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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”