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Chapter 9 -Density logs
Lecture notes for PET 370
Spring 2012
Prepared by: Thomas W. Engler, Ph.D., P.E.
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Density Log
• Porosity/Lithology Determination
– Mineral identification in evaporates
– Gas detection (w/ other logs)
– Estimate mechanical properties (w/ sonic)
– Evaluation of shaly sands and complex lithologies
• other
–
Determination of hydrocarbon density – Determination of oil shale yield
– Identify overpressured zones
Uses
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Density Log
Logging speed:
15 to 30 fpm
Depth of investigation:
3 to 6”
Tool Schematic
Vertical
Resolution ~ 1 ft
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Density Log Principle of Operation
Bassiouni, 1994
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Density Log
Density logging is based on measuring the attenuation
of gamma rays caused by compton scattering, (CPS per
energy)
Theory
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Density Log
1. Compton scattering depends only on the electron density of the formation(# of electrons/cc)
2. The electron density is related to the bulk density by,
Where for common elements in sedimentary rocks,
But for H2 the ratio is double.
3. To account for variations in Z/A, the tool is calibrated in a freshwater filledlimestone formation.
Where rba is the apparent bulk density.
1883.0e0704.1 ba rr
Calibration
rrA
Z2
be
00.1
A
Z2
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Density Log
• r ba r b for most common sedimentary rocks
• For other formations (salt, anhydrite, coal) corrections are necessary.
Calibration
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Density Log FDC-Borehole Correction
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Density LogTeague (Mckee) Field
La Munyon #76
Caliper
Incorrect
Density
Readings
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Density Log
dolofor2.86
lmsfor2.71
ssfor2.65
densitymatrixma
where
f ma
bma
or
f
ma)1(
b
oncontributi
fluid pore
oncontributi
matrix
density bulk
Measured
r
rrrr
rrr
Porosity
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Density Log
• Define rf as average density of fluid in pore space
• Depth of investigation of density tool is shallow (invaded zone), thus
• In practice,
Mud Type rmf (gm/cc)oil 0.9
Fresh water 1.0
Salt water 1.1
•In water-bearing zones,
Sxo = 1 and rf = rmf
• In oil-bearing zones, use same rf = rmf Assumes Sxo is large and rh rmf .
h)
xoS1(
mf xoS
f rrr
Porosity
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Density Log
The density porosity is usually calculated assuming rf
= 1 gm/cc.Compare this apparent porosity, a, to the true porosity, t , thatcorresponds to a bulk density of 2.1 gm/cc in the following:
(1). a water-bearing sandstone invaded by a mud filtrate of 1.05 gm/cc
density
Porosity-Example
%3.330.165.2
1.265.2
f ma
bma
a
rr
rr
%4.3405.165.2
1.265.2
f ma
bma
t
rr
rr
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Density Log
The density porosity is usually calculated assuming rf
= 1 gm/cc.Compare this apparent porosity, a, to the true porosity, t , thatcorresponds to a bulk density of 2.1 gm/cc in the following:
(2). a 0.8 gm/cc oil-bearing sandstone characterized by Sor = 30%.
Porosity-Example
%3.330.165.2
1.265.2
f ma
bmaa
rr
rr
975.0
8.0*3.005.1*7.0
h)
xoS1(
mf xoS
f
rrr
%8.32975.065.2
1.265.2
f ma
bma
t
rr
rr
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Density Log
The density porosity is usually calculated assuming rf
= 1 gm/cc.Compare this apparent porosity, a, to the true porosity, t , thatcorresponds to a bulk density of 2.1 gm/cc in the following:
(3). a low pressure gas-bearing sandstone with 30% residual gas
saturation.
Porosity-Example
%3.330.165.2
1.265.2
f ma
bma
a
rr
rr
735.0
0*3.005.1*7.0
h)
xoS1(
mf xoS
f
rrr
%7.28735.065.2
1.265.2
f ma
bma
t
rr
rr
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Density Log
The density porosity is usually calculated assuming rf
= 1 gm/cc.Compare this apparent porosity, a, to the true porosity, t , thatcorresponds to a bulk density of 2.1 gm/cc in the following:
(4). a change in matrix density to 2.68 gm/cc.
Porosity-Example
%3.330.165.2
1.265.2
f ma
bma
a
rr
rr
%5.340.168.2
1.268.2
f ma
bma
t
rr
rr
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Density Log
Example:
mf = 1.1 gm/cc
ma = 2.71 gm/cc (reference)
A
B
C
Porosity?
rb = 2.81
rb = 2.68
rb = 2.40
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Density Log
• Photoelectric absorption depends on both re and the average atomic number
of the formation.
•Two independent parameters, re and U, are measured in a low energy window
LDT - measurement
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Density Log
The absorption rate depends on the absorption coefficient per electron (Pe) and re;
1883.0 b
U0704.1
eP
density,electronforSubstitute
3cm
electronse*
electron
barnseP3cm
barns U
r
r
LDT - measurement
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Density Log
Advantage:
The Pe curve distinguishes mineralogy regardless of porosity and fluid type inthe pore space.
Common Pe values: quartz 1.81
calcite 5.08
dolomite 3.14
anhydrite 5.05
LDT - measurement
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Density Log
LDT –Example
mf = 1.0 gm/cc
Zone Y
Pe = ?
rb = ?
rma = ?
= ?
Zone X
Pe = ?
rb = ?
rma = ?
= ?
Dog Canyon (Wolfcamp) Field
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Density LogDog Canyon (Wolfcamp) Field
Blackbird BKR St #2
Pe curve
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Density Log
LDT• Detector counts high energy gamma rays (compton scattering) and low
energy region (photoelectric effect)
• Measures both bulk density and Pe
• Pe is strong function of matrix
• Lower statistical variation
•
Better vertical resolution
FDC• Detects only high energy gamma rays
• Measures only bulk density
•
Strong function of porosity, matrix, and fluids
LDT/FDC comparison
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Density Log
Overpressured shale
Overpressured
Normal
Pressuregradient
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Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE Textbook
Series, Vol. 4, (1994)
Chapter 2, Sec 6 – 8
Chapter 8
Schlumberger, Log Interpretation Charts, Houston, TX (1995)
Schlumberger, Log Interpretation and Principles, Houston, TX (1989)
Western Atlas, Log Interpretation Charts, Houston, TX (1992)
Western Atlas, Introduction to Wireline Log Analysis, Houston, TX (1995)
Halliburton, Openhole Log Analysis and Formation Evaluation, Houston, TX (1991)
Halliburton, Log Interpretation Charts, Houston, TX (1991)
Density Log References
Teague (Mckee) Field
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Density LogTeague (Mckee) Field
La Munyon #76
Caliper
Pe curve
0 10