PASSIVE MEASUREMENTS – NATURAL GAMMA

FORMATION EVALUATION

PETE 663

Summer 2010

Dr. David Schechter

PASSIVE MEASUREMENTS• Caliper• Spontaneous Potential• Gamma Ray

– Natural– Spectral

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

GR

Too

l

1. The gamma ray tool records the natural radioactivity of the formation without regard to the source

2. The spectral gamma ray tool identifies the source 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

API: (1/200) OF THE DIFFERENCE IN LOG READING BETWEEN A HOT AND A COLD ZONE

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 gamma

counts 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

NATURAL GR PRINCIPLE• Cause

– Unstable isotopes in formation

– Isotopes decay– Emit GR’s (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 Series1.76

SOURCES OF PASSIVE GAMMA RAYS

1. Clays– Kaolinite (very little K [potassium])– Illite (4-8% K)– Montmorillonite (<1% K)

2. Sand and Silt– Potassium (K) feldspar– Heavy minerals– Volcanoclastics

3. Natural Cements– Fracture-filling

4. Uranium Ores

1. Gamma rays interact with scintillation crystal

2. Electrons excite phosphor atoms, which in turn decay by 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

SCINTILLATION DETECTOR

SHALE WASHOUT

From Dresser Atlas, 1982

CORRECTED ANDUNCORRECTED

GAMMA RAYCURVES

IN WASHOUT

From Dresser Atlas, 1982

STATISTICAL ISSUES• Measurement problem

– GR emissions random– Tool moving

• Results– Imprecise measurement– Details smeared out

• Procedures– New tools better

detectors– Limit logging speed

• Old tools 1800 fph• New tools 3600 fph

– Exercise care interpreting boundaries

Shale

4ftsand

Shale

5,400 ft/hr

1,800 ft/hr

600 ft/hr

API0 120

EFFECTS OFLOGGING

SPEED AND FILTER LENGTH

ON GAMMARAY LOG

GR 2.25 FILTER 100 FPM

GR 2.25 FILTER13 FPM

GR UNFILTERED13 FPM0 150 0 150

0 150

High-resolution loggingfor thin bed, .i.e. coal, is usually

done at low speed tobetter define bed boundaries

and partings

Are these reversed?

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, Chart Book

• Units– GR calibrated to standard– Response in “mid-continent

shale” equals 200 API units– Calibration pits

0 50 100 API units

Shale

Shaly sand

Very shaly sand

Clean limestone

Dolomite

Shale

Clean sand

CoalShaly sand

Anhydrite

SaltVolcanic ash

Gypsum

sand

silt

dry clay

HC

free water

bound waterφt

φe

Vsh

Unit volume of rock

WHAT IS Vshale?

• Fraction of rock made up of shale

• Why calculate Vsh in Sandstone?– 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)

VOLUME OF SHALE

Gamma Ray Index MINMAX

MINSH 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

CALCULATING CLAY CONTENT (VSHALE)

• Shale Index

• Calculating Vsh– Numerous models– Always have Vsh < Ish

– May only apply locally

minmax

minGRGR

GRGRIsh −−

=

)12(33.0

)34/()2/(

2 −=

−=−=

=

− shIsh

shshsh

shshsh

shsh

V

IIVIIV

IV

90 GAPIGR (max)

GR

GR(min)

15 GAPI

48 GAPI

90 GAPI

0 GR (API) 100

Shale

Shalysand

Cleansand

Shale

GR

Too

l

Some Models:

EXAMPLE PROBLEM

Choose value for GRmax and GRminand compute Vsh in sand “C” using linear, Clavier, and Steiber methods

SOLUTION

GRmin = 10API

GRmax =132

Grlog =50 API

V SH RELATIONSHIPS

minmax

minGRGR

GRGRIsh −−

=

15901548

−−

=shI

44.0=shI

0.44

20%26%

Example from Slide 22

Example from Slide 24

minmax

minGRGR

GRGRIsh −−

=

101321050−−

=shI

327.0=shI0.327

14%17.5

SOLUTION

GRmin = 10 API

GRmax = 132 API

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

Linear Vsh = 0.327

Clavier Vsh = 0.175

Steiber Vsh = 0.139

SPECTRAL GR ANALYSIS

• Gives the individual quantities of uranium,

potassium, and thorium

• Good fracture detector, because uranium

tends to precipitate with fracture-filling

minerals

• A sharp uranium peak may indicate fractures

• Good for mineral identification

SPECTRAL GR• Th, U, and K different energies• Tool measures

– counts– energies

• Output– K, Th, U contents.– Th + K gives CGR

• no-uranium GR curve• better measure for Vsh

CGR

SGR

ThK

U

SPECTRAL ANALYSIS PRINCIPLE

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

SPECTRAL GAMMA RAY LOG

URANIUM

THORIUM POTASSIUM

From Dresser Atlas, 1982

SPECTRAL GAMMARESPONSE IN

MESOZOICCARBONATESAND SHALES,

EAST-CENTRALTEXAS

From Halliburton

From Halliburton

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 sedimentary processes and environments

• Exploration for radioactive rocks– Uranium, potassium chloride

• Fracture detection– Some fracture-filling mineral deposits are “hot”