Estimation of anisotropic permeability in fractured reservoirs from seismic AVOZ analysis

Abstract

With the increased emphasis on the management of fractured reservoirs, understanding the spatial variation of anisotropic permeability through the use of discrete fracture models is becoming a crucial issue. Nevertheless, the link between the azimuthal seismic response and the underlying fluid flow properties is still poorly understood. In this paper we use a consistent rock physics model for the effective permeability and stiffness tensors of fractured reservoirs to investigate whether the anisotropic permeability of a simple reservoir model (involving a single set of vertical fractures) can in principle be estimated from the azimuthal variation of seismic amplitudes. We have developed an inclusion‐based method for inverting seismic amplitude versus offset and azimuth (AVOZ) data with respect to the fracture density and azimuthal orientation, which also provides uncertainties (via Monte Carlo simulation). We have tested this inversion method on synthetic seismic data and found that many of the parameters needed for the estimation of anisotropic permeability can be recovered with good accuracy. The main uncertainty comes from the fracture aperture (or aspect ratio), which can be important for the fluid flow properties but normally has little effect on the seismic response (in the absence of squirt flow). From this analysis, we conclude that it may be possible to obtain improved permeability estimates from seismic anisotropy, if one makes certain assumptions about the discrete fracture geometry.