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ABSTRACT: The Response of Shale Ultrasonic Properties to Stress Path, Stress State and Pore Pressure

Dewhurst, Dave1, Tony Siggins1, Bailin Wu1, and Kevin Dodds2
(1) CSIRO Division of Petroleum Resources, Glen Waverley, Australia 
(2) CSIRO Petroleum, Bentley, Australia

We present results from a laboratory investigation into the ultrasonic response of shales under simulated in-situ stress and pore pressure conditions. Our aim is to assess the effects of stress path and stress state on ultrasonic compressional and shear response under rigorously defined stress conditions. We attempt to assess the validity of the conventional Biot and Terzaghi effective stress laws for ultrasonic responses of shales using a hydrostatic stress path. A second test using a burial (Ko) stress path will investigate the effects of stress anisotropy on the elastic and physical properties of shales. The effect of pore fluid pressure on ultrasonic response is monitored under normally compacted and overcompacted conditions. This will allow us to simulate pressure/stress conditions that may result from disequilibrium compaction or fluid expansion mechanisms. We report here on experimental procedures employed, shale microfabric and progressive change of shale petrophysical/acoustic properties with degree of compaction. Comparisons of acoustic parameters between stress paths (isotropic, anisotropic) and stress states (under-, normal and over-compaction) will be made to assess which ultrasonic attributes are good indicators of pore pressure and stress conditions. Transverse isotropy of P-wave and S-wave velocities has been noted and modelling predicts a rotation of S-wave polarisation due to microfracturing under overpressured conditions. This may occur in shales when overpressure generation post-dates compaction. This work is part of the Australian Petroleum Cooperative Research Centre's "Abnormal Geopressure Research Program" which aims to enhance current pre-drill techniques used for pore pressure prediction by the petroleum industry.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia