Dynamic seismic S-wave anisotropy in vertically fractured rocks: Modeling and field application

ABSTRACT Due to overburden and tectonic stresses, many deep formation rocks contain vertically oriented fractures, resulting in significant seismic S-wave anisotropy and attenuation, as commonly observed from borehole acoustic anisotropy measurements in deep wells. Understanding the fracture-induced phenomena is of theoretical interest and practical importance. By applying a fracture scattering theory to a fluid-saturated rock containing vertical fractures, we investigate the fracture-induced effects on dynamic S-wave moduli, anisotropy, and attenuation anisotropy in the rock medium. This includes the effects of fracture-background wave-induced fluid flow and elastic scattering on the fractured rock’s stiffness. The results indicate that fracture elastic scattering has a significant influence on the S-wave moduli and attenuation anisotropy. Compared with the velocity response of elastic scattering, Rayleigh scattering increases the S-wave anisotropy, whereas Mie scattering decreases it. The elastic scattering-dominated S-wave anisotropy and attenuation anisotropy are significantly affected by fracture size (diameter) and density. We use the theoretical results to interpret field acoustic anisotropy and attenuation anisotropy data acquired in a deep well, wherein the theory and measured data are in good agreement.