Shear-Wave VSP'S: A Powerful New Tool for Fracture and Reservoir Description

Abstract

Summary Seismic shear waves contain much more information than P waves about the internal structure of the rock along the ray path, which can be interpreted in terms of the crack and stress geometries within the rock mass. Recent field data sets show that shear-wave vertical seismic profiles (VSP's), where a surface source is recorded on geophones downwell, are powerful new tools for detecting fracture orientation and reservoir description. They can provide detailed estimates of the internal crack and stress structure of the reservoir rock. Introduction Several major oil companies recently presented substantial shearwave data sets from reflection surveys gathered for hydrocarbon E and P purposes that reveal crack alignment in the rock mass. Similar oil company data from VSP surveys in the Paris basin, France, and the Silo field, WY, have been reported, Earthquake seismologists have seen the same type of phenomenon, called shearwave splitting, above small earthquakes in many parts of the world (see Ref. 10). There is a now a substantial body of observational evidence that shear-wave splitting is caused by propagation through stress-aligned fluid-filled inclusions. We know that there are fluid-filled fractures, cracks, microcracks, and pores within most rocks in the crust. These fluid-filled inclusions are the most compliant parts of the rock mass and will be preferentially oriented by the current stress field acting on the rock mass. These aligned inclusions are effectively anisotropic to seismic waves, so shear waves that are most sensitive to anisotropy display shear-wave splitting. Confirmation of this interpretation has been provided by recent observations of temporal changes in shear-wave splitting before and after the North Palm Springs earthquake in California and before and after hydraulic fracturing, as reported in this paper. No other explanation for the observed anisotropy displays changes with time in response to comparatively small changes of low-level stress in the comparatively cool upper crust. Shear-wave splitting indicates that the "cracks" are typically aligned by stress into nearly parallel, nearly vertical orientations, striking parallel to the horizontal direction of the current maximum compressional stress (or, more correctly, striking perpendicular to the horizontal direction of current minimum compressional stress). These results are particularly important for reservoir and production engineering. Analysis of shear-wave VSP's can provide information about the internal structure of the reservoir. In particular, analysis of shear-wave splitting yields estimates of the in-situ crack and stress geometries within the reservoir. This improved understanding of the physics of the rock mass is expected to result in improved hydrocarbon recovery. Shear-Wave Splitting In the past, most seismic investigations used seismic P waves to examine the rock mass. P waves arrive before the shear waves because shear waves travel about half as fast as P waves. P waves have longitudinal polarization parallel to the ray path, as illustrated schematically in Fig. 1a. The arrival time, amplitude, and frequency spectrum are the only information in the P wave train. In contrast, shear waves have transverse polarizations (transverse particle motion, Fig. 1b), which carry much more information. In rock without any internal structure (isotropic, uncracked rock, where the properties are the same in all directions), any shear-wave polarization radiated by the source will propagate without significant change of waveform (Fig. 1b). Any two orthogonal polarizations radiated from the same source position will propagate at the same velocity (in isotropic uncracked rock) and the initial polarization of the shear wave will be essentially preserved.