Chapter-4_gr.pdf

Chapter-4 GR Log (Gamma Ray Log) By

Dr. Jorge Salgado Gomes 3/4/2013

Chap -4

Duration of this chapter: 3 classes1(135’)

Core Gamma

Educational Outcomes • • • • • •

Review the concepts of formation radioactivity Review the most radioactive lithologies/minerals Use of GR log for correlations Use of GR to match logs with cores The use of GR log in sequence stratigraphy The use of GR to detect water entries in cased holes across perforation – NORM – Natural Occurring Radioactive Minerals

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Electromagnetic Radiation Spectrum

Use of GR Log • Detect pay from non-pay zones – Pay = reservoir; non-pay= shales (non reservoir)

• Correlations between wells (sequence stratigraphy) • Indication of lithology and source rock type (marine, continental) • Picking coring points • Useful for geosteering with MWD-Gamma • Determine the net to gross ratio for volumetric calculations • If faults are present, the fault throw detection is easier than other conventional logs. • Detect water breakthroughs in cased holes • Detect casing leaks (cross-flow behind pipe) 3/4/2013

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How the tool works ? spectral

n

Condition: E (MeV)

open or cased hole

selective

K

water/mud or dry hole

U

Th

1,3 ... 1,6 ... 2,4 ... 2,8 MeV channels

integral

detector all impulses above a treashold of energy

Unit: MeV (Million electron Volts) Recording speed: 1800 ft/hr – satisfactory definition of a 4 ft bed

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Schematic Scintillation Detector

API GR Calibration Pit UNIVERSITY of HOUSTON

Integral Gamma Measurement Integral activity is effect of 3 contributions

I = (K + U + Th) Unit: API-unit API facility is constructed of concrete with an admixture of radium to provide 238U decay series, monazite ore as a source of thorium, and mica as a source of potassium.

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The complex GR response • Determine different minerals • For complex mineral identification

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TYPICAL GAMMA-RAY RESPONSE TO SEDIMENTARY ROCKS

Gamma-ray API Values for Minerals (S - data from Schlumberger 1989; H - data from a Table of Hearst, Nelson, 1985, made on the basis of values from Brom and Driedonks 1981, Edmundson, Raymer, 1979, Fertl , Frost 1980, Patchett 1975, Reeves 1981, Tixier, Alger 1970)

Mineral

Quartz, Dolomite, Calcite (clean) Plagioclase (Albite, Anorthite) Alcali feldspar (Orthoclase, Anorthoclase, Microcline) Micas (Muscovite, Biotite) Shale Kaolinite Chlorite Illite Montmorillonite Sylvite Carnallite

 - ray (API)

Ref

0 0 220

H; S S S

270 80 …150 80 …130 180 … 250 250 … 300 150 … 200 500 200

S H S S S S H; S H; S

Application 1: shale – sand separation

1. Plot sand line (minimum) 2. Plot shale line (maximum) 2. Design lithologic profile Note: Consider the caliper

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Application 2: Shale content calculation •

Basis: correlation between shale content and gamma activity

•

Assumption: only shale and clay are radioactive components in rock, no other radioactive minerals First step: Calculation of “gamma ray shale index”

GR I GR

GR  GRcn  GRsh  GRcn

log response in zone of interest

GR

log response in a GRcn zone considered clean (shale free) response in a GRsh log shale zone

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Application 2: Shale content calculation Second step: Select & apply a relationship IGR vs. Vsh

Vsh  I GR

Linear relationship (upper limit)

Vsh  0.083  (23.7IGR  1)

Tertiary clastics (Larionov, 1969)

Vsh  0.33  (2 2IGR  1)

Mesozoic & older rocks (Larionov, 1969)

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Application 2: Shale content calculation Second step: Select & apply a relationship IGR vs. shale content Vsh

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Note: Sand with Mica or Feldspar - "radioactive sandstone"

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Baker Atlas, 2002

Application 3: Clay Mineral Identification

• Clay minerals show different Th/K ratios for different mineral composition • Used for clay mineral identification • Combination with other properties (Pe, neutron) recommended

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Mineral Identification from Spectral gammalog 100% Illite

12

K (%)

Ave. Feldspar Line Potassium Evaporites Feldspars 10 Glauconite Micas Illite Clays 8 6

Baker Atlas, 2002

Smectites and Mixed Layer Clays

4

Kaolinite

2 Chlorite 0 3/4/2013

Ave. 100% Clay Line

5

10

Heavy Thorium Minerals 15 20 Th (ppm)

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Chap -4

30 Baker Atlas, 2002 18

Note: Uranium Content of Source Rocks 4.65 ppm Uranium in acid igneous rocks •

forms soluble salts (uranyle), transported in water (sea water 3ppb dissolved Uranium)

•

three ways passes into sediments (Serra, 1979):

-

chemical precipitation in acid (pH 2.5 - 4.0) reducing environment

-

adsorption by organic matter, or living plants and animals

-

chemical reaction in phosphorites

Stagnant, anoxic waters, low rate of sediment deposition, which typically produce black shales (North Sea Jurassic „hot shales‘) 3/4/2013

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NGS Response to Trace Uranium in Clean Sand

Th/U indications Th/U indicator for environment: • Th/U > 7 continental, oxydizing • Th/U < 7 marine, grey ... green shales • Th/U < 2 marine, black shales, phosphates.

Source rock indication from spectral gammalog, Baker Atlas document

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Influences and corrections • absorption of radiation – influence of caliper, mud density, casing

– influence of tool position (centralized or sidewall) • bed thickness (thin beds show reduced effect) • formation density (influences depth of investigation) • logging speed – influences statistics – influences vertical resolution

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QC - Quality Control Gammalog The gamma curve should agree with other shale indicators (except in „radioactive beds“) Shale values should be similar to those in nearby wells Repeatability: curves should have the same shape and character as those from previous runs or repeated sections Cross-check the curve character with other logs from the same logging run.

Adapted after Krygowski, 2004

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3.6.2.6 Natural Radioactivity Summary Natural Gamma-activity controlled by U-, K- and Th- content Two techniques are applied – integral measurement – spectral measurement Gammalog is a typical „lithology log“ based on the measurement of the natural gammaradioactivity of a formation.

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Summary • K, Th, and U as source of radioactivity are concentrated in shale  shale has high gamma reading. • Shale-free („clean“) rocks (sandstones and carbonates) usually have low gamma intensity. • Gammalog can be applied for lithologic profile design, shale content estimate, and well-to-well-correlation. • Other shale indicators: Spontaneous potential, Density-NeutronCombination • Attention: Feldspathic, glauconitic, or micaceous sandstone show high gamma radiation (K); organic matter shows high radiation (U)

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Running GR Log along cores • To be able to match core-log depth mismatch

Core inside

Baker Atlas, D. Georgi

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Core Gamma

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Core-Log Gamma Ray Correlation Open Hole GR

Top of Core

Core GR

Core Gamma Ray is best correlation curve for Clastics.

Core porosity is best correlation curve for Carbonates ? Let’s think about this !

Example from Core Labs

Baker Atlas, D. Georgi

Gamma-Gamma Log Interaction of incident radiation (source) with electrons - gives information about density  porosity - gives information about lithology detector

source

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Gamma Ray Interactions with Rocks 3 effects of interaction

 Photoelectric effect  Compton effect

energy loss

 Pair production

probability depends on • energy of radiation and • atomic number of target material

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Photoelectric Effect 

e-

An incident low-energy gamma photon (< 0.2 MeV) collides with an atom If the energy of the gamma photon equals or exceeds the "binding energy" of an orbital electron, then • the gamma photon gives up all of its energy • the electron leaves its orbit, • and has a kinetic energy Ekin= gamma ray energy - electron binding energy

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Compton Effect 

Compton electron

An incident intermediate-energy gamma photon (gamma ray) collides with an atom: • it ejects an electron (“Compton or recoil electron”) from an outer shell and leaves with a lower energy;

• the scattered gamma energy is a function of the angle of scattering

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Pair Production nucleus

e-



e+

electron

positron

An incident high-energy gamma photon (gamma ray energy > 1.02 MeV)1 can be converted into a electron - positron pair when it is near a nucleus. The electron slows down The positron interacts with an ordinary electron. They annihilate one another and produce two gamma-rays. 11.02

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MeV is exactly twice the rest mass of an electron (mc2)

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Gamma-ray energy as result of scattering (Photoeffect and Compton effects) Pe

Photoelectric effect

Z

mineralogy Compton effect

Gamma radiation

electron density

cps

density

r1 increasing Z

Pe

r2 < r1

density measurement

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Energy of gamma radiation 33

Gamma Ray Absorption Mechanisms In the energy range between 0.5 and 5 MeV for most abundant elements the COMPTON-effect dominates.

Rock forming elements

Cs Co 3/4/2013

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Absorption of Radiation Interactions result in attenuation (absorption) of radiation, described by absorption coefficient a

I0

I(x)

IGG(x) = I0 exp (-a x)

x The absorption coefficient is • connected with the absorption cross section • related to the effect of interaction: ac

- absorption coefficient for Compton effect

aPe

- absorption coefficient for Photoelectric effect (Pe)

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Photoelectric Effect For many elements the photoelectric cross section shows the proportionality to atomic number Z3.6

sPe  Z 3.6 on this basis a effective photoelectric index Pe (average photoelectric cross section per electron) is defined:

Pe = (Z/10) 3.6 Pe depends on elemental composition (lithology) - see table.

Pe - unit: b/e barns per electron

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Mean values for density r, electron density re , ratio Z/A, and photoelectric absorption index Pe Substance

r (g/cm3)

re (g/cm3)

Z/A

Pe (b/e)

quartz

2.654

2.650

0.499

1.806

calcite

2.710

2.708

0.500

5.084

dolomite

2.870

2.864

0.499

3.142

halite

2.165

2.074

0.479

4.65

gypsum

2.320

2.372

0.511

3.420

anhydrite

2.97

2.96

0.499

5.05

kaolinite

2.44

2.44

0.50

1.83

illite

2.64

2.63

0.499

3.45

barite

4.48

4.09

0.446

266.8

water (fresh) 1.000

1.110

0.555

0.358

oil3/4/2013

0.948 Chap -4

0.558

0.125

0.850

37

note • Pe can help to discriminate between Quartz, Calcite, and Dolomite, • Pe is one component in mineralogyporosity crossplot technique • Pe is extremely sensitive with respect to barite (mud!)

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Compton effect  Bulk density and electron density Number of orbiting electrons e control probability of Compton effect

e=Z But bulk density is controlled by

-

+++ + +

A=Z+N

-

Z/A  0.5

-

Compton effect controlled by bulk density 3/4/2013

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Compton Effect Probability for Compton scattering is proportional the number of electrons per unit volume

e  N 

Z r A

where N - Avogadro's number (6.026 1023 ) Z - atomicnumber A - atomicmass number r - density

For practical purposes we define an „electron density“

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Chap -4

Z re  2   r A

40

Electron density is related to the number of electrons per molecule (Z), and bulk density is related to the total atomic mass per molecule (A). For most common Earth minerals, the ratio is constant

Z  0.5 A and thus

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Z rb  2     r e  r e  A Chap -4

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Mean values for density r, electron density re , ratio Z/A, and photoelectric absorption index Pe Substance

r (g/cm3)

re (g/cm3)

SZ/SM

Pe (b/e)

quartz

2.654

2.650

0.499

1.806

calcite

2.710

2.708

0.500

5.084

dolomite

2.870

2.864

0.499

3.142

halite

2.165

2.074

0.479

4.65

gypsum

2.320

2.372

0.511

3.420

anhydrite

2.97

2.96

0.499

5.05

kaolinite

2.44

2.44

0.50

1.83

illite

2.64

2.63

0.499

3.45

barite

4.48

4.09

0.446

266.8

water (fresh) 1.000

1.110

0.555

0.358

oil (med. gr.) 0.79

0.80

0.57

0.125

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Gamma Ray Interactions - Summary  For practical log applications two effects are important  Compton effect  Photoelectric effect  Density determination by nuclear measurements applies Compton effect; the correlation between density and electron density bases on a nearly constant ratio Z/A.  Determination of Pe applies Photoelectric effect and gives an information about mineral composition by the strong correlation to atomic number Z

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Gamma-Gamma-Measurement: Tool and Calibration

count rate axis logarithmic density axis linear

Count rate

Short spacing

Detector 1

Long spacing

Detector 2

source 3/4/2013

density Chap -4

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Gamma-Gamma-Density Primary calibration of density tools usually freshwater saturated limestones of high purity, Secondary calibration aluminium, sulfur, concrete blocks

Corrections:

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• Caliper and rugosity • Mud density • Deviation from Z/A = 0.5 (mineralogy) • Barite Chap -4

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SUPPORT MATERIAL

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Geiger-Muller Tube