Chapter 7 Gamma Ray (GR) log
Lecture notes for PET 370 Spring 2012 Prepared by: Thomas W. Engler, Ph.D., P.E.
GR Log
Background
• The Gamma Ray log is a continuous measurement of the natural radioactivity emanating from the formations? True or False?
GR Log
Background
• The Gamma Ray log is a continuous measurement of the natural radioactivity emanating from the formations. True • The principal isotopes emitting radiation are Potassium40, Uranium, and Thorium (K40, U, Th) ? True or False?
GR Log
Background
• The Gamma Ray log is a continuous measurement of the natural radioactivity emanating from the formations. True • Principal isotopes emitting radiation are Potassium-40, Uranium, and Thorium (K40, U, Th) True • Isotopes are concentrated in sands; thus higher radioactivity occurs in sandstones than other formations. True or False
GR Log
Background
• The Gamma Ray log is a continuous measurement of the natural radioactivity emanating from the formations. True • Principal isotopes emitting radiation are Potassium-40, Uranium, and Thorium (K40, U, Th) True • Isotopes are concentrated in clays; thus higher radioactivity is in shales than other formations. False • Sensitive detectors count the number of gamma rays per unit of time and record in “API Units” which is 1/200th of the calibrated, standard response. True or False?
GR Log
Background
• The Gamma Ray log is a continuous measurement of the natural radioactivity emanating from the formations. True • Principal isotopes emitting radiation are Potassium-40, Uranium, and Thorium (K40, U, Th) True • Isotopes are concentrated in clays; thus higher radioactivity is in shales than other formations. False • Sensitive detectors count the number of gamma rays per unit of time and record in “API Units” which is 1/200th of the calibrated, standard response. True
GR Log
General GR Response
Uses
GR Log
Write down five uses of the GR log? • Estimate bed boundaries, stratigraphic correlations • Estimate shale content • Perforating depth control • Identify mineral deposits of potash, uranium, and coal
• Monitor movement of injected radioactive material
Bed boundaries and stratigraphic correlation
LaRica (Morrow) Pool, SENM
Perforating Depth Control Casedhole log Openhole log
Perforating Depth Control Openhole log
Casedhole log
Monitor movement of injected radioactive material
Two-Isotope Tracking in a Two-Stage Fracture Treatment
Identify mineral deposits of potash, uranium, and coal
Gamma ray, density, and micrologresistivity data measured in Well Southern Ute-Mobil 36-1.
Coal intervals are characterized by densities < 1.75 g/cm3 and lower (20 to 60API) gamma Ray values. Notice the significant microlog separation in the coal intervals and the lack of separation in the noncoal intervals.
GR Log
Vsh estimation
• Vsh is the bulk volume of shale (precisely the volume of silt, dry clay, and bound water) to bulk volume. • Calculate shale index, IRA , by RA RA min I RA RA sh RA min where RAmin is clean zone reading RAsh is shale zone (max) reading RA is reading in zone of interest • GR correlations based on: – shale distribution type – age of shale (tertiary or older) – local area • Disadvantage: Contamination from non-shale radioactive sources.
GR Log
Shale Distribution
Define: fe = ft
fe = ?ft
What is the value Of Vsh?
Vsh= ?
Vol (str gr)
Name a type of structural shale grain?
VB
GR Log
Shale Distribution
Define: fe = ft
fe = ?ft -Vsh
What is the value Of Vsh?
Vsh= ?Vol (dis) VB
Name a type of dispersed shale?
GR Log
Shale Distribution x= fsh
Define: fe = ft
fe = ?ft -xVsh
What is the value Of Vsh?
Vsh= ?
Vol (lam) VB
Vsh estimation
GR Log 1.0 0.9
Shale Volume, V
sh
0.8 0.7 laminated
0.6 0.5
Larionov (older rocks)
0.4 Stieber
Clavier, et al
0.3 0.2
Larionov (tertiary rocks)
0.1 0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Radioactivity Index, IRA
0.8
0.9
1.0
Vsh estimation
GR Log 1.0 0.9
Shale Volume, V
sh
0.8 0.7 laminated
0.6 0.5
Larionov (older rocks)
0.4 Stieber
Clavier, et al
0.3 0.2
Larionov (tertiary rocks)
0.1 0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Radioactivity Index, IRA
RAmin clean sand zone = ? RAsh shale zone = ? RA in zone of interest ,Very shaly sand = ? I RA
RA RA min
RA sh RA min
?
0.9
1.0
GR Log
SP/GR Comparison
GR Log
Factors affecting tool response
Write down five factors that affect GR tool response:
(1). Radiation intensity of the formation (2). Counter’s efficiency
(3). Time constant (rc) (4). Logging speed (5). Borehole environment
GR Log
Time constant/logging speed
Gamma Ray Logs never repeat exactly! The minor variations are statistical fluctuations due to the random nature of the radioactive pulses reaching the detector.
Typical ranges are 5 - 10 API Units in shales, and 2 - 4 units in clean formations
Reduce statistical fluctuations by optimizing the time constant and logging speed.
GR Log Effect of Time Constant and Logging Speed on bed resolution
Time constant/logging speed
GR Log
Time constant/logging speed
The faster the logging speed, the less time the tool can sufficiently react and properly count the radiation intensity. Two effects: 1. The tool response is shifted in the direction the tool is moving. This lag or critical thickness (hc) is given by hc = n tc; where n is logging speed (ft/sec) and tc is the time constant (sec). 2. The log cannot properly respond when h < hc
Time constant/logging speed
GR Log
The time constant and logging speed are regulated so that the GR log is representative of the formation radioactivity. 1. By experience, hc = 1 ft., avoids excessive distortion of the GR curve. 2. Common combinations are:
n (ft/hr)
3,600 1,800 1,200 900
tc (seconds)
1 2 3 4
GR Log
Borehole effects
Function of tool type, borehole size, mud weight, eccentricity
Correction typically ignored except for quantitative analysis such as shale volume calculations.
Borehole effects
GR Log
Examples: (1). A GR-CNL-LDT combination is run decentralized. What is the corrected response if the log response is 40 API units in a 9” hole with 8.3 ppg mud? ...16” hole ...?
GRcor/Grlog 1.4
GRcor/Grlog 1.0
Borehole effects
GR Log
Examples: (2). A GR - BHC combination is run centered. What is the corrected response if the log response is 40 API units in a 9” hole with 16 ppg mud? ....16” hole....?
GRcor/Grlog 4.5
GRcor/Grlog 1.75
GR Log
References
Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE Textbook Series, Vol. 4, (1994) Chapter 2, Sec 2.1 – 2.5 Chapter 7, Sec all Schlumberger, Log Interpretation Charts, Houston, TX (1995) Western Atlas, Log Interpretation Charts, Houston, TX (1992)
