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IMPEDANCE MICROSTRUCTURE OF KEROGEN SHALES
Manika PrasadRock Abuse Laboratory
Colorado School of Mines
Presented at the 27th Oil Shale Symposium at CSM on October 17, 2007
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OUTLINE
• Motivation• Working Principles• Quantitative Studies• Ultrasonic Impedance Analyses• Impedance Microstructure • Conclusions
October 17, 2007 27th Oil Shale Symposium at CSM
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MOTIVATION
• Microstructural investigations based on impedance changes
• Quantitative impedance mapping• Mapping with 1 - 100 μm resolution
– quantify textures as impedance values– quantify textural changes with maturation
• Non-destructive evaluation
October 17, 2007 27th Oil Shale Symposium at CSM
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POROSITY - VELOCITY RELATION
27th Oil Shale Symposium at CSM
2
3
4
5
6
7
0 0.1 0.2 0.3Fractional porosity
Vp
(km
/s)
sandstones
High porosity shales
Low porosity shales
Data from Vernik and Liu, 1997
• High porosity kerogen shalesresemble sandstones
• Low porosity kerogen shalesrequire different approach
Increasingshaliness
Sandstones
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2
3
4
5
6
7Vp
(km
/s)
Stage II III - IVa IV V - VI
high porosity
MATURATION - VELOCITY RELATIONVp increases with maturity in low porosity kerogen shales
Maturity stage
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ACOUSTIC MICROSCOPY
HF pulse generator
circulator
Receiver Image storage
Monitor
Scanning system
sample
water
transducer
Rayleigh wavesZ
Y
X
C-scan
B-scanA-scan
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QUANTITATIVE INFORMATIONGray scale calibration with materials of known impedance. Gray color in image of unknown sample gives its AI values
October 17, 2007 27th Oil Shale Symposium at CSM
200 μm
18.2
1000 μm
30.3
17.3
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MATURITY GRADE: II
October 17, 2007 27th Oil Shale Symposium at CSM
Elongated grains Very low acoustic impedance
1000 μm100 μm8.2
62.5 μm7.4Bakken
Formation
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MATURITY GRADE: II
October 17, 2007 27th Oil Shale Symposium at CSM
200 μm200 μm11.4
100 μm13-198.3
Elongated grains Very low acoustic impedance
Woodford Formation
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1000 μm500 μm 1000 μm
MATURITY GRADE: III
October 17, 2007 27th Oil Shale Symposium at CSM
BazhenovFormation Fine grained, elongated grains
Low acoustic impedance
238.310.9
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MATURITY GRADE: III
BakkenFormation
500 μm1000 μm 200 μm7.5 9.57.7
Elongated grains, connected texturesLow acoustic impedance
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MATURITY GRADE: IVa
October 17, 2007 27th Oil Shale Symposium at CSM
BazhenovFormation
Coarser grained Higher acoustic impedance
1000 μm200 μm15 8.3
10
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MATURITY GRADE: IVb
October 17, 2007 27th Oil Shale Symposium at CSM
BakkenFormation
Coarser grained, elongated textureHigher acoustic impedance
312 μm1000 μm 312 μm19-23 8.89.3
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312 μm1000 μm 312 μm
MATURITY GRADE: V
Woodford Formation
9.0 13-179.4
Coarser grained, elongated textureHigher acoustic impedance
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HYDROGEN INDEX - ACOUSTIC TEXTURE
Transition from kerogen load-bearing (immature) to grain supported (mature)
1 mm
BakkenShales
294319
122175
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HYDROGEN INDEX - ACOUSTIC TEXTURE
Transition from kerogen load-bearing (immature) to grain supported (mature)
62 µmBakkenShales
294319
122175
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2
3
4
5
6
0 200 400 600 800Hydrogen Index
Vp(
km/s
)HYDROGEN INDEX - Vp RELATION
Vpincreases with increasing shale maturity
Fast
Slow
BakkenShales
mature immature
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4
6
8
10
12
0 200 400 600 800Hydrogen Index
Impe
danc
e (M
rayl
s)HYDROGEN INDEX - IMPEDANCE
Micro-Impedance
Bulk Impedance
• Impedance increases with increasing shale maturity.
• Ultrasonic values match well with AM values
BakkenShales
mature immature
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MICRO- AND MACRO IMPEDANCE
October 17, 2007 27th Oil Shale Symposium at CSM
• Bulk impedance increases with maturation• Microstructural changes• Micro-impedance increases with maturation
Remote detection of kerogen maturity
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Statistical tools for texture analysis
[ ][ ]{ }II mnymxImyxIEnmR −++−= ),(),(),(
∫∫ +−= dxdyykxkiyxIkkI yxyx )}(exp{),(),(ˆ
∗•= IIkkS yx ˆˆ),(
Autocovariance function (ACF)
Fourier transform
Power spectrum
S R
Heterogeneity – Coeff. of variation (CV)
CV = std. dev{I(x,y)} / mean{I(x,y)}
Mukerji and Prasad, 2005
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Autocovariance Function & Textures
From Mukerji and Prasad, 2005
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Textural anisotropy ratio (AR)
1/e ~ 0.37
amin amaxar = amax/amin
lag
AC
F
From Mukerji and Prasad, 2005
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Texture analysis
Textural Heterogeneity – Coeff. of variation CVLarger contrast of heterogeneity leads to high values of CV
Textural Anisotropy – Anisotropy Ratio ARTextural anisotropy leads to a directional dependence of the ACF
Textural Scale – Mean correlation lengthLarger sized heterogeneities lead to larger correlation lengths
From Mukerji and Prasad, 2005
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Textural anisotropy & scales• Textural anisotropy
(AR) increases with increasing correlation length
• With depth (= maturity), textural anisotropy increase is lower while the mean correlation length increase is larger Deeper samples have lower anisotropy but larger heterogeneities
DEEP
SHALLOW
Increasing Maturity
From Mukerji and Prasad, 2005
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Textural heterogeneity and scales - depth dependence
Increasing Maturity
SHALLOW
DEEP
• Mean correlation length decreases as textural heterogeneity (CV) increases deeper samples have larger heterogeneities with higher contrast
From Mukerji and Prasad, 2005
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Textural heterogeneity & shale maturity
0.06
0.08
0.1
0.12
0.14
0 10 20 30
TOC (%)
Coe
f. of
var
iabi
lity
Increasing maturityBakken
0.06
0.08
0.1
0.12
0.14
0.1 0.3 0.5 0.7Volumetric kerogen content
Coe
f. of
var
iabi
lity
Increasing maturityFrom Mukerji and Prasad, 2005
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Textural anisotropy and maturity-depth & scale dependence
Increasing maturity
Bakken
1
1.2
1.4
1.6
1.8
0.1 0.2 0.3 0.4 0.5 0.6Volumetric kerogen content
Mea
n A
niso
trop
y 1000 µm
100 µm100 µm
1000 µm
From Mukerji and Prasad, 2005
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Results from SAM Image Analysis• The coefficient of variation (CV) (a measure of impedance
heterogeneity) ranges from 7% to about 12%. • The mean correlation length tends to increase with increasing
heterogeneity.• Textural heterogeneity, elastic impedance, velocity, and density
increase with increasing shale maturity.• The textural spatial correlation length varies with direction. • The textural anisotropy (AR) ranges from 10% to about 70% and
tends to decrease with increasing depth & maturity.
Quantifiable and consistent patterns linking - Texture, - Shale maturity, and - Wave propagation properties
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CONCLUSIONS
• Acoustic impedance in kerogen shalesincreases with shale maturity
• Bulk impedance matches well with impedance measured on a micrometer scale
• With increasing maturity, there is a transition from kerogen supported to grain supported framework
October 17, 2007 27th Oil Shale Symposium at CSM
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ACKNOWLEDGEMENTS
The experimental work was done at the Frauenhofer Institute, Saarbrücken in Germany
This research is supported by NSF, PRF, and the Fluids Consortium.
October 17, 2007 27th Oil Shale Symposium at CSM