AVO Sensitivity to Shale Reservoirs

Hamad Alghenaim - NTNU

Outline

Objectives

Laboratory Experiment

Data and Model

Results

Conclusion

Next

Acknowledgment

Objectives

Examine unconventional shale reservoir sensitivity to:

Saturation

Stress

Frequency

Anisotropy

Cracks

Laboratory Experiments

• Two experiments:• Constant stress and

varying saturation. (done previously – data available)

• Constant saturation and varying confining and axial stresses. (April 2015 – data processing)

• Measured ultra sonic velocities and low frequency velocities calculated from strain measurements. (E, ʋ)

• Three different orientations in each expirement to calculate the stiffness tensor elements and anisotropy parameters.

Data and Model

Saturation Frequency Crack density

Stress

Saturation

Frequency

Crack density

Stress

Data and Model

Oveburden clay rich shale

Reservoir silt rich shale

Vp=3000 m/s, Vs =1500 m/s, ,density=2.4 g/cc, epsilon=0.2 and delta=0.1

Lab data

X Shale

Mancos Shale

Results

Two freq. (1 Hz & 21 Hz) while varying saturation

0 5 10 15 20 25 30 35 40 45 50-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1AVO Analysis

Offset (m)

Refe

ctivit

y

1 Hz S=01 Hz S=121 Hz S=12 Anis1 Hz S=331 Hz S=551 Hz S=751 Hz S=861 Hz S=86 Anis1 Hz S=10021 Hz S=021 Hz S=1221 Hz S=12 Anis21 Hz S=3321 Hz S=5521 Hz S=7521 Hz S=8621 Hz S=86 Anis21 Hz S=100

0-6 deg

6-20 deg

20+ deg

Increasing sensitivity to saturation

Shale samples measured at 1 Hz

0 5 10 15 20 25 30 35 40 45 50-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1AVO Analysis

Offset (degrees)

Refe

ctivit

y

1 Hz S=01 Hz S=121 Hz S=12 Anis1 Hz S=331 Hz S=551 Hz S=751 Hz S=861 Hz S=86 Anis1 Hz S=100

• Dimming with increasing saturation

• Trend starts at S=55

• Lower saturations are random

• Higher separation between isotropic and anisotropic curves with increasing saturation

Shale samples measured at 21 Hz

0 5 10 15 20 25 30 35 40 45 50-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1AVO Analysis

Offset (degrees)

Refe

ctivit

y

21 Hz S=021 Hz S=1221 Hz S=12 Anis21 Hz S=3321 Hz S=5521 Hz S=7521 Hz S=8621 Hz S=86 Anis21 Hz S=100

Similar observation to the previous slide but the whole curves are higher meaning brighter at low angles and dimmer at higher angles with increasing frequency

Data with measured anisotropic parameters

0 5 10 15 20 25 30 35 40 45 50-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1AVO Analysis

Offset (degrees)

Refe

ctivit

y

1 Hz S=121 Hz S=12 Anis1 Hz S=861 Hz S=86 Anis21 Hz S=1221 Hz S=12 Anis21 Hz S=8621 Hz S=86 Anis

• Higher zero crossing for higher saturation 17 deg vs 25 deg

• Dimming with angle:• Increasing S• Increasing Frequency• anisotropy

Data and Model

Oveburden clay rich shale

Reservoir silt rich shale

Lab data

Lab data

Pierre Shale

Mancos Shale

3 Freq. (1 Hz,21 Hz, 500kHz), 2 Sat. (12%,86%)

0 5 10 15 20 25 30 35 40 45 50-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3AVO Analysis

Offset (degrees)

Refe

ctivit

y

1 Hz S=121 Hz S=8621 Hz S=1221 Hz S=86500 kHz S=12500 kHz S=86

• Intercept separation

• Change in polarity

1 Hz and varying saturation

0 5 10 15 20 25 30 35 40 45 50-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25AVO Analysis

Offset (degrees)

Refe

ctivit

y

1 Hz S=121 Hz S=861 Hz S=01 Hz S=331 Hz S=751 Hz S=100

100

86

75

• Confirms Batzle 2010

Plots from Batzle and Sarker 2010

Sensitivity to Cracks

Tsvankin 1997Rathore 1995

Adding vertical cracks to 1 Hz, S=12% & 86%

0 5 10 15 20 25 30 35 40 45 50-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3AVO Analysis

Offset (degrees)

Refe

ctivit

y

1 Hz no crack 121 Hz with crack 12 y1 Hz with crack 12 x1 Hz no crack 861 Hz with crack 86 y1 Hz with crack 86 x

Varying crack density (1 Hz, S=12%,86%)

0 5 10 15 20 25 30 35 40 45 50-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3AVO Analysis

Offset (degrees)

Refe

ctivit

y

1 Hz no crack 121 Hz with crack 12 y 0.021 Hz with crack 12 x 0.021 Hz with crack 12 y 0.011 Hz with crack 12 x 0.011 Hz no crack 861 Hz with crack 86 y 0.021 Hz with crack 86 x 0.021 Hz with crack 86 y 0.011 Hz with crack 86 x 0.01

Adding 500 kHz

0 5 10 15 20 25 30 35 40 45 50-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3AVO Analysis

Offset (degrees)

Refe

ctivit

y

1 Hz no crack 121 Hz with crack 12 y 0.021 Hz with crack 12 x 0.021 Hz with crack 12 y 0.011 Hz with crack 12 x 0.011 Hz no crack 861 Hz with crack 86 y 0.021 Hz with crack 86 x 0.021 Hz with crack 86 y 0.011 Hz with crack 86 x 0.01500 kHz no crack 12500 kHz with crack 12 y 0.02500 kHz with crack 12 x 0.02500 kHz with crack 12 y 0.01500 kHz with crack 12 x 0.01500 kHz no crack 86500 kHz with crack 86 y 0.02500 kHz with crack 86 x 0.02500 kHz with crack 86 y 0.01500 kHz with crack 86 x 0.01

Conclusion

Sensitivity to saturation at angles higher than 20

Higher affect of anisotropy with higher saturation

Dimming reflectivity with increasing saturation for angles higher than 20

Lack of saturation trend for low saturations until ~S=55%

The higher the frequency the brighter the reflectivity at low angles and the dimmer it gets at higher angles

At higher saturations, larger separations between zero intercepts of the different frequencies

Small influence of cracks at angles higher than 30

Next

Include the stress varying data in the analysis after processing

Plot all the data on intercept-gradient for further analysis

Acknowledgment

PhD student: Dawid Szewczyk

Supervisors: Rune Holt / Per Avseth

Sintef