Numerical study on the impact of coal fractures on seismic wave dispersion and attenuation: anisotropic WIFF effects

Coalbed methane, a key resource in natural gas exploration, is closely linked to the fracture networks within coal beds. Unlike shale or tight sandstone reservoirs, coalbed methane reservoirs feature a dual-porosity/fracture system consisting of matrix pores and cleats. While prior studies have examined the impact of cleat orientation on P-wave velocity and permeability anisotropy, joint frequency-direction analyses of dispersion and attenuation are limited. This study fills this gap by analyzing dispersion and attenuation characteristics in coalbed methane reservoirs, focusing on the cleat system's role in reservoir characterization and gas extraction. Using Biot's quasi-static pore elastic consolidation equation coupled with finite element modeling and numerical upscaling, we investigated the cleat system's influence on seismic signals at the representative elementary volume (REV). Digital rock samples incorporating cleat features were constructed, and their attenuation and velocity dispersion were calculated perpendicular to butt and face cleat directions. A sensitivity analysis assessed the effect of cleat characteristics on seismic signals. Results show that seismic wave dispersion and attenuation are primarily driven by the wave-induced fluid flow (WIFF) effect, with the cleat system's characteristics influencing seismic signal propagation. Directional variations in seismic wave propagation significantly affect dispersion and attenuation, with fracture aspect ratio and infills playing key roles. These findings enhance our understanding of seismic wave anisotropy and provide a foundation for interpreting seismic data in porous media.