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Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones

Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones
Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones

Seismic and electromagnetic properties generally are anisotropic, depending on the microscale rock fabric and the macroscale stress field. We have assessed the stress-dependent anisotropy of poorly consolidated (porosity of approximately 0.35) sandstones (broadly representative of shallow reservoirs) experimentally, combining ultrasonic (0.6 MHz P-wave velocity, VP, and attenuation 1/QP) and electrical resistivity measurements. We used three cores from an outcrop sandstone sample extracted at 0°, 45°, and 90° angles with respect to the visible geologic bedding plane and subjected them to unloading/loading cycles with variations of the confining (20-35 MPa) and pore (2-17 MPa) pressures. Our results indicate that stress field orientation, loading history, rock fabric, and the measurement scale all affect the elastic and electrical anisotropies. Strong linear correlations (R2>0.9) between VP, 1/QP, and resistivity in the three considered directions suggest that the stress orientation similarly affects the elastic and electrical properties of poorly consolidated, high-porosity (shallow) sandstone reservoirs. However, resistivity is more sensitive to pore-pressure changes (effective stress coefficients n>1), whereas P-wave properties provide simultaneous information about the confining (from VP, with n slightly less than 1) and pore pressure (from 1/QP, with n slightly greater than 1) variations. We found n is also anisotropic for the three measured properties because a more intense and rapid grain rearrangement occurs when the stress field changes result from oblique stress orientations with respect to rock layering. Altogether, our results highlighted the potential of joint elastic-electrical stress-dependent anisotropy assessments to enhance the geomechanical interpretation of reservoirs during production or injection activities.

anisotropy, resistivity, ultrasonic
0016-8033
MR271-MR283
Falcon-suarez, Ismael Himar
f14858f6-d086-4761-9dc5-ba09bd89d95b
North, Laurence
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Callow, Ben
19f1a5fe-cabd-4c49-a84f-dc7de0e3c462
Bayrakci, Gaye
e0b89aa5-d514-4ecb-91b1-8ed8bd472eda
Bull, Jon
974037fd-544b-458f-98cc-ce8eca89e3c8
Best, Angus
fd094b23-2f48-41d3-a725-fb2bef223a8a
Falcon-suarez, Ismael Himar
f14858f6-d086-4761-9dc5-ba09bd89d95b
North, Laurence
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Callow, Ben
19f1a5fe-cabd-4c49-a84f-dc7de0e3c462
Bayrakci, Gaye
e0b89aa5-d514-4ecb-91b1-8ed8bd472eda
Bull, Jon
974037fd-544b-458f-98cc-ce8eca89e3c8
Best, Angus
fd094b23-2f48-41d3-a725-fb2bef223a8a

Falcon-suarez, Ismael Himar, North, Laurence, Callow, Ben, Bayrakci, Gaye, Bull, Jon and Best, Angus (2020) Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones. Geophysics, 85 (5), MR271-MR283. (doi:10.1190/geo2019-0612.1).

Record type: Article

Abstract

Seismic and electromagnetic properties generally are anisotropic, depending on the microscale rock fabric and the macroscale stress field. We have assessed the stress-dependent anisotropy of poorly consolidated (porosity of approximately 0.35) sandstones (broadly representative of shallow reservoirs) experimentally, combining ultrasonic (0.6 MHz P-wave velocity, VP, and attenuation 1/QP) and electrical resistivity measurements. We used three cores from an outcrop sandstone sample extracted at 0°, 45°, and 90° angles with respect to the visible geologic bedding plane and subjected them to unloading/loading cycles with variations of the confining (20-35 MPa) and pore (2-17 MPa) pressures. Our results indicate that stress field orientation, loading history, rock fabric, and the measurement scale all affect the elastic and electrical anisotropies. Strong linear correlations (R2>0.9) between VP, 1/QP, and resistivity in the three considered directions suggest that the stress orientation similarly affects the elastic and electrical properties of poorly consolidated, high-porosity (shallow) sandstone reservoirs. However, resistivity is more sensitive to pore-pressure changes (effective stress coefficients n>1), whereas P-wave properties provide simultaneous information about the confining (from VP, with n slightly less than 1) and pore pressure (from 1/QP, with n slightly greater than 1) variations. We found n is also anisotropic for the three measured properties because a more intense and rapid grain rearrangement occurs when the stress field changes result from oblique stress orientations with respect to rock layering. Altogether, our results highlighted the potential of joint elastic-electrical stress-dependent anisotropy assessments to enhance the geomechanical interpretation of reservoirs during production or injection activities.

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More information

Accepted/In Press date: 12 May 2020
e-pub ahead of print date: 28 July 2020
Published date: 1 September 2020
Keywords: anisotropy, resistivity, ultrasonic

Identifiers

Local EPrints ID: 445201
URI: http://eprints.soton.ac.uk/id/eprint/445201
ISSN: 0016-8033
PURE UUID: ed7d8159-ee6c-44db-b4e3-d23e2937dd6f
ORCID for Ben Callow: ORCID iD orcid.org/0000-0003-2296-1702

Catalogue record

Date deposited: 25 Nov 2020 17:30
Last modified: 25 Nov 2021 16:47

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Contributors

Author: Ismael Himar Falcon-suarez
Author: Laurence North
Author: Ben Callow ORCID iD
Author: Gaye Bayrakci
Author: Jon Bull
Author: Angus Best

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