8/10/2019 Pete 663 Pass Gr

1/30

PASSIVE MEASUREMENTS NATURAL GAMMA

FORMATION EVALUATION

PETE 663

Summer 2010

Dr. David Schechter

8/10/2019 Pete 663 Pass Gr

2/30

PASSIVE MEASUREMENTS

Caliper Spontaneous Potential

Gamma Ray

Natural

Spectral

8/10/2019 Pete 663 Pass Gr

3/30

GAMMA RAY LOGS Uses

Correlation

Lithology indicator; exploration

for radioactive materials Mineral identification

Open or cased hole; any fluids

Evaluation of shale content Paleoenvironmental indicator

Fracture detection

Properties

Measures natural gamma

radiation

Random fluctuations

Rock Formations

GRTool

8/10/2019 Pete 663 Pass Gr

4/30

1. The gamma ray tool records the natural

radioactivity of the formation without regard to

the source

2. The spectral gamma ray tool identifies thesource and gives the contribution of each

elements (potassium , uranium, and thorium )

to the overall spectrum. Also, it is useful in

identifying fractures

GAMMA RAY TOOLS

8/10/2019 Pete 663 Pass Gr

5/30

8/10/2019 Pete 663 Pass Gr

6/30

HOT AND COLD ZONES

The Gamma tool is placed in the hot zone (200

API)

and the gamma counts are recorded.

It is then placed in the cold zone and the gammacounts are recorded. The difference in counts is

converted by a gain factor to represent 200 API.

API UNIT: (1/200) OF THE DIFFERENCE IN

LOG READING BETWEEN A HOT ZONE

AND A COLD ZONE

GAMMA CALIBRATION

8/10/2019 Pete 663 Pass Gr

7/30

NATURAL GR PRINCIPLE

Cause Unstable isotopes in

formation

Isotopes decay

Emit GRs (various energies)

Three main contributors

K40 with half-life 1.3x109 yrs

Th232 with half-life 1.4x1010

yrs

U238

with half-life 4.4x109

yrs Sources

K40 feldspar, mica, illite

Th232

heavy minerals, clays U238 organic material

Thorium Series2.62

Potassium

1.46Probability of Emission per Disintegration

Gamma Ray Energy (MeV)0 0.5 1 1.5 2 2.5 3

Uranium-Radium Series

1.76

8/10/2019 Pete 663 Pass Gr

8/30

SOURCES OF PASSIVE

GAMMA RAYS

1. Clays Kaolinite (very lit tle K [potassium])

Illite (4-8% K)

Montmorillonite (

8/10/2019 Pete 663 Pass Gr

9/30

1. Gamma rays interact with scintillation crystal

2. Electrons excite phosphor atoms, which in turn decayby emission of light

3. These photons interact with the photocathode of the

p.m tube producing electrons

4. Ejected electrons are focused into photomultiplier

string

5. Electrons are accelerated through successive dynodes

producing multiplication at anode (1e = 106 e)

SCINTILLATION DETECTORS

8/10/2019 Pete 663 Pass Gr

10/30

SCINTILLATION DETECTOR

8/10/2019 Pete 663 Pass Gr

11/30

SHALE WASHOUT

From Dresser Atlas, 1982

8/10/2019 Pete 663 Pass Gr

12/30

8/10/2019 Pete 663 Pass Gr

13/30

STATISTICAL ISSUES

Measurement problemGR emissions random

Tool moving

Results Imprecise measurement

Details smeared out

ProceduresNew tools better

detectorsLimit logging speed Old tools 1800 fph

New tools 3600 fph

Exercise care interpretingboundaries

Shale

4ft

sand

Shale

5,400 ft/hr

1,800 ft/hr

600 ft/hr

API

0 120

8/10/2019 Pete 663 Pass Gr

14/30

EFFECTS OFLOGGING

SPEED AND

FILTER LENGTHON GAMMA

RAY LOG

GR 2.25 FILTER

100 FPM

GR 2.25 FILTER

13 FPM

GR UNFILTERED

13 FPM0 150 0150

0 150

High-resolution logging

for thin bed, .i.e. coal, is usually

done at low speed tobetter define bed boundaries

and partings

Are these

reversed?

8/10/2019 Pete 663 Pass Gr

15/30

GR RESPONSE IN COMMON FORMATIONS

Shales often radioactive Clays

Trace and heavy minerals

Sandstones may be radio-active

Non-clay minerals, e.g., mica,feldspar

Clays

See Appendix B, ChartBook

Units GR calibrated to standard

Response in mid-continent

shale equals 200 API units Calibration pits

0 50 100API units

Shale

Shaly sand

Very shaly sand

Clean limestone

DolomiteShale

Clean sand

Coal

Shaly sand

Anhydrite

SaltVolcanic ash

Gypsum

8/10/2019 Pete 663 Pass Gr

16/30

sand

silt

dry clay

HC

free water

bound watert

e

Vsh

Unit volume of rock

WHAT IS Vshale?

Fraction of rock made up ofshale

Why calculate Vsh inSandstone? Delimit reservoir quality rock

Shale = clays in FE

Clays reduce perm and porosity

Estimates of Sw too large

Shales reduce net pay

Vsh definitionmatrix (silt + dry clay)

+

fluid (bound water)

8/10/2019 Pete 663 Pass Gr

17/30

VOLUME OF SHALE

Gamma Ray Index

MINMAX

MIN

SH

GRGR

GRGRI

=

RELATIONSHIP EQUATION

Linear Vsh = Ish

Clavier Vsh= 1.7-(3.38-(Ish+.7)2 )1/2

Steiber Vsh= 0.5*(Ish/(1.5-Ish))

Bateman Vsh= Ish(Ish +GRFactor)

GRFactor = 1.2 1.7

8/10/2019 Pete 663 Pass Gr

18/30

CALCULATING CLAY CONTENT

(VSHALE) Shale Index

Calculating Vsh Numerous models

Always have Vsh < Ish

May only apply locally

minmax

min

GRGR

GRGRIsh

=

)12(33.0

)34/(

)2/(

2=

=

=

=

shIsh

shshsh

shshsh

shsh

V

IIV

IIV

IV

90 GAPIGR (max)

GR

GR

(min)15 GAPI

48 GAPI

90 GAPI

0 GR (API) 100

Shale

Shaly

sand

Clean

sand

Shale

GRTool

Some Models:

8/10/2019 Pete 663 Pass Gr

19/30

8/10/2019 Pete 663 Pass Gr

20/30

SOLUTION

GRmin

= 10API

GRmax=132

Grlog=50 API

8/10/2019 Pete 663 Pass Gr

21/30

V SH RELATIONSHIPS

minmaxminGRGR

GRGR

Ish

=

1590

1548

=sh

I

44.0=sh

I

0.44

20%

26%

Example from Slide 22

Example from Slide 24

minmax

min

GRGR

GRGRIsh

=

10132

1050

=sh

I

327.0=shI 0.327

14%17.5

8/10/2019 Pete 663 Pass Gr

22/30

SOLUTION

GRmin = 10 API

GRmax = 132 API

Choosing a depth in SAND C , say GR =50 API

Linear Vsh = 0.327Clavier Vsh = 0.175

Steiber Vsh = 0.139

8/10/2019 Pete 663 Pass Gr

23/30

8/10/2019 Pete 663 Pass Gr

24/30

8/10/2019 Pete 663 Pass Gr

25/30

SPECTRAL ANALYSIS PRINCIPLE

The radioactivities of the 3 elements differ, based on theenergy level peaks

8/10/2019 Pete 663 Pass Gr

26/30

SPECTRAL GAMMA RAY LOG

URANIUM

THORIUM POTASSIUM

8/10/2019 Pete 663 Pass Gr

27/30

From Dresser Atlas, 1982

SPECTRAL GAMMA

RESPONSE INMESOZOIC

CARBONATESAND SHALES,

EAST-CENTRAL

TEXAS

8/10/2019 Pete 663 Pass Gr

28/30

8/10/2019 Pete 663 Pass Gr

29/30

SOME GR APPLICATIONS

8/10/2019 Pete 663 Pass Gr

30/30

SOME GR APPLICATIONS -

VERSATILE TOOL

Lithology indicator

Reservoir descrimination

Vsh cutoff

Correlation Well-to-well

Open hole to cased hole

Core-to-log

Depth control

Depositional Environment

Uses curve shape, log responses, and characteristis of

bedding contacts to infer grain sizes and sedimentaryprocesses and environments

Exploration for radioactive rocks

Uranium, potassium chloride

Fracture detection Some fracture-fil ling mineral deposits are hot