-1 for untreated and treated sludge in Bahja, Nimr and Marmul sludge farms, and ambient soil readings

Sample type Dose rates (µSv h-1) Number

Mean Median Range of readings

Untreated sludge piles

Bahja 0.702 ± 0.250 0.637 0.281 - 1.116 23

Nimr 0.166 ± 0.026 0.166 0.126 – 0.238 14

Marmul 0.345 ± 0.454 0.146 0.072 – 1.781 16

Treated sludge strips

Bahja 0.084 ± 0.004 0.084 0.076 – 0.091 12

Nimr 0.109 ± 0.084 0.080 0.072 – 0.426 17

Marmul 0.115 ± 0.068 0.091 0.063 – 0.362 31

Ambient soil 0.086 ± 0.014 0.082 0.074 - 0.105 4

Table 7.1 (c) presents 222Rn exhalation rates and radon exhalation to radium

concentration ratios for the various samples analysed in this study. The

geometric mean of ambient soil exhalation rate for area surrounding the sludge

was 7.90.11.3 mBq m-2 s-1. Radon exhalation rates reported in oil waste products

were all higher than the ambient soil value and varied over three orders of

magnitude. Rn-222 exhalation to 226Ra concentration ratios for sludge farm

treated and untreated sludge were similar to the ambient soil value, whereas the

oil scale and pond sediment values were twice as high. The reason for this

difference might be that the scale grains are finer than the sludge grains,

resulting in a higher emanation fraction. In the pond sediment, the 226Ra

(originating from produced water) is thought to be present as a surface coating

on the sediment grains, resulting in a non-uniform distribution, which may

explain the greater fraction of 222Rn emanation per 226Ra activity concentration.

163

In contrast, the oily sludge had a low radon exhalation to radium concentration

ratio. Because radon is known to be soluble in water and organic liquids, the

results suggest a greater proportion of recoiled radon in the oily sludge samples

is absorbed and retained, hence resulting in the lower 222Rn:226Ra ratio.

164

Table 7.1 (c): Maximum, minimum and geometric mean of 222Rn exhalation rates in mBq m-2 s-1 and the geometric mean of radon exhalation to radium concentration ratio in mBq m-2 s-1/Bq kg-1 for the various sample types analysed in this study Sample type 222Rn exhalation rate(mBq m-2 s-1) 222Rn:226Ra ratio Number

Maximum Minimum Geomean Geomean of readings

Oil scales 5107 274 46355721628 42.0

11.021.0 7

Pond sediments 120 31 1324980 62.0

06.019.0 6

Untreated sludge piles 598 12 1541547 29.0

05.012.0 36

Treated sludge strips 37 6 26915 37.0

03.010.0 19

Barrel stored sludge 52 26 1292759 020.0

011.0015.0 6

Ambient soil 8.2 0.7 7.90.11.3 26.0

03.009.0 4

165

A total of five site visits, each ranging from 3-17 days, were made for in-situ

measurements and sample collection. Due to a number of difficulties, only a

limited amount of time could be spent in the field. For example, limited

transport was available for both the 260 kg of equipment and researchers to get

to the site, and finding accommodation in the remote mining camps was not

always easy. Also, in order to avoid transporting the radioactive materials back

to Australia, all of the collected samples had to be analysed in the Medical

Physics Laboratory of Sultan Qaboos University (SQU), Muscat, using both the

SQU facilities and the equipment transported from QUT.

Many factors also needed to be taken into consideration when designing the

measurement and analysis procedures used in the study. For example, where

portable gamma spectroscopy is used for detecting activity concentration of

228Ra through its progeny, it was important to understand that, in petroleum

waste products, its immediate daughter, 228Th, is found in transient rather than

secular equilibrium with 228Ra, while in ambient soils, the equilibrium is

secular.

Despite these difficulties, many of the features of this study are novel and

unique. Firstly, most of the previously published studies relate to offshore

operations. Therefore this study is unique in being the first large-scale onshore

study on petroleum exploration and production radiological assessment and in

being conducted in the Sultanate of Oman. Therefore, the information provided

in this thesis may be used as an important reference for NORM activity

concentration assessments in the oil industry. In addition, because this study

166

was conducted in the Southern Oman Directorate, the findings would also be a

good source of knowledge for possible future assessments in oil rig operations

of the Northern Oman Directorate as well.

Further, this was the first ever study to perform an assessment of radon

(222Rn) exhalation from oil sludge samples. It was also the first ever study to

quantify the presence of 227Ac in oil sludge and scales, and to date, it is only the

second study to ever quantify the presence of 227Ac in gas scales. Ac-227 has

high inhalation and ingestion dose coefficients compared to 226Ra, 228Ra and

210Pb, particularly since its committed effective dose coefficient for inhalation is

two orders of magnitude higher than 226Ra, 228Ra and 210Pb. Therefore, proper

radiological protection measures should be adhered to during the

decontamination of scaled pipes, as well as during the disposal of scale and

sludge.

An empirical relation was also derived between petroleum sludge activity

concentrations and gamma dose rates. The coefficients of this relationship

turned out to be different to those reported by various authors who investigated

such relationships of infinite thickness and infinite dimension slabs of soils. The

reasons for the lower conversion coefficients obtained in this study are thought

to be due to: (1) the presence of heavy metal sediments and corrosive particles

in the petroleum sludge, leading to a greater attenuation coefficient; (2) the

radionuclides may not have reached equilibrium at the time of measurement;

(3) the ambient soil measurements are usually conducted on flat extended land,

whereas untreated sludge piles are in small heaps; and (4) the greater soil

167

density of ambient soil compared to less compacted untreated and treated

sludge.

7.2 Future directions

As outlined above, this was the first ever study to perform an assessment of

radon (222Rn) exhalation from oil sludge samples, and the results of this study

suggest the samples with an appreciable amount of oily hydrocarbon content

tended to exhale less 222Rn. As such, further investigations are recommended to

establish this hypothesis. In addition, Ac-227 was detected for the first time in

oil scales and sludge, possibly as a result of the residues left behind from the

produced water. However, further investigations into the formation and ultimate

fate of Ac-227 are also required, in order to develop a better understanding of its

presence in the petroleum industry.

It is well known that the measurement capabilities of HPGe systems vary

significantly, and unfortunately, the SQU pop-up detector used in this study had

a poor sensitivity for the 210Pb gamma energy of 46.5 keV, resulting in high

uncertainties and/or no detection of 210Pb in a number of samples. As such, it is

recommended that future studies use a planner HPGe system instead, in order to

increase the detection capabilities for 210Pb.

Scaling sedimentation and deposition in various sections of the extraction,

transport and storage equipment is a major problem in the petroleum industry,

costing the oil production companies significant time and money, due to

168

maintenance and replacement costs, as well as losses in efficiency and the down

time associated with its removal. Thus, the ability to section the scale, and

measure the age of individual sections (using 228Ra:226Ra and/or 228Th:228Ra

activity ratios) would contribute greatly to existing knowledge on the scale

deposition process. However, field conditions during this study were such that

sectioning the depositions was not possible, leaving many opportunities for

further measurements and analysis in the future.

To summarise, the findings of this study highlight that considerably more

work is still required and that there are many gaps in knowledge yet to be

explored, including:

• Radiological health impact due to the petroleum mining activities

• Investigation of dust re-suspension pathway in the arid desert

environment

• Investigation of the social aspects of bedwen life style to model the

likely radiation dose to critical groups in the long term

• Modelling the likely radiation dose to be incurred by direct gamma,

inhalation and injection pathways during future oil and gas scale

removal activities

• Further investigation on 227Ac migration, deposition and fate in the oil

and gas systems

• Investigation of scale deposition rate by use of inherent radiological

behaviour of 228Ra:226Ra and/or 228Th:228Ra activity ratio(s)

169

• Assessment of 220Rn activity flux, since the presence of 228Ra may

make it an inhalation hazard, especially in confined areas such as

separation tanks

• Measurement of 220Rn and 222Rn in the air

• Use of a planner HPGe system for improved detection of 210Pb

• Detection of 210Po and its applications for health hazard and/or

estimating the age of 210Pb depositions

• Determination of petroleum samples moisture and hydrocarbon

content and its implications on radon exhalation

• Determination of sample mineralogy using X-ray fluorescence

spectrometer (XRF) and diffractometer (XRD)

170

References

Ahmad, M., Tasneem, M. A., Rafiq, M., Khan, I. H., Farooq, M. & Sajjad, M. I.

(2003) Interwell tracing by environmental isotopes at Fimkassar

Oilfield, Pakistan. Applied Radiation and Isotopes, 58, 611-619.

Ajayi, O. S. (2002) Evaluation of absorbed dose rate and annual effective dose

equivalent due to terrestrial gamma radiation in rocks in a part of

southwestern Nigeria. Radiation Protection Dosimetry, 98, 441-444.

Al-Futaisi, A., Jamrah, A., Yaghi, B. & Taha, R. (2007) Assessment of

alternative management techniques of tank bottom petroleum sludge in

Oman. Journal of Hazardous Materials, 141, 557-564.

Al-Masri, M. S. (2006) Spatial and monthly variations of radium isotopes in

produced water during oil production. Applied Radiation and Isotopes,

64, 615-623.

Al-Masri, M. S. & Aba, A. (2005) Distribution of scales containing NORM in

different oilfields equipment. Applied Radiation and Isotopes, 63, 457-

463.

Al-Masri, M. S. & Shwiekani, R. (2008) Radon gas distribution in natural gas

processing facilities and workplace air environment. Journal of

Environmental Radioactivity, 99, 574-580.

Al-Shaibany, S. (2007) Oman sees oil production rising to 1 mln bpd. Reuters.

Muscat.

APPEA (2002) Guidelines for Naturally Occurring Radioactive Materials.

Arora, H. S., Cantor, R. R. & Nemeth, J. C. (1982) Land treatment: A viable

and successful method of treating petroleum industry wastes.

Environment International, 7, 285-291.

171

Bader, M. S. H. (2006) Sulfate scale problems in oil fields water injection

operations. Desalination, 201, 100-105.

Barillon, R., Özgümüs, A. & Chambaudet, A. (2005) Direct recoil radon

emanation from crystalline phases. Influence of moisture content.

Geochimica et Cosmochimica Acta, 69, 2735-2744.

Battaglia, A. & Bramati, L. (1988) Environmental radiation survey around a

coal-fired power plant site. Radiation Protection Dosimetry, 24, 407-

410.

Bouville, A. & Lowder, W. M. (1988) Human population exposure to cosmic

radiation. Radiation Protection Dosimetry, 24, 293 - 299.

Brown, J. L., Syslo, J. L., Lin, Y.-H. L., Getty, S. L., Vemuri, R. L. & Nadeau,

R. L. (1998) On-Site Treatment of Contaminated Soils: An Approach to

Bioremediation of Weathered Petroleum Compounds. Journal of Soil

Contamination, 7, 773-800.

Burton, E. F. (1904) Petroleum radioactive gas. Phys Zs, 5, 511 - 516.

Carrero, E., Queipo, N. V., Pintos, S. & Zerpa, L. E. (2007) Global sensitivity

analysis of Alkali-Surfactant-Polymer enhanced oil recovery processes.

Journal of Petroleum Science and Engineering, 58, 30-42.

Carter, M. & Sonter, M. (2003) Gamma doserates over land containing NORM.

Australian Radiation Protection Society Conference (ARPS 28). Hobart,

Tasmania, Australia.

Chen, S. Y. (1991) Calculation of effective dose-equivalent responses for

external exposure from residual photon emmitters in soil. Health

Physics, 60, 411-426.

Chierici, G. L. (1992) Economically improving oil recovery by advanced

reservoir management. Journal of Petroleum Science and Engineering,

8, 205-219.

172

Clark, M. J., Burgess, P. H. & McClure, D. R. (1993) Dose quantities and

instrumentation for measuring environmental gamma radiation during

emergencies. Health Physics, 64.

Couillard, D., Tran, F. T. & Tyagi, R. D. (1991) Process for the In Situ

restoration of oil-contaminated soils. Journal of Environmental

Management, 32, 19-34.

Deworm, J. P., Slegers, W., Gillard, j., Flemal, J. M. & Clust, J. P. (1988)

Survey of the natural radiation of Belgian territory as detemined by

different methods. Radiation Protection Dosimetry, 24, 347-351.

Doscher, T. M. & Wise, F. A. (1976) Enhanced crude oil recovery potential - an

estimate. Journal of Petroleum Technology, 575 - 585.

Dutkiewicz, A., Ridley, J. & Buick, R. (2003) Oil-bearing CO2-CH4-H2O fluid

inclusions: oil survival since the Palaeoproterozoic after high

temperature entrapment. Chemical Geology, 194, 51-79.

Dyer, S. J. & Graham, G. M. (2002) The effect of temperature and pressure on

oilfield scale formation. Journal of Petroleum Science and Engineering,

35, 95-107.

Faheem, M. & Matiullah (2008) Radon exhalation and its dependence on

moisture content from samples of soil and building materials. Radiation

Measurements, In Press, Corrected Proof.

Gazineu, M. H. P., Araujo, A. A. d., Brandao, Y. B., Hazin, C. A. & Godoy, J.

M. d. O. (2005) Radioactivity concentration in liquid and solid phases of

scale and sludge generated in the petroleum industry. Journal of

Environmental Radioactivity, 81, 47-54.

Gazineu, M. H. P. & Hazin, C. A. (2007) Radium and potassium-40 in solid

wastes from the oil industry. Applied Radiation and Isotopes, In Press,

Corrected Proof.

173

Goddard, C. C. (2002) Measurement of outdoor terrestrial gamma radiation in

the Sultanate of Oman. Health Physics, 82, 869-874.

Godoy, J. M. & Petinatti da Cruz, R. (2003) 226Ra and 228Ra in scale and sludge

samples and their correlation with the chemical composition. Journal of

Environmental Radioactivity, 70, 199-206.

Gomez Escobar, V., Vera Tome, F. & Lozano, J. C. (1999) Procedures for the

determination of 222Rn exhalation and effective 226Ra activity in soil

samples. Applied Radiation and Isotopes, 50, 1039-1047.

Greeman, D. J. & Rose, A. W. (1996) Factors controlling the emanation of

radon and thoron in soils of the eastern U.S.A. Chemical Geology, 129,

1-14.

Hamilton, I. S., Arno, M. G., Rock, J. C., Berry, R. O., Poston, J. W., Sr.,

Cezeaux, J. R. & Park, J.-M. (2004) Radiological assessment of

petroleum pipe scale from pipe-rattling operations. Health Physics, 87,

382-396.

Hamlat, M. S., Djeffal, S. & Kadi, H. (2001) Assessment of radiation exposures

from naturally occurring radioactive materials in the oil and gas

industry. Applied Radiation and Isotopes, 55, 141-146.

Hamlat, M. S., Kadi, H., Djeffal, S. & Brahimi, H. (2003a) Radon

concentrations in Algerian oil and gas industry. Applied Radiation and

Isotopes, 58, 125-130.

Hamlat, M. S., Kadi, H. & Fellag, H. (2003b) Precipitate containing norm in the

oil industry: modelling and laboratory experiments. Applied Radiation

and Isotopes, 59, 95-99.

Harley, N. H. (2000) The 1999 Laurison S. Taylor Lecture-Back to

Background: Natural Radiation and Radioactivity Exposed. Health

Physics, 79, 121-128.

174

Hart, K. P. & Levins, D. M. (1986) Steady-state Rn diffussion through tailings

and multiple layers of coverings materials. Health Physics, 50, 369 -

379.

Heaton, B. & Lambley, J. (1995) TENORM in the oil, gas and mineral mining

industry. Applied Radiation and Isotopes, 46, 577-581.

Himstedt, F. (1904) Radioactive emanation. Phys Zs, 5, 210 - 213.

Holdsworth, S. J. & Akber, R. A. (2004) Diffusion length and emanation

coefficient of 220Rn for a Zircon and a Monazite sample. Radiation

Protection in Australasia, 21, 7-13.

IAEA (2003) Radiation protection and the management of radioactive waste in

the oil and gas industry. IAEA-Safety reports series No. 34. Vienna.

ICRP68 (1994) Dose Coefficients for Intakes of Radionuclides by Workers.

Annals of the Internation Commission on Radiological Protection, 24.

ICRP (1991) Annual limits on intake of radionuclides by workers based on the

1990 recommendations. ICRP publication 61. Pergamon Press, Oxford.

Jerez Vegueria, S. F., Godoy, J. M. & Miekeley, N. (2002a) Environmental

Impact in Sediments and Seawater due to Discharges of Ba, 226Ra,

228Ra, V, Ni and Pb by Produced Water from the Bacia de Campos Oil

Field Offshore Platforms. Environmental Forensics, 3, 115-123.

Jerez Vegueria, S. F., Godoy, J. M. & Miekeley, N. (2002b) Environmental

impact studies of barium and radium discharges by produced waters

from the "Bacia de Campos" oil-field offshore platforms, Brazil. Journal

of Environmental Radioactivity, 62, 29-38.

Jerez Veguería, S. F., Godoy, J. M. & Miekeley, N. (2002) Environmental

Impact in Sediments and Seawater due to Discharges of Ba, 226Ra,

228Ra, V, Ni and Pb by Produced Water from the Bacia de Campos Oil

Field Offshore Platforms. Environmental Forensics, 3, 115-123.

175

Jha, S., Khan, A. H. & Mishra, U. C. (2000) A study of the 222Rn flux from

soil in the U mineralised belt at Jaduguda. Journal of Environmental

Radioactivity, 49, 157-169.

Jonkers, G., Hartog, F. A., Knaepen, A. A. I. & Lancee, P. F. J. (1997)

Characterisation of NORM in the oil and gas production (E&P) industry.

International symposium on radiological problems with natural

radioactivity in the non-nuclear industry. Amsterdam.

King, C.-Y. & Minissale, A. (1994) Seasonal variability of soil-gas radon

concentration in central California. Radiation Measurements, 23, 683-

692.

Kocher, D. K. & Sjoreen, A. L. (1985) Dose-rate conversion factors for external

exposure to photon emmitters in soil. Health Physics, 48, 193-205.

Kolb, W. A. & Wojcik, M. (1985) Enhanced radioactivity due to natural oil and

gas production and related radiological problems. The Science of the

Total Environment, 45, 77-84.

Lieser, K. H. (1995) Radionuclides in the Geosphere: Sources, Mobility,

Reactions in Natural Waters and Interactions with Solids. Radiochemica

Acta, 70, 355-375.

Lieser, K. H., Hill, R., Muhlenweg, U., Sing, R. N., Shu-De, T. & Steinkopff, T.

(1991) Actinides in the Environment. Journal of Radioanalytical and

Nuclear Chemistry-Articles, 147, 117-131.

Losana, M. C., Magnoni, M. & Righino, F. (2001) Comparison of different

methods for the assessment of the environmental gamma dose.

Radiation Protection Dosimetry, 97, 333-336.

Lovborg, L., Botter-Jensen, L., Kirkegaard, P. & Chritiansen, E. M. (1979)

Monitoring of Natural Soil Radioactivity with Portable Gamma-Ray

Spectrometers. Nuclear Instruments and Methods, 167, 341-348.

176

Martin, P. & Akber, R. A. (1999) Radium isotopes as indicators of adsorption-

desorption interactions and barite formation in groundwater. Journal of

Environmental Radioactivity, 46, 271-286.

Mater, L., Sperb, R. M., Madureira, L. A. S., Rosin, A. P., Correa, A. X. R. &

Radetski, C. M. (2006) Proposal of a sequential treatment methodology

for the safe reuse of oil sludge-contaminated soil. Journal of Hazardous

Materials, 136, 967-971.

Matta, L. E., Godoy, J. M. & Reis, M. C. (2002) Ra-226, Ra-228 and Th-228 in

scale and sludge samples from the Campos Basin oilfield E&P activities.

Radiation Protection Dosimetry, 102, 175-178.

Menetrez, M. Y., Mosley, R. B., Snoddy, R. & Brubaker, S. A. (1996)

Evaluation of radon emanation from soil with varying moisture content

in a soil chamber. Environment International, 22, 447-453.

Morawska, L. & Phillips, C. (1992) Dependence of the radon emanation

coefficient on radium distribution and internal structure of the material.

Geochimica et Cosmochimica, 57, 1783-1797.

NHMRC (1996) Australian Drinking Water Guidelines - Summary. National

Water Quality Management Strategy.

Nielson, K. K., Rogers, V. C. & Holt, R. B. (1996) Measurements and

calculations of soil radon flux at 325 sites throughout Florida.

Environment International, 22, 471-476.

Oil&Gas_UK (2008) Key environmental issues, produced water. IN

HTTP://WWW.OILANDGASUK.CO.UK/ISSUES/ENVIRONMENT/P

RODUCEDWATER.CFM (Ed.) London.

Oman (2008) Major economic and social indicators, Sultanate of Oman

Ministry of National Economy. IN

HTTP://WWW.MONEOMAN.GOV.OM/BOOK/MB/APR2008/T1.PD

F (Ed.).

177

Omar, M., Ali, H. M., Abu, M. P., Kontol, K. M., Ahmad, Z., Ahmad, S. H. S.

S., Sulaiman, I. & Hamzah, R. (2004) Distribution of radium in oil and

gas industry wastes from Malaysia. Applied Radiation and Isotopes, 60,

779-782.

Othman, I., Al-Masri, M. S. & Suman, H. (2005) Public and regulatory

acceptability of NORM contaminated soil disposal: The Syrian

experience. E&P NORM Workshop. Sultanate of Oman, Muscat.

Paranhos Gazineu, M. H., de Araujo, A. A., Brandao, Y. B., Hazin, C. A. & de

O. Godoy, J. M. (2005) Radioactivity concentration in liquid and solid

phases of scale and sludge generated in the petroleum industry. Journal

of Environmental Radioactivity, 81, 47-54.

Paschoa, A. S. (1997) Naturally occurring radioactive materials (NORM) and

petroleum origin. Applied Radiation and Isotopes, 48, 1391-1396.

Paschoa, A. S. (2003) Radiological Impact due to Oil and Gas Extraction.

Business Briefing: Exploration and Production.

Paschoa, A. S. & Godoy, J. M. (2002) The areas of high natural radioactivity

and TENORM wastes. International Congress Series, 1225, 3-8.

PDO (2001) Health, safety & environment report.

PDO (2006) Petroleum Development Oman Annual Report.

http://www.pdo.co.om/pdoweb/NEWSLIBRARY/PublicationReports/Ann

ualReports/tabid/114/Default.aspx.

Petroleum Development Oman, P. D. O. (2005) Health, Safety and Environment

Specifications: Naturally Occurring Radioactive Materials (N.O.R.M.).

SP-1170. http://www.pdo.co.om/hseforcontractors/health/NORM.htm.

Prabhu, C. (2007) Concession agreements signed for two oil blocks. Oman

Daily Obsever. Muscat.

Prado-Jatar, M., Correa, M., Rodriguez-Grau, J. & Carneiro, M. (1993) Oil

Sludge Landfarming Biodegradation Experiment Conducted At A

178

Tropical Site In Eastern Venezuela. Waste Management & Research, 11,

97-106.

Rajaretnam, G. & Spitz, H. B. (2000) Effect of leachability on environmental

risk assessment for naturally occurring radioactive materials in

petroleum oil fields. Health Physics, 78, 191-198.

Rood, A. S., White, G. J. & Kendrick, D. T. (1998) Measurement of Rn-222

flux, Rn-222 emanation, and Ra-226,Ra-228 concentration from

injection well pipe scale. Health Physics, 75, 187-192.

Saito, K. & Jacob, P. (1995) Gamma ray field in the air due to sources in the

ground. Radiation Protection Dosimetry, 58, 29-45.

Salih, F. M., Pillay, A. E. & Jayasekara, K. (2005) Levels of radium in oily

sludge. International Journal of Environmental and Analytical

Chemistry, 85, 141-147.

Satterly, J. & McLennan, J. C. (1918) The radioactivity of natural gases of

Canada. Trans. Royal Canada, 12, 153-160.

Semkow, T. M. (1990) Recoil-emanation theory applied to radon release from

mineral grains. Geochimica et Cosmochimica Acta, 54, 425-440.

Shawky, S., Amer, H., Nada, A. A., Abd El-Maksoud, T. M. & Ibrahiem, N. M.

(2001) Characteristics of NORM in the oil industry from Eastern and

Western deserts of Egypt. Applied Radiation and Isotopes, 55, 135-139.

Shweikani, R., Giaddui, T. G. & Durrani, S. A. (1995) The effect of soil

parameters on the radon concentration values in the environment.

Radiation Measurements, 25, 581-584.

Smith, A. L. (1987) Radioactive-scale formation. Journal of Petroleum

Technology, 697-706.

Smith, K. P., Arnish, J. J., Williams, G. P. & Blunt, D. L. (2003) Assessment of

the disposal of radioactive petroleum industry waste in nonhazardous

179

landfills using risk-based modeling. Environmental Science &

Technology, 37, 2060-2066.

Smith, K. P., Blunt, D. L. & Arnish, J. J. (1998) Potential Radiological Doses

Associated with the Disposal of Petroleum Industry NORM Via

Landspreading. Fossil Energy.

Sonter, M., Akber, R. & Holdsworth, S. (2002) Radon flux from rehabilitated

and unrehabilitated uranium mill tailings deposits. Australasian

Radiation Protection Society, 19, 36 - 48.

Spehr, W. & Johnston, A. (1983) The measurement of radon emanation rates

using activated charcoal. Radiation Protection in Australasia, 1, 113-

116.

Spitz, H., Lovins, K. & Becker, C. (1997) Evaluation of residual soil

contamination from commercial oil well drilling activities and its impact

on the naturally occurring background radiation environment. Journal of

Soil Contamination, 6, 37-59.

Stranden, E., Kolstad, A. & Lind, B. (1984) Radon exhalation: Moisture and

temperature dependence. Health Physics, 47, 480-484.

Sun, H. & Furbish, D. J. (1995) Moisture content effect on radon emanation in

porous media. Journal of Contaminant Hydrology, 18, 239-255.

Tan, G. T. (2005) Proposed Disposal Routes for NORM Contaminated Soil and

Sludges. PDO NORM workshop. Muscat.

Tanner, A. (1964) Radon Migration in the ground: A review. The Natural

Radiation Environment. Chicago, University of Chicago Press.

Tanner, A. B. (1980) Radon migration in the ground: A supplementary review.

The Natural Radiation Environment. Springfield, VA, National

Technical Information Service.

180

Testa, C., Desideri, D., Meli, M. A., Roselli, C., Bassignani, A., Colombo, G. &

Fresca Fantoni, R. (1994) Radiation protection and radioactive scales in

oil and gas production. Health Physics, 67.

Trujillo, A. J. (2004) Petro-state constraints on health policy: guidelines for

workable reform in Venezuela. Health Policy, 67, 39-55.

UNSCEAR (2000) Sources and effects of ionising radiation. United Nations

Scientific Committee on the Effects of Atomic Radiation. Report to the

general assembly with annexes. New York, United Nations.

UNSCEAR (2001) Exposures from natural radiation sources. New York, United

Nations.

USEPA (2005) List of drinking water contaminants & their maximum

contaminant levels (MCLs). Ground water & drinking water.

USGS (1997) Research on saline waters co-produced with energy resources.

http://pubs.usgs.gov/fs/1997/fs003-97/FS-003-97.html.

Vandenhove, H. (2002) European sites contaminated by residues from the ore-

extracting and -processing industries. IN SERIES, I. C. (Ed.)

International Congree Series.

Vasudevan, N. & Rajaram, P. (2001) Bioremediation of oil sludge-contaminated

soil. Environment International, 26, 409-411.

White, G. J. & Rood, A. S. (2001) Radon emanation from NORM-contaminated

pipe scale and soil at petroleum industry sites. Journal of Environmental

Radioactivity, 54, 401-413.

WHO (2004) Guidelines for drinking-water quality, third edition. Geneva.

Wilson, A. J. & Scott, L. M. (1992) Characterization of Radioactive Petroleum

Piping Scale with an Evaluation of Subsequent Land Contamination.

Health Physics, 63, 681-685.

181

Wilson, A. J. & Scott, L. M. (1993) Characterization of Radioactive Petroleum

Piping Scale with an Evaluation of Subsequent Land Contamination

(Vol 63, Pg 681, 1992). Health Physics, 64, 443-443.

Worden, R. H., Manning, D. A. C. & Lythgoe, P. R. (2000) The origin and

production geochemistry of radioactive lead (210Pb) in NORM-

contaminated formation waters. Journal of Geochemical Exploration,

69-70, 695-699.

Zielinski, R. A., Otton, J. K. & Budahn, J. R. (2001) Use of radium isotopes to

determine the age and origin of radioactive barite at oil-field production

sites. Environmental Pollution, 113, 299-309.

Zikovsky, L. (2001) Temperature dependence of adsorption coefficient of 222Rn

on activated charcoal determined by adsorption-desorption method.

Health Physics, 80, 175-176.