A new shale gas evaluation method using isotropic approximation based on modified Xu-Payne model

Anisotropy is regarded as a key issue for shale gas due to partial alignments of clay minerals especially illite and bedding-parallel of organic-matter within shale. In high-quality reservoir prediction, however, the isotropic assumption is routinely adopted instead of the anisotropic assumption. Moreover, when the anisotropy is beyond the scope of weak anisotropy, the answer remains to be unknown. This paper mainly modifies Xu-Payne model under the isotropic approximation and make it feasible for strong anisotropic high quality reservoir prediction, and a detailed discussion on the performance of isotropic approximation is carried out. Firstly, continuous kerogen distribution is obtained using CARBOLOG method constrained by the chemical analysis. Then modified Xu-Payne model for organic-rich shale is proposed to predict velocity, based on which the effects of kerogen on the elastic properties are discussed. Finally, an integrated analysis is performed to detect the high quality reservoir interval, and the results agree well with the anisotropy analysis and SEM (Scanning Electron Microscope) observations of cracks, verifying the feasibility and effectiveness of the isotropic approximation.