ABSTRACT:
A standard assumption in the geophysics literature is that shear wave polarization and splitting occurs due to stress-aligned structure. This structure is considered by some to be stress aligned microcracks, by others with reservoir interests, as a desirable ‘open’ set of sub-vertical conducting fractures that are also assumed to be parallel or sub-parallel to the maximum horizontal stress. Geomechanics modeling demonstrates that unless fractures are rather rough and wall strength rather high, or there is over-pressure, there are likely to be only very small hydraulic apertures at several kilometers depth. Deep-well measurements demonstrate that fractures that are under differential shear stress are more likely to be water conducting, and those that are principally under normal stress are less likely to be water conducting. In this paper, alternative interpretations of shear-wave polarization directions are examined, including the contribution of two, maybe unequal joint sets, intersected by the major stress, having different compliances, and possibly with pre-peak non-linear shear strength and dilation contributions, to their enhanced permeability.
1 INTRODUCTION
The use of polarized shear waves, for indicating the presence of aligned and perhaps fluid-conducting structure at depths in petroleum reservoirs, has been a topic of interest for at least 20 years. The classic assumption has been that the aligned structure that causes frequency-dependent shear-wave anisotropy, is usually parallel or sub-parallel to the major stress, and may also be caused by stress-aligned microcracking. In fact it has been shown in an extensive review of the literature (Barton 2006), that deviation between the assumed stress and apparent structure, or deviation between the axes of maximum attenuation and the assumed stress, may each occur, each being more likely when no longer close to the surface.