PASSIVE MEASUREMENTS – NATURAL GAMMA

FORMATION EVALUATION

PASSIVE MEASUREMENTS

• Caliper

• Spontaneous Potential

• Gamma Ray– Natural– Spectral

GAMMA RAY LOGS• Uses

– Correlation– Lithology indicator; exploration

for radioactive materials– Evaluation of shale content– Paleoenvironmental indicator– Open or cased hole; any fluids– 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

1.46

Probability of Emission per Disintegration

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

Uranium-Radium Series

1.76

Gamma ray is corrected for borehole effects

1. Hole size

2. Mud density

3. Tool position in hole (centering)

4. Casing diameter

5. Casing size and weight

6. Cement thicknessDepth of investigation 12 inches - 90% from the first 6 inches

GAMMA RAY CORRECTIONS

EXAMPLE

• Gr log = 67 API

• Hole size = 8 inches

• Mud weight = 16 lbs/gal

• Tool is centered

• Od. of the tool = 3-3/8 inches

C.F=1.8

16

SOLUTION

GR COR /GR LOG = 1.8

GR COR = 1.8 * GR LOG

= 1.8 * 67

= 120.6 API

SHALE WASHOUT

From Dresser Atlas, 1982

CORRECTED ANDUNCORRECTED

GAMMA RAYCURVES

IN WASHOUT

From Dresser Atlas, 1982

WA

SH

OU

T

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 FILTER 13 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

• 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

Coal

Shaly sand

Anhydrite

Salt

Volcanic ashGypsum

BO

RE

HO

LE

ZONATION• Zonation - Defining intervals of similar lithologic and fluid properties

to identify lateral and vertical changes in reservoir properties• Criteria

– Lithology (correlation)– Fluids– Porosity and permeability

• Lithology - Identify correlation markers– Distinctive shale spikes– Distinctive log patterns– Above and below interval of interest (bracket)

• Begin with coarse zonation– Initially, well-to-well correlation of thick (several hundred ft)

sedimentary packages between distinctive markers– Next, correlate finer intervals (100 - 300 ft)– Finally, detailed evaluation of sedimentary facies (5 - 60 ft thick)

• Considerations– Subtle lithologic (facies) changes– Fluid changes– Types of logs available

PASSIVE LOGCORRELATION

• GR, SP, and CAL– often correlate– different

measurements– different reasons

• Correlation helps– GR instead of SP in

OBM– Easier detection of

shales– Facilitates

“zonation”

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

minGRGRGRGR

Ish

)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

Shalysand

Cleansand

Shale

GR

Too

l

Some Models:

V SH RELATIONSHIPS

Linea

r

Clavi

er (~

Consolid

ated

Rock

s)

Steib

er (~

Tertia

ry C

last

ics)

minmax

minGRGRGRGR

Ish

1590

1548

shI

44.0shI

0.44

20%

26%

Example from Slide 28

Example from Slide 31

minmax

minGRGRGRGR

Ish

10132

1050

shI

327.0shI 0.327

14%

EXAMPLE PROBLEM

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

SOLUTION

GRmin = 10API

GRmax =132

Grlog =50 API

V SH RELATIONSHIPS

Linea

r

Clavi

er (~

Consolid

ated

Rock

s)

Steib

er (~

Tertia

ry C

last

ics)

minmax

minGRGRGRGR

Ish

1590

1548

shI

44.0shI

0.44

20%

26%

Example from Slide before

Example from Slide before

minmax

minGRGRGRGR

Ish

10132

1050

shI

327.0shI 0.327

14%

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