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Abstract

AAPG Bulletin, V. 86, No. 8 (August 2002),Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Geomechanical, microstructural, and petrophysical evolution in experimentally reactivated cataclasites: Applications to fault seal prediction

David N. Dewhurst,1 Richard M. Jones2

1CSIRO Petroleum, Australian Petroleum Cooperative Research Centre, 26 Dick Perry Avenue, Kensington, Western Australia, 6151, Australia; email: [email protected]
2National Centre for Petroleum Geology and Geophysics, Australian Petroleum Cooperative Research Centre, University of Adelaide, Adelaide, South Australia, 5005, Australia; current address: Woodside Energy Ltd., 1 Adelaide Terrace, Perth, Western Australia, 6000, Australia; email: [email protected]

AUTHORS

Dave Dewhurst obtained a Ph.D. in 1991 from the University of Newcastle upon Tyne, England. Postdoctoral work at the University of Birmingham, the University of Newcastle upon Tyne, and Institut Francais du Petrole, together with a three-year stint at Imperial College, London, focused on compaction, faulting, and fluid flow in mudrocks. He joined CSIRO Petroleum in 1998 and is responsible for geomechanical, geophysical, and petrophysical inputs to projects involving overpressure prediction and cap/fault seal potential.

Richard Jones received a Ph.D. in 1995 from the University of Keele, England. After positions at GeoChem and Rock Deformation Research, Richard joined the National Centre for Petroleum Geology and Geophysics (NCPGG) in 1999 as fault seals project leader for the research program, Hydrocarbon Sealing Potential of Faults and Cap Rocks. His current position is as an exploration geologist working for Woodside Energy. He has worked extensively in the area of fault and top seal evaluation and has been involved with seals research programs in Europe, the United States, and Australia. His current interests include structural/seal evaluation and wellbore stability.

ACKNOWLEDGMENTS

This work was performed under the aegis of the Australian Petroleum Cooperative Research Centre (APCRC) program, Hydrocarbon Sealing Potential of Faults and Cap Rocks. Origin Energy kindly provided data and samples for the study. The sponsors of this APCRC program (Woodside Energy, Origin Energy, OMV, JNOC, Globex, Santos, Exxon/Mobil, BHP, Chevron, and Phillips) are thanked for their financial support and permission to publish. The advice and technical expertise of Leo Connelly was invaluable during the geomechanical testing program. Colin MacRae and Cameron Davidson from CSIRO Minerals Division are gratefully acknowledged for their help with and specimen preparation for the scanning electron microscopy analyses carried out in the course of this research. Thanks are also due to Richard Hillis for comments on an early version of the manuscript, Jon Olsen and Amgad Younes for thoroughly constructive reviews, and also to editors Bruce Trudgill and John Lorenz for their clear and detailed summary of the suggested revisions.

ABSTRACT

Failure envelopes for well-lithified cataclastic fault rocks from the Otway Basin, Australia, where fault reactivation is a significant risk to trap integrity, have been determined through triaxial testing. Geomechanical analyses indicate that cemented cataclasites exhibit significant cohesive strength and that fault reactivation and trap breach is influenced by the development of shear, tensile, and mixed-mode fractures. The mechanics of the fracturing process are influenced by grain strength and cataclasite morphology. Cemented cataclasites are more prone to failure than are reservoir sandstones under low differential stress conditions, as a result of a relatively low cohesive strength and higher friction coefficient. As such, the geomechanical property differential between cataclastic faults and undeformed reservoir strata may impact significantly on seal integrity during reactivation. Intact cataclasite seal capacity exceeds 2400 psi (16.5 MPa). Following reactivation seal capacity is reduced about 95% as a result of the development of a highly connected fracture network. The tensile strength of these cataclastic faults allows failure to occur by shear, tensile, and mixed-mode fracturing. This suggests that geomechanical tools used to predict trap breaching by reactivation that assume cohesionless frictional failure may significantly underestimate seal risk. Determination of fault seal risk can, therefore, be significantly enhanced by multidisciplinary research efforts combining field- and laboratory-scale geomechanical analysis with microstructural and petrophysical property description.

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