Joint Fluid Factor and Brittleness Index Inversion of a Coal-Measure Gas Reservoir with Vertical Transverse Isotropy

A coal-measure gas reservoir can be practically approximated as a medium with vertical transverse isotropy (VTI), accounting for the anisotropy caused by near-horizontal bedding and fractures. However, previous studies inversed the elastic and reservoir parameters using the isotropic assumption-based traditional amplitude variation with offset (AVO) method, which is unacceptable in high-precision evaluation and prediction for coal-measure gas reservoirs. To address this research gap, we performed a joint inversion of fluid factor and brittleness index (BI) for the VTI coal-measure gas reservoir. A PP-wave reflection coefficient approximation equation, expressed in terms of the fluid factor and BI for a VTI medium (F-BI-VTI equation), was derived using Rüger’s approximate equation. Two typical interface models were employed to evaluate and analyze the precision and suitability of the F-BI-VTI equation. Subsequently, we tested the AVO inversion method through synthetic angle gathers generated from well-logging data and demonstrated its application using a cross-well prestack seismic profile. The results show excellent consistency between inversion outputs and original, calculated, and estimated logging curves for noise-free synthetic data. Although inversion precision slightly decreased for synthetic data with SNR = 2, the method maintained robust performance. Application to cross-well prestack data yielded inversed fluid factor, BI, and density consistently matched the original, calculated, and estimated logging curves at the well location. However, the precisions of the two inversed anisotropy parameters were reduced to a relatively lower level. Nevertheless, the proposed method can effectively inverse the fluid factor and BI of VTI coal-measure gas reservoirs, thus facilitating future identification of optimal resource and engineering conditions.