Summary

A kaolinite-rich shale core from the North Sea was characterized in terms of mineralogical composition and physical properties. Subsampled plugs were then subjected to undrained multi-stage triaxial tests close to failure in order to determine the shear failure envelope. During undrained loading, ultrasonic waveforms were recorded parallel to, perpendicular to and at off-axis angles to bedding, such that the evolution of the full elastic tensor could be monitored with increasing stress anisotropy. Results indicate the importance of fabric elements and their orientation with respect to the prevailing stress field.

Introduction

Our understanding of dynamic elastic behaviour in shales is limited, due in part to lack of well preserved samples (see Wang, 2002a) and also to the time involved for testing due to their low permeability. In addition, many velocity measurements previously acquired on shales have been made without control of pore pressure which is critical if relating velocities to changes in effective stress conditions. Shale anisotropy has been known to be a significant problem for many years in terms of depth conversion for seismic exploration (e.g. Banik, 1984), imaging of structures in both surface seismic and cross-hole tomography domains (e.g. Carrion et al., 1992; Meadows and Abriel, 1994) and also for amplitude variation with offset (AVO) analysis (e.g. Wright, 1987). Failure to account for overburden anisotropy may lead to misidentification of fluid type. In addition, anisotropy can cause significant error in estimates of dynamic Poisson’s ratio(s) (Thomsen, 1986). Few laboratory determinations of the full elastic tensor and resultant anisotropy have been made for shales with full pore pressure control (e.g. Jakobsen and Johansen, 2000; Domnesteanu et al., 2002; Wang 2002a). Such studies generally show shales to be transversely isotropic (TI; e.g. Wang, 2002a,b), with the degree of anisotropy dependent on a number of factors such as porosity, kerogen content and microfractures. High anisotropy is noted for example in tight, low porosity organic shales (Vernik and Liu, 1997). Stress effects in shale overburden as a result of reservoir depletion also have a significant effect on time-lapse (4D) seismic response in these low permeability rocks and are not well understood at present. Factors affecting velocity and anisotropy in shales include stress state, stress history, smectite content, organic content, microstructure and physicochemical interactions with pore fluids (e.g. Vernik and Liu, 1997). While it is difficult to know all these parameters in the field, controlled laboratory experiments on well-characterized shales under in situ stress and pore pressure conditions can shed light on the influence of some of these factors on shale velocity and anisotropic response.

Definitions

Geologists and geophysicists have developed slightly different terminology with respect to stress nomenclature over the years.

Methodology

Consolidated undrained (CU) multi-stage triaxial tests were performed on shale core plugs at successively greater confining pressures to determine a failure envelope. In CU tests, samples are consolidated isotropically in a drained state to a set level of confining pressure with drainage at both ends to ensure pore pressure equilibration throughout the sample.

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