Seismic azimuthal amplitude variation with offset response analysis in coalbed methane reservoirs with aligned fractures using Biot theory

The development of coalbed methane (CBM) relies on high-precision reservoir prediction and lithological inversion. Seismic amplitude variation with offset (AVO) technology is an important tool for fine-scale reservoir characterization. However, the seismic AVO response of CBM reservoirs is complex and is affected by seismic rock physics parameters at different scales. Microscopically, aligned fractures in CBM reservoirs produce complex anisotropy due to formation inclination. At the macroscopic scale, the thickness of CBM reservoirs within seismic frequency bands is comparable to the seismic wavelength and should therefore be treated as a layered medium. In addition, pore fluid significantly affects seismic wave propagation. Consequently, determining the azimuthal AVO response of CBM reservoirs in relation to seismic rock physics parameters at different scales can support high-precision reservoir prediction and lithological inversion. In this study, the primary–primary wave reflection coefficient for a two-phase layered medium was derived using Biot’s theory. Using this model, the response characteristics of the reflection coefficients with respect to seismic azimuth, aligned fracture parameters, reservoir thickness, and seismic main frequency were analyzed. A rotated staggered–grid finite–difference algorithm was employed to simulate wavefield characteristics separately for coal seams and surrounding strata. Seismic attributes were then used to characterize the seismic AVO response. The Z-direction seismic amplitude attributes and reflection coefficients showed similar trends in their responses to seismic rockphysical parameters. This study contributes to establishing a more precise seismic AVO response framework to support CBM reservoir prediction and high-quality lithological inversion.