Three-dimensional discrete element simulation of a transversely isotropic rock considering initial cracks

Shale, as a reservoir rock, exhibits obvious anisotropy in both its physical and mechanical properties. An in-depth understanding of its mechanical behaviors considering the pre-existing cracks is vital for the shale gas extraction. In this study, a three-dimensional discrete element model of transversely isotropic rock considering initial cracks was developed to investigate the nonlinear crack-closure behavior and failure characteristics of shale under uniaxial compression. The results show that the nonlinear crack-closure and failure behaviors of shale are greatly affected by the anisotropic angle ((1), microcrack density and width in rock matrix. The crack-induced strain of shale specimen is mainly dependent on the change of microcrack width in soft rock matrix, which increases with (1. Compared with the specimen without considering initial cracks, regardless of (1, the specimen considering initial cracks is characterized by a lower crack-initiation stress and a higher brittleness index, which is consistent with the evolution of microcracks formed in specimen. The failure mechanism of specimen is further revealed by the stress transfer between stiff and soft rock matrices. With increasing (1, the axial stress in soft rock matrix increases, whereas that in stiff rock matrix decreases first and then increases, resulting in the transition of failure mode of specimen. Fully considering the influences of weak planes and initial cracks, the proposed model can well capture the variation of rock brittleness and predict the critical breakout pressure of boreholes, thus providing a more reliable basis for analyzing engineering problems under complex stress conditions in shale gas extraction.