Study on Mechanical Behavior of Weak Bedding of Deep Shale Based on Discrete Element Simulation

Bedding influences fracture mechanisms and mechanical properties of shale. To investigate the mechanical behavior of deep shale with different bedding angles, conventional triaxial tests and direct shear tests were carried out on specimens of shale with high quartz content, then the failure process of shale was inversed based on discrete element method (DEM). The results show that the shale has high strength and obvious brittleness characteristics, whose rock strength decreases firstly and then increases with the increasing bedding angle. When the loading direction is parallel to the bedding direction shale mainly produces fractures along bedding and all have residual strength (110 ~ 150 MPa). Instead, when their angles are at right angles, shale mainly produces penetrating cracks and appearance of residual strength needs higher confining pressure (30 MPa). The difference between them is caused by anisotropy of grain displacement. There are a lot of tensile microcracks with high angle (70° ~ 90°). According to the DEM simulation based on direct shear, when the shear direction is not parallel to bedding, the shale produced serrated fractures, which is seriously worn and has a large amount of particle slip. On the contrary, it is flat and the particle slip is small along the bedding. In addition, the higher bedding density shale has, the lower the cohesion and shear strength of rock it has. Understanding the mechanical behaviour of deep shale with different bedding angles can provide basic guidance for wellbore instability control and fracturing optimization in deep shale reservoirs.