Sensitivity of seismic attenuation and phase velocity to intrinsic background anisotropy in fractured low-permeability formations

摘要

Several authors studied the impact on anisotropic seismic attenuation and dispersion of fracture networks embedded in an isotropic background. While this is a reasonable assumption in many scenarios, in the practically important case of fractured shales considered in this study, we do, however, have to account for its well known intrinsic transverse anisotropy. In this work, we explore how seismic anisotropy is affected by the intrinsic elastic and hydraulic anisotropy of the background. Modelling both the background and the fractures as poroe-lastic media, we use numerical oscillatory relaxation tests to determine the effective anisotropic seismic properties of the probed fractured rock. For a shale containing two orthogonal hydraulically connected fractures, we find that the behaviors of both P- and S-waves are significantly modified with respect to an isotropic background. The anisotropy in the stiffness of the background not only plays a fundamental role in the pressure diffusion equilibration between the fractures and the background, but also affects the wave-induced fluid flow between connected fractures. Conversely, the anisotropy of the background permeability mainly affects the characteristic frequency at which the maximum attenuation due to wave-induced fluid flow between fractures and background occurs.