Abstract: Accurate estimation of effective pore aspect ratio for shale reservoir is significant in rock-physics modeling and shear-wave velocity prediction. In order to estimate pore structure and S-wave velocity, a rock-physics-based method is proposed for effective pore aspect ratio and shear-wave velocity prediction in shale. We build a quantitative relationship among P-wave velocity, S-wave velocity, pore aspect ratio, porosity and mineral compositions through rock-physics model. Then, through finding the best estimation of pore aspect ratio by minimizing the error between theoretical predictions and field measurements, we invert the effective pore aspect ratio and the shear wave is predicted with this constraint. The inversion results of the well logging data show that the aspect ratio remains stable in Longmaxi shale formation, while the aspect ratio of the surrounding rock varies in wide range. It indicates that the micro-structure in shale is more stable than in surrounding rock. The error between the predicted and measured shear wave velocities is small. In addition, for shale formation lacking mineral components data, the error in prediction of shear wave velocity calculated by average mineral compositions is still small. This shows that compared with mineral compositions, Longmaxi shale is more sensitive to pore aspect ratio. In conclusion, the effective pore aspect ratio and shear-wave velocity can be predicted accurately by the method proposed in this study.

Key words: shale, rock-physics model, pore aspect ratio, prediction of shear wave velocity