Investigation on mechanical response and fracture behavior of initially damaged shale based on multi-level PB-GBM method

Research on the failure behavior of Longmaxi shale is vital for shale reservoir reconstruction. Shale inherently contains some initial micro-cracks, which significantly affect its strength and failure behavior. In this paper, a refined boundary multi-level parallel bonded grain-based model (multi-level PB-GBM) in Particle Flow Code (PFC2D) was developed, and the effect of inherent initial damage on shale strength and failure behavior was quantitatively investigated. The results showed that inherent initial damage significantly influences the failure pattern and mechanical properties of shale. The newly generated cracks of the initially damaged samples are significantly self-organized compared with those of the undamaged samples, indicating that the inherent initial damaged cracks induce the orientation and aggregation of micro-cracks. High initially damaged samples mainly demonstrate by splitting-shear coupled fracture as a result of the co-evolution of primary and secondary micro-cracks. Generally, rock strength gradually decreases as the initial damage increases. When the inherent initial damage within the sample is low, the rock strength is greatly influenced by confining pressure, whereas when the initial damage is high enough, the initial damage contributes more to the rock strength.