Dynamic seismic signatures of fluid-saturated porous rocks with horizontal planar fractures embedded in a vertical transversely isotropic background

ABSTRACT Wave-induced fluid flow (WIFF) is a crucial mechanism influencing seismic dispersion, attenuation, and frequency-dependent anisotropy in fractured reservoirs. Most previous studies of WIFF assume an isotropic rock background. However, fractured reservoirs, such as shale, often exhibit intrinsic anisotropy in the rock background. To explore the effects of this background anisotropy on dynamic seismic signatures, we derive an analytical model for the effective elastic properties of fluid-saturated rocks with horizontal planar fractures embedded in a vertical transversely isotropic (VTI) background. This model is derived from the solution for periodically layered rocks composed of VTI layers, treating the planar fractures as thin, highly porous isotropic layers. We apply the model to a fractured tight sandstone sample with a VTI background. Our results indicate that background anisotropy significantly affects seismic wave velocities and the magnitude of velocity anisotropy. However, the impact on the magnitude of seismic dispersion and attenuation is relatively small. Correspondingly, the frequency-dependent features of velocity and attenuation anisotropy are also less influenced by background anisotropy. To validate our model, we compare our theoretical predictions with the experimental measurements on a fractured shale sample. By accounting for the background anisotropy effects, our model’s predictions agree well with the experimental data. Our model is concise and can be easily applied to characterize unconventional reservoirs with background anisotropy, such as shale reservoirs.