3 - gamma ray
DESCRIPTION
3 - GAMMA RAYTRANSCRIPT
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