Materi 3

GAMMA RAY

LOG

STT MIGAS BALIKPAPAN

28 Oktober 2014

OUTLINE

Tujuan Dari Gamma Ray Log

Theorytical Background

Log Display

GR Log Interpretation and Calculation

Environmental Effect Correction

Exercise

Spectral GR Log

OBJECTIVE

Mengidentifikasi Lithologi Secara General

GR Log kita gunakan untuk menentukan lapisan

mana yang merupakan reservoir dan lapisan mana

yang merupakan shale

Mengidentifikasi Kuantitas dari Shaliness

Meskipun GR Log memberikan respon bahwa suatu

lapisan itu reservoir, tapi belum tentu reservoir

tersebut murni sand

Depth Matching

Membandingkan respon dari cased hole log

dengan open hole log

Correlation Between Wells

Mengkorelasikan well yang memiliki respon log

yang sama.

THEORYTICAL BACKGROUND

Electron (negative charge)

Proton (positive charge)

Neutron (no electric charge)

Number of protons = Number of electrons

Mass of proton Mass of neutron

Mass of electron is negligible

Nucleus (protons + neutrons)

Electron shells

NATURAL RADIOACTIVITY

Stable Atom

The number of protons, Z, and

the number of neutrons, N, are

balanced.

Unstable Atom

The numbers of protons and

neutrons are not balanced. The

atom naturally evolves to a

stable element.

Radioactivity (U,Th,K)

The spontaneous transformation

of some elements into other

elements, accompanied by the

emission of , , or radiation.

LOG DISPLAY

The gamma ray is displayed in the left-hand track of the paper log.

Log scale is 0 to 150 API units.

A scale of 0 to 200 can also be used if log values are very high.

LOG INTERPRETATION AND CALCULATION

High gamma ray values (about 100 API units) are mostly

encountered in shales.

Organic shales may have much higher gamma ray values.

Clean reservoirs (without shale) normally have low

gamma ray values (15 to 25 API units).

Evaporites (halite, anhydrite) have low GR.

Coal beds have low GR

The gamma ray log is used to identify clean layers.

RADIOACTIVE RESERVOIRS

"Clean" formations are sometimes very radioactive:

Clean sands may contain feldspars, micas or otherradioactive minerals.

Clean carbonates may contain uranium salts.

Radioactive salts get deposited near the wellboreover perforated intervals.

In this cases, the gamma ray overestimates the clay

content of the formation.

The spectral gamma ray log can help, as well as other

shale indicators.

GAMMA RAY RESPONSE IN CLASTICS

GAPI 1500

Shale

Coal

Shal

e

Clean Sand

Shaly Sand

Carbonate

GR min

GR max

Organic Shale

SHALINESS FROM GAMMA RAY

A normalized gamma ray, or gamma ray index, IGR, is

computed from the measured gamma ray and the

minimum (sand) and maximum (shale) values.

IGR = (GR GRmin) / (GRmax GRmin)

Empirical transforms are used to convert this index to

shale fraction, Vsh:

- Linear

- Larionov (Older Rocks)

- Clavier et al.

- Stieber

- Larionov (Tertiary Rocks)

SHALINESS VS. GAMMA RAY INDEX

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Gamma Ray Index

Sh

ale

Fra

cti

on

, V

sh

Linear Scaling

Larionov (Older Rocks)

Clavier et al.

Stieber

Larionov (Tertiary Rocks)

CASED-HOLE CORRELATION TO OPENHOLE

Openhole Gamma Ray

One string of casing

Cased-hole Gamma Ray

CASED-HOLE CORRELATION TO OPENHOLE

Openhole Gamma Ray

Cased-hole Gamma Ray

Two strings of casing

WELL CORRELATION

LOG QUALITY CONTROL

Log is affected by:

Tool position (eccentered or centered),

Borehole size,

Mud weight,

Mud composition, in particular potassium,

Casing and cement.

Correction charts and software settings exist to correct

the GR log for these effects.

If the GR log is primarily used for correlation, corrections

are not always applied.

GAMMA RAY BOREHOLE CORRECTION

Reference Conditions:

Hole Diameter = 8 inches

Borehole Fluid = 10 lb/gal mud

Tool eccentered

Actual Conditions:

Hole Diameter > 8 inches


Tool eccentered

Some of the radioactive formation is missing: Gamma ray log is too

low.

Actual Conditions:


Borehole Fluid = heavy mud

Tool eccentered

Some gamma rays are absorbed by the mud: Gamma ray log is too

low.

Actual Conditions:



Tool centered

Some gamma rays are absorbed by the mud: Gamma ray log is too

low.

Halliburton Chart GR-1



Schlumberger Chart GR-1

2

)2.54(d

2

)2.54(d

8.345

Wt sondeholemud

(mud weight in g/cc, diameter in cm)

2

d-dWt sondeholemud

(mud weight in lbm/gal, diameter in

inches)

Wmud = 10 lbm/gal

dhole = 8 inches

t = 7.04


t = 1.40 (14.5 3.375) 2.54 / 2 = 19.8 g/cm2

Correction Factor = 1.35

Compute gamma ray correction factor using

Schlumberger Chart GR-1 for the following conditions:

Mud Density = 1.40 g/cc

Borehole Diameter = 14.5 inches

Sonde Diameter = 3.375 inches

Sonde is eccentered



2

ddWt sondeholemud

CASED HOLE GAMMA RAY CORRECTION

Reference Conditions:


Borehole Fluid = 10 lbm/gal mud

Mud Casing Cement

Formation


Schlumberger Chart GR-3(mud weight in lbm/gal, diameter in inches)

Borehole Diameter = 8.5 in

Casing OD = 7 in

Casing Weight = not given, assume 26 lb/ft

Casing ID = 6.276 in

Sonde Diameter = 3 3/8 in = 3.375 in

Mud Density = 1.79 g/cc lb/gal

Casing density = 7.96 g/cc

Cement density = 2.0 g/cc


Borehole Diameter = 8.5 in

Casing OD = 7 in

Casing Weight = not given, assume 26 lb/ft

Casing ID = 6.276 in

Sonde Diameter = 3 3/8 in = 3.375 in

Mud Density = 1.79 g/cc lb/gal

Casing density = 7.96 g/cc

Cement density = 2.0 g/cc

t = 18 correction factor = 1.7


Cased-Hole GR

Op

en

-Ho

le G

R

Slope = 1.66

Empirical Gamma Ray Normalization

Cased-hole log

Open-hole log

In this case, we have both

openhole and cased-hole gamma

ray logs over the same interval.

Why is GR Correction Important?

Correcting the gamma ray log for casing

effect is very important when splicing logs

recorded over separate drilling phases.

Is there a reservoir here?

Gamma Ray Splicing Final log aftersplicing and

patching.

LOGGING SPEED

SPECTRAL GR LOG

This log records the individual concentration of

U, Th, K

Applications:

Clay identification

Complex lithology analysis

Organic matter identification

Better estimation of clay contents

GAMMA RAY ENERGY WINDOWS

Gamma ray energy (MeV)

Potassium

Thorium

Uranium

0 0.5 1 1.5 2 2.5 3

Scale

X 10

W1 W2 W3 W4 W5

K+U+Th

Materi 3

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