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

OUTLINE

• Motivation• Working Principles• Quantitative Studies• Ultrasonic Impedance Analyses• Impedance Microstructure • Conclusions

October 17, 2007 27th Oil Shale Symposium at CSM

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

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

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

ACOUSTIC MICROSCOPY

HF pulse generator

circulator

Receiver Image storage

Monitor

Scanning system

sample

water

transducer

Rayleigh wavesZ

Y

X

C-scan

B-scanA-scan

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

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

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

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

MATURITY GRADE: III

BakkenFormation

500 μm1000 μm 200 μm7.5 9.57.7

Elongated grains, connected texturesLow acoustic impedance

MATURITY GRADE: IVa

October 17, 2007 27th Oil Shale Symposium at CSM

BazhenovFormation

Coarser grained Higher acoustic impedance

1000 μm200 μm15 8.3

10

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

312 μm1000 μm 312 μm

MATURITY GRADE: V

Woodford Formation

9.0 13-179.4

Coarser grained, elongated textureHigher acoustic impedance

HYDROGEN INDEX - ACOUSTIC TEXTURE

Transition from kerogen load-bearing (immature) to grain supported (mature)

1 mm

BakkenShales

294319

122175

HYDROGEN INDEX - ACOUSTIC TEXTURE

Transition from kerogen load-bearing (immature) to grain supported (mature)

62 µmBakkenShales

294319

122175

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

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

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

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

Autocovariance Function & Textures

From Mukerji and Prasad, 2005

Textural anisotropy ratio (AR)

1/e ~ 0.37

amin amaxar = amax/amin

lag

AC

F

From Mukerji and Prasad, 2005

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

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

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

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

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

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

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

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