Frequency-dependent seismic anisotropy and its implication for estimating fracture size in low porosity reservoirs

Abstract

The use of seismic anisotropy for characterizing subsurface fracture orientations and intensity has become increasingly popular. However, the reluctance of reservoir engineers to accept seismic anisotropy as a routine technique for fracture characterization is partly because of its inability to provide information about sizes and volume of fractures. Although both grain-scale micro-cracks and formation-scale macro-fractures are considered causes of seismic anisotropy, reservoir engineers are more interested in the latter as permeability in many hydrocarbon reservoirs is believed to be dominated by formation-scale fluid units (on the order of meters). We intend to fill this gap by developing new practical applications of seismic anisotropy including new theoretical models and new analysis methods. Frequency-dependent anisotropy has been observed and can be explained by two mechanisms: seismic scattering by heterogeneities and fluid flow in fractured porous rock. In this article, we present a synthetic study demonstrating the dependence of seismic anisotropy on fracture sizes using a newly proposed multiscale fracture model. We then use this model to invert fracture sizes from field multicomponent shear-wave VSP data acquired in Bluebell-Altamonta, Utah. Our study has indicated a great potential of fracture size estimation in low porosity fractured reservoir, and thus we may potentially go beyond the conventional application of seismic anisotropy to predict fracture sizes and fracture volumes. The two most popular models for describing the elastic properties of fractured media are the so-called Thomsen's equant porosity model, and Hudson's thin-crack model. Thomsen's model assumes perfect fluid pressure equalization between the cracks and the surrounding rock, whereas Hudson's model assumes that the cracks are isolated with respect to fluid flow. Both models predict frequency independent behavior. In both models the magnitude of anisotropy is related to the fracture density, although the precise dependence is different in each case. This fracture density is defined as e= …