ABSTRACT:

The results are reported of a study of elastic-wave propagation and permeability measurements on a slate before cracking and after oriented sets of cracks had been introduced. Three loading cycles on a servo-controlled testing machine were performed on the rock sample in order to create the cracks in planes parallel to the sample axis. Compressional - and two shear-wave velocities, having particle motion parallel and perpendicular to sets of cracks, were determined concurrently with permeability measured in the same direction. Significant changes in both permeability and elastic-wave velocity, due to stress-induced cracks, were observed after each loading cycle. The velocity magnitudes conformed basically to the predictions of Hudson's theory. A near-linear relationship between the velocity of shear waves polarized perpendicular to the stress-induced cracks and the logarithm of the measured permeability exist. This indicates an encouraging means of monitoring permeability variation in a fractured reservoir rock by using seismic data.

INTRODUCTION

The elastic behaviour of the crust is significantly affected by the presence of cracks, joints and fractures. King (1969) and Anderson, Minster and Cole (1974) reported that a preferred orientation of microcracks has a marked effect on elastic-wave velocities, with a major reduction in velocity when particle motion is perpendicular to the plane of microcracks. Many authors (e.g. Crampin 1984, 1985) have shown that an isotropic solid permeated by aligned cracks or fractures will behave in an anisotropic manner. It is generally found that the microcracks, joints, fractures and faults are aligned by tectonic stresses (Nur and Simmons 1969, Babuska and Pros 1984, Engelder 1982) perpendicular to the direction of the minimum principal stress. Crampin (1984) therefore suggests that the azimuthal anisotropy in the crust, particular at shallow depths, is frequently the result of the alignment of cracks or other inhomogeneities. The use of seismic data to monitor the occurrence of cracks and other discontinuties is of particular interest to geophysicists, petroleum engineers and rock mechanics engineers. In earlier studies, crack parameters were only estimated from observation of the P-wave anisotropy (Crampin and Bamford 1977, Bamford and Nunn 1979, Crampin, McGonigle and Bamford 1980). However, when it was realised that shear wave velocities were more sensitive to changes in crack parameters of a cracked body (Crampin 985), attention was drawn to the importance of studies of polarized shear waves and shear-wave birefringence. King, Myer and Rezowalli (1986) studied acoustic velocities of P and S waves in a jointed basaltic rock mass and observed a very strong seismic anisotropy, approximately 18% for P waves and 25% for S waves. Besides the velocity information provided by shear waves, the polarisation of shear waves provides important information about crack geometry. This is because the faster shear wave is always polarized parallel to aligned cracks. Roberts and Crampin (1986) employed shear-wave polarizations to monitor the natural stress-aligned microcracks and artificial hydraulic fractures at the Hot Dry Rock Geothermal Project in Cornwall, England, and found that the orientation of hydraulic fractures could be predicted from polarizations of shear waves.

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