Summary

Shales are the most abundant sedimentary rock type, and have a very important role in oil and gas drilling operations, where they make up for a great part of all the drilled sections. Few experimental are available reporting their mechanical and rock physics characteristics. This study aims at characterizing a preserved shale recovered from the Northwest shelf of Australia in terms of composition, microstructure, mechanical and rock physics properties, plus evolution of ultrasonic anisotropy under triaxial loading. Preserved shale specimens cored normal and parallel to bedding were tested to evaluate the evolution of the ultrasonic wave velocities with increasing stress conditions and how anisotropy is affected by different loading angle with respect to the bedding plane. An array of ultrasonic transducers allowed us to measure five independent wave velocities on a single core plug, which were used to calculate the full elastic tensor of the shale assuming it to be a transversely isotropic medium. Results indicate that P- and S-wave velocities vary monotonically with increasing mean effective stress. The shale has small intrinsic P-wave anisotropy which tends to increase up to 5% with increasing mean effective stress, while S-wave anisotropy decreases from ~40% to ~30% over the same stress increment. Intrinsic anisotropy is related to the initial composition and fabric of the sediment and the presence of microfratures, while changes in elastic anisotropy result from the applied stresses, their orientation to the rock fabric and the degree of stress anisotropy.

Introduction

Despite many years of research and their important industrial implications, the elastic properties of shales are poorly understood. In particular, the monitoring of elastic wave velocities and anisotropy in shales under loading is relatively uncommon, while it would be helpful in resolving ambiguities in seismic profile interpretation and seismic signature of fluids.

Most of the experimental determinations of elastic wave velocities on shale samples have been performed under hydrostatic conditions (Best and Katsube, 1995; Hornby, 1998; Johnston and Christensen, 1995; Johnston and Toksöz, 1980; Jones and Wang, 1981; Stanley and Christensen, 2001). However, elastic wave velocity measurements on shale are reported by Yin (1992) under triaxial and polyaxial loading, and by Podio et al. (1968) under uniaxial loading.

Many of the laboratory studies are conducted without control of pore pressure or in undersaturated conditions not reflecting the in-situ settings of the sediments. Only few studies (e.g. Dewhurst and Siggins, 2006) report elastic wave velocity data on saturated shale samples under triaxial loading.

This study investigates the acoustic response of a Northwest shelf (NWS) shale to changing stress conditions. The shale is first described in terms of texture and microstructure as well as composition so that its fabric elements can be linked to the measured elastic parameters.

Methods

Consolidated undrained multi-stage triaxial tests were performed on preserved shale samples using a Terratek triaxial testing machine (described by Dewhurst and Siggins, 2006). The equipment comprises a high stiffness load frame, a pressure vessel, and independent stepping motor pumps for cell and pore pressure control, as well as for axial load.

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