2012-078

SPWLA 53rd

Annual Logging Symposium, June 16-20, 2012

1

Measuring Isolation Integrity in Depleted Reservoirs

Long Jiang, Dominique Guillot, Milad Meraji, Puja Kumari, and Benoit Vidick, Schlumberger

Bill Duncan, Gamal Ragab Gaafar, and Salmi B Sansudin, PETRONAS Carigali Sdn. Bhd.

Copyright 2012, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors This paper was prepared for presentation at the SPWLA 53rd Annual Logging Symposium held in Cartagena, Colombia, June 16-20, 2012.

ABSTRACT

Isolating through proper cementing is critical in

depleted reservoirs, which are often near or very close

to higher pressure water zones. These same reservoirs

are often multilayered but production is not allowed to

be comingled, making isolation through cement

integrity even more important. Failing to ensure proper

isolation increases the likelihood of crossflow between

adjacent zones during the life of the well. Even when

the parameters necessary for a perfect cementing job

(centralization, mud removal, etc.) are well known,

interpreting cement logs (sonic or ultrasonic) in order to

assess well integrity is still a challenge in many cases.

Endless discussions will take place to determine if

zonal isolation is ensured or if remedial perforation and

cement squeeze operations should be performed.

In this study, cement evaluation logs were recorded and

interpreted in several depleted reservoirs. Nearly

perfect correlations between cement sonic and

ultrasonic logs and formation logs (total water

saturation, for instance) are clearly shown. Different

scenarios were looked at using all information possible

(logs, permeability, porosity, water saturation, etc.).

Based on detailed analysis and laboratory experiments,

a mechanism explaining those correlations was

proposed with possible solutions. This mechanism links

the decrease in the hydrostatic pressure during the

cement setting to the oil invasion or influx from the

formation. The possible effect of the gas invasion

through the cement slurry has also been reviewed in

this study. Numerous examples were obtained in the

same field with all data related to the formation

properties and cementing operations.

INTRODUCTION

During recent redevelopment of a 35-year-old field,

five new wells were drilled and completed. The

reservoirs are depleted at different levels. The first well

was cemented without any operational issues reported.

However, the cement bond logs showed poor cement

bond across the hydrocarbon-bearing reservoirs and

good cement bond in wet sands. The question was

whether zonal isolation was achieved around planned

perforations. The interpretation and corrective actions

taken would have direct impact on subsequent wells.

The aim was to improve the cement bond.

CEMENT EVALUATION

Before attempting to interpret the cement log, slurry

design, casing centralization, operational records, and

other information related to each cementing job were

reviewed. There were no design defects or operational

issues that could have affected the cement bond quality.

Cement evaluation was performed with conventional

sonic and ultrasonic logs. The conventional sonic logs

included the cement bond log (CBL) and variable-

density log (VDL). The ultrasonic log was acquired

with an ultrasonic imaging tool. The logs were run 2

days after the cementing job. Even though the cement

did not reach its full strength, the cement has reached a

high enough acoustic impedance to interpret the logs.

The log in Figure 1 is an example from the first well.

From left to right, track 1 is depth; track 2 shows the

formation evaluation results including fluid types; track

3 shows maximum, minimum, and average acoustic

impedance; and track 4 shows the cement map from

ultrasonic tool. This map is derived from acoustic

impedance. The yellow to dark brown coding represents

cement, green is microdebonded cement, blue is liquid,

and red is gas or dry microannulus. The bond index

map in track 5 is the color mapping of computed

indexes. The solid black line is the CBL. Track 6 is the

VDL.

The log response clearly indicated a good cement bond

in water sands and poor cement bond in hydrocarbon-

bearing reservoirs. This correlation was observed in

entire well. A fluid channel around 6,300 ft is most

probably due to formation fluid migration occurring

during the liquid to solid transition of the cement. The

hole size is constant and very close to bit size.

The impact of formation type on cement bond logs has

been discussed by other authors (Vidick. B. &

Krummel. K. in 2005). The hydrocarbon effect on the

cement bond is also a common issue in the industry.

However, it is very rare to see such good correlation in

the entire wellbore.

SPWLA 53rd


2

Fig. 1 Good cement bond in water sand and poor

cement bond in depleted hydrocarbon reservoir.

Figure 2 is from a shallower section of the same well.

The logs show a similar trend. However, the cement

map indicates more microdebonded cement in less

depleted reservoirs. The microdebonding logic is well

explained in Butsch (1995).

Fig. 2 Microdebonded cement in less depleted

reservoirs.

Figure 3 shows the most critical part of the cemented

section because this is where the casing is to be

perforated. The presentation is the same as in Figure 1

except the formation tops are displayed on the left, and

the well sketch including the depth of the perforations

is added to depth track.

Both CBL and VDL logs show a very poor cement

bond. However, the ultrasonic cement map indicates

patchy cement. This is interpreted as a microannulus

effect. Microannulus effect on cement logs is also

described by Jutten and Hayman (1993). The question

now was whether there is hydraulic zonal isolation

between those perforations.

Fig. 3 Microannulus in depleted reservoirs.

To answer the question, the individual acoustic

impedances were plotted with scale of 0 to 10 MRay

(Figure 4). The ultrasonic tool was acquired in high-

resolution mode with 36 radial measurements and a 1.5-

in. vertical sample rate. The plot verified there are

many sections in which acoustic impedance is greater

than fluid threshold (~1.8 MRay). Based on field

experience, the minimum interval for zone isolation in

9 5/8-in. casing is 15 ft (assuming a bond index of 0.8).

On this basis one can conclude the ultrasonic log shows

there is hydraulic zonal isolation between the top

perforations at M5.0/5.5 reservoirs and the bottom

perforation at M7.0. The permanent downhole gauges

installed on the completion string confirmed there is no

SPWLA 53rd


3

communication between M5.0/5.5 and M7.0 reservoirs.

The production tests also did not show an evidence of

lack of zonal isolation between the two sets of open

perforations.

The main concern was potential water production

through the microannulus. The logs did not indicate

apparent channels. Any squeeze attempt would be

difficult. The perforated zones are separated by sliding

sleeves. The decision was made to proceed without any

squeeze job.

In the case of a single-zone completion, one may

consider different options to prevent any potential water

production through the microannulus.

Fig. 4 Acoustic impedance display for reservoirs shown

in Figure 3.

To further investigate the possible causes of the poor

cement bond, the composite display including

ultrasonic bond index, total water saturation, and

depletion is shown in Figure 5. The depletion is the

pressure reduction from the initial reservoir pressure.

On the left track, the bond index in green shows very

good correlation with total water saturation in blue. The

reservoir depletion is shown as red dots. The only

difference on the right track is the ultrasonic bond index

included microdebonded cement. One can clearly see

more microdebonded cement in less depleted

reservoirs.

As discussed above, the poor cement bond is linked to

microannulus across the hydrocarbon-bearing zones.

However, the cement bond is good in water sands. This

phenomenon cannot be explained by a simple hydraulic

pressure difference between drilling mud and

completion fluid.

Fig. 5 Composite display of water saturation, bond

index, and depletion.

POSSIBLE MECHANISM

To be able to propose a possible mechanism explaining

why when the different pressure was high, thus

allowing a microannulus to be created, it is necessary to

go through a detailed analysis of the cementing job.

The slurry placement was executed as per design.

Figure 6 shows a comparison between the simulated

wellhead pressure (WHP, blue curve) and the acquired

WHP (green curve) during the job. Both curves do

match very well during the displacement which takes

place when the time exceeds 150 minutes. The higher

WHP acquired is due to a small difference (less than 50

ft) in the top of cement. The analysis does not show any

losses during cement placement.

SPWLA 53rd


4

Fig. 6 Wellhead pressure comparison between

calculated (blue) and acquired (green) pressures.

The next parameter to look at is the mud removal. Is the

mud effectively displaced by the spacer and the spacer

by the slurry?

Figure 7 shows that for the interval logged, there was

good separation between all the fluids during slurry

placement. It also shows that from a depth of 5,000 ft

and above, it is possible for some mud to be left in the

narrow side of the annulus. But this is above the zone of

interest.

Fig. 7 Mud removal plot.

The slurry was initially tested against the API

requirements for gas tightness i.e. a gel strength

development from the critical static gel strength (CSGS

in lbf/100 sqft) to 500 (lbf/100 sqft) in less than 45

minutes and an API fluid loss of less than 50 ml/30

min. From the well design and the top of cement, the

CSGS was found to be above 500 lbf/100 sqft,

indicating very low risk of gas migration during cement

setting. The gel strength development analysis showed

that only 21 minutes were required for development

from 100 to 500 lbf/100 sqft. The risk of gas migration

was therefore very low, and the slurry was, in any case,

gas tight (as per API specifications).

What made this cementing job different (at least for the

bottom section where a microannulus is observed) is the

extremely large differential pressure between the

hydrostatic pressure in the annulus (at the end of the

displacement when the slurry is still liquid) and the

formation pressure. This differential pressure exceeds

2,500 psi at 6,200 ft.

The amount of water the slurry can lose to the

formation was measured as per API specifications (i.e.,

after conditioning the slurry for 20 minutes at bottom

hole circulating temperature, BHCT, and under 1000

psi differential pressure only). The value was 36 ml/30

min. Due to safety and technical constraints, it was not

possible to measure the fluid loss at 2,500 psi

differential pressure. Outmans (1963) gave an equation

linking the filtrate to the square root of pressure as

follows:

Kdep = deposition constant

Figure 8 is the relationship between API filtrate volume

and the square root of pressure.

Fig. 8 Relationship between API filtrate and square

root of pressure.

SPWLA 53rd


5

An extrapolation to 2,500 psi increases the API

measured volume from 36 ml/30 min to 62 ml/30 min.

This figure might look small. However, one needs to

consider two important points. First, there are no data to

support this extrapolation through physical

measurements. Second, the open hole size is 12 ¼ in.

This makes a surface area of 462 square inches per foot

of length. The fluid-loss test is done in the laboratory

through a grid of roughly 2 in.2. So one could imagine

that the amount of fluid the slurry could lose with an

API fluid loss of 62 ml is very important. This very

large amount of water to the formation will create bulk

cement shrinkage. This shrinkage, if high, can induce

the formation of radial cracks (tensile failure) along the

cement column. Those cracks could allow the flow

(from formation fluids or gas) from the formation to the

interface cement/casing (once the cement is set) and

affect the cement bond quality as seen on the cement

evaluation logs.

The changes in the ultrasonic log versus depth are

abrupt (i.e., that within a foot, the log quality changes

from good to bad and vice versa). This indicates that

those changes are induced by phenomena occurring

after the placement of the slurry. The log variations

would be smoother and occurring over hundreds of feet

if those changes were induced by the placement (mud

removal, channeling, or bad centralization).

Across a zone where there is a high differential pressure

between the annulus hydrostatic and the formation

pressure, during the period where the cement is static

until it sets, fluid (water) is lost from the slurry into the

formation. This creates bulk shrinkage and radial

cracks. Once the cement is set, the cement pore

pressure has dropped to a value well below the

equivalent water density. Therefore, formation fluids or

gas will then flow from the formation to the slurry.

If radial cracks are present, then formation fluids or gas

can enter a microannulus and induce potential loss of

zonal isolation. When the differential pressure is not

large enough to generate radial cracks, formation fluids

or gas will still migrate through the set cement matrix

(set cement is not an impermeable material), and a

microdebonding (heterogeneous cement) was seen on

the log.

SOLUTION ATTEMPTED

A few options were available to improve the quality of

cement bond and therefore zonal isolation.

One solution would have been to use light-weight

cement. This would reduce the differential pressure

and, if the mechanism is correct, prevent the formation

of a microannulus. This was not done for economic

reasons. The cost of light-weight cement was found to

be excessive.

Another option would have been to improve

dramatically the fluid-loss control. If the amount of

fluid lost to the formation is reduced, bulk shrinkage

should also be reduced and the formation of radial

cracks and microannulus prevented. Preliminary

laboratory testing showed that it is possible to reduce

the fluid-loss value to less than 10 ml/30 min. However,

the amount of fluid-loss additive required excessively

increased the costs of the slurry, making it uneconomic,

similar to the case of light-weight cement.

The third option, used on the next four wells, was to

include expanding additive into the cement slurry. The

expansion takes place after the cement is set and is

induced by a chemical reaction in between the additive

and the water from the cement.

The expansion was measured in the API expansion cells

in water and oil baths. The results are presented in

Figure 9. They indicate that the expansion can take

place in presence of water and oil.

Fig. 9 Expansion tests in water and oil baths.

RESULTS

The well logs from well drilled after the first well

showed very good results. The short section shown in

Figure 10 is a log example across the oil zone. Both

ultrasonic and sonic logs confirmed the cement bond

quality is excellent.

The bond index does not correlate with total water

saturation. The trend observed in the first well does not

exist in the subsequent wells. The cement bond quality

has been improved in hydrocarbon bearing zones.

SPWLA 53rd


6

Fig. 10 Good cement bond across oil reservoirs.

All four wells are displayed in Figure 11. The overall

cement bond is good. The occasional low bond index is

related to gas reservoirs. The gas was trapped in gas

block cement. The zonal isolation is obtained in those

wells.

Fig. 11 Combined display of water saturation and bond

index.

CONCLUSIONS

A correlation in the first well shows good cement bond

in water sands and poor cement bond in hydrocarbon-

bearing reservoirs. The poor cement bond was caused

by the development of a microannulus in depleted

reservoirs.

Microdebonded cement was present in less depleted

reservoirs. Across gas reservoirs, gas was trapped in gas

block cement.

A detailed analysis of the log and of the cement job

execution (slurry placement and post-placement)

indicates that the loss of fluid from the slurry into the

formation (due to the extreme differential pressure)

would create cement bulk shrinkage. The shrinkage

would generate radial cracks allowing the flow of

gas/liquid through the cement to the casing/cement

interface. When the differential pressure is lower,

heterogeneous cement is seen with no loss of zonal

isolation.

The use of an expanding additive on four wells has

improved the quality of the cement bond and of zonal

isolation supporting the proposed mechanism.

NOMENCLATURE

A = filtration surface area, m2

Kfc = filtercake permeability, m2

Kdep = deposition constant, unitless

Δp = differential pressure, Pa

SWT = total water saturation, unitless

t = time, s

V = cumulative filtrate volume, m3

μfilt = filtrate viscosity, Pa.s

ACKNOWLEDGMENT

The authors thank PETRONAS Carigali Sdn Bhd and

Schlumberger for permission to publish this paper.

REFERENCES SECTION

Butsch, R.J., 1995, Overcoming interpretation problems

of gas-contaminated cement using ultrasonic cement

logs: SPE 30509 presented at SPE Annual Technology

Conference and Exhibition, Dallas, Texas, USA.

Jutten, J.J. and Hayman, A.J., 1993, Microannulus

effect on cementation logs: Experiments and case

histories: SPE 25377 presented at SPE Asia Pacific Oil

and Gas Conference and Exhibition, Singapore.

SPWLA 53rd


7

Outmans, H.D., 1963, Mechanics of static and dynamic

filtration in the borehole: SPE 491 presented at Drilling

and Rock Mechanics Conference in Austin, Texas,

USA.

Vidick, B., Krummel, K., and Kellingray, D.S., 2005,

Impact of formation type on cement bond logs: SPE

96022 presented at SPE/IADC Middle East Drilling

Technology Conference and Exhibition, Dubai, UAE.

ABOUT THE AUTHORS

Long Jiang joined Schlumberger as a field engineer in

November 1984, after graduating from the Petroleum

University in China with a BS degree. He has worked

in many areas including Asia, the North Sea, and

Canada. In 2000 he was assigned as senior

petrophysicist in Calgary. He moved to Bangkok in

2004 as senior petrophysicist for the region of

Thailand, Myanmar, and Vietnam. Long has been

principal petrophysicist in Kuala Lumpur, Malaysia

since October 2007. He is the author of SPE and

SPWLA papers and is a member of both societies.

Dominique Guillot is the Domain Manager for Well

Integrity/Cementing in Schlumberger. He received a

degree in civil engineering and completed his PhD

thesis on flow through porous media. Dominique has

more than 30 years with Schlumberger. He has had

various positions, mainly in engineering, during which

he contributed to the development of new technology

for cementing and hydraulic fracturing. He is the co-

editor of the Well Cementing book. He is currently

serving as a technical editor for the SPE Drilling &

Completion Journal and as the program chairman of the

SPE R&D Technical Section.

Milad Meraji holds his bachelor’s degree in civil

engineering from American University of Sharjah, and

his master’s degree in petroleum engineering from

Herriot Watt University. His paper on “Water

Resources and Possible Hazards Faced in UAE” won

the third place in Green Gulf Conference and was

published in 2006. He joined Schlumberger in August

2006 as a wireline field engineer and worked in various

land and offshore locations throughout Indonesia and

Iran, including Jakarta, Papua, Central Sulawesi, and

Kish Island. He is currently one of the Lead Wireline

Field Engineers in Labuan, Malaysia.

Puja Kumari holds her bachelor’s degree in

mechanical engineering and master’s degree in

chemistry from Birla Institute of Technology and

Science Pilani, India. She joined Schlumberger in

December 2007 as a cementing field engineer and

worked in various land and offshore (shallow and

deepwater) locations in Australia and Egypt. She

currently works as a technical engineer for cementing in

Kuala Lumpur, Malaysia.

Benoit Vidick joined Schlumberger 25 years ago. A

chemical engineer with a PhD in chemistry, he has

worked in different parts of the world including the

North Sea, North America, North Africa, and Asia. He

holds different positions in between operations,

marketing, engineering, and research centres. He is now

an Advisor for Well Services Schlumberger in Kuala

Lumpur, Malaysia. He was and is involved in HTHP

projects in the North Sea, North Africa, and Asia.

Benoit has experience in drilling fluid, cementing, and

sand control.

Bill Duncan graduated from the University of

Manitoba in 1976 with a BS degree in geological

engineering. After a 6-year stint with Schlumberger of

Canada as a Logging and Marketing Engineer, Bill

joined Canadian Superior Oil, subsequently Mobil Oil

Canada, in 1982. After 9 years working as an

Operations Log Analyst in the Geological Operations

Department of Mobil Oil Canada, Bill joined

PETRONAS Carigali Sdn Bhd as a Senior

Petrophysicist in Miri, Sarawak, East Malaysia in

March 1991. After 15 years as Team Senior

Petrophysicist for numerous Asset Teams, Bill

transferred to Kuala Lumpur in March 2006 as Staff

Petrophysicist in the Production Enhancement Group in

PCSB. Bill is still in KL serving in the role of

Specialist Petrophysicist in the Logging Operations and

Production Enhancement groups.

Gamal Ragab Gaafar, PhD, is a staff Petrophysicist

with PETRONAS Exploration. He holds a BSc in

geology, MSc in sedimentology, and PhD in

hydrodynamics and reservoir evaluation. He has almost

26 years of experience in the oil industry in the area of

formation evaluation and petrophysics; currently, he is

the focal point of core analysis with PETRONAS. He

is a vesting professor with University Technology of

PETRONAS (UTP), teaching a formation evaluation

course for master’s students. He published many papers

in the field of formation evaluation and reservoir

characterization. He is a member of SPWLA and SPE.

Salmi Sansudin joined PETRONAS Carigali (PSCB)

as a Petrophysicist in September 2006. He graduated

from University Technology of PETRONAS (UTP) in

2006 with a degree in mechanical engineering. After

two years of working as an Operations Petrophysicist in

many areas, including Malaysia, Myanmar, Northern

Africa, and Venezuela, he transferred to the Production

Enhancement Group in PCSB, where he working as a

Senior Petrophysicist.

SPWLA 53rd


8

2012-078

Documents