Shale mineral particles directional arrangement and its effect on the mesoscopic failure mechanism

Quantitative characterization of the heterogeneity of shale mineral particles and determination of the relationship between the heterogeneity and the anisotropy of shale meso-mechanical behavior is difficult. Based on the Advanced Mineral Identification and Characterization System (AMICS) mineral analysis system, the distribution of shale minerals grain has been obtained. The coordinate population variance and heterogeneity coefficient are innovatively proposed to quantitatively characterize the directional arrangement characteristics of shale particles. The anisotropic mesoscopic failure characteristics of shale with different burial depths in the Longmaxi formation, Southern Sichuan, China, were obtained by X-ray nano-CT (computed tomography) real-time monitoring, and the relationship between shale mineral grain arrangement and meso-mechanical behavior is discussed. The results show that both coefficients defined in this study can reflect the anisotropic distribution of mineral particles, and assist in finding the direction of mineral particle deposition. Based on the calculation of two parameters, we found that the studied shale from three different depths have different mineral grain distributions along the bedding orientation, which controls the fracture evolution characteristics and the macroscopic mechanical behavior of the vertical bedding of shale. Fracture propagation of the horizontal bedding shale is related to micro-cracks. Micro-cracks are usually developed in the periphery of high-density brittle minerals, and cracks under external loadings are generated by the expansion of these micro-cracks.