Effects of Fractured Reservoir Elastic Properties on Azimuthal Avo

Aligned vertical fractures introduce velocity anisotropy which is directly related to parameters, such as fracture density, fracture shape and fracture contents. Effective medium models allow us to study qPand qS-wave velocity anisotropy in rocks with aligned vertical fractures, intersecting fracture sets and aligned fractures with smallscale porosity. Spatially varying fracture density distributions result in velocity spatial variations. Stochastic modeling is used to quantify velocity heterogeneity in a fractured reservoir, where the fracture density field is modeled as a stationary Gaussian random field specified by a covariance function describing the amplitude, orientation, characteristic wavenumbers, and roughness of a fracture density field. The goal of the modeling is to relate the stochastic forward model to the statistics of the velocity and seismic reflectivity fields. Three-dimensional finite difference modeling has been used to investigate the seismic response of fractured reservoirs in P-wave seismic data, and the effects of background Vs/Vp contrasts and anisotropic overburden. The numerical results indicate that background Vs/Vp contrasts across the reflecting boundary, and the presence of anisotropy above the reservoir, have significant effects on azimuthal AVO response at the top of fractured reservoirs. Although a larger Vs/Vp contrast gives rise to a strong AVO response in isotropic media, it is not necessary to give rise to a strong azimuth AVO response. Our numerical modeling shows that the smaller Vs/Vp contrast model gives rise to a strong azimuthal AVO response. Anisotropic overburden always modifies the azimuthal AVO response.