Abstract
Summary Frequency-dependent anisotropy has been observed in seismic data and it can be explained by fluid flow in fractured porous rock. In this study we use a new equivalent medium theory that considers fluid movement due to a squirt-flow mechanism at two scales: the grain scale, where the pore space consists of micro-cracks and equant matrix porosity, and formation-scale fractures. The theory models velocity dispersion and frequency dependence of anisotropy with the fracture length being one of the key parameters. The model is first tested and calibrated against laboratory data. Then we present the analysis and modeling of frequency-dependent shear-wave splitting from multi-component VSP data. By extracting the change in time delay between the split shear waves with frequency, we are able to invert for the fracture size. The derived length scale, which is a crucial parameter for mapping flow units in the reservoir, matches independent observations from borehole data very well.
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