Influence of Bedding Orientation on Fracture Properties and Fracture Propagation Modes of Shale

The shale SCB samples of southern Sichuan basin with the angle θ of bedding and sample base being 0°, 30°, 45°, 60° and 90° are tested subjected to three-point bending loading. The axial load-displacement curves and fracture load were obtained, and the fracture toughness K_IC and fracture energy of shale were calculated. Combined with shale reservoir fracturing engineering, the influence of bedding orientation on fracture tortuosity of shale reservoir is discussed. The results show that: (1) As θ increases, the fracture toughness of shale samples decreases, and the average K_IC value decreases from 1.88 to 0.43 MPa·m^0.5. The bedding orientation has a significant control on the fracture toughness of shale. (2) The axial load-displacement curves of shale samples under three-point bending include defect compression, elastic compression deformation and rupture unloading stage at the initial loading stage. After the axial load increases to the critical fracture load, the shale SCB samples are unloaded rapidly, which reflects the obvious brittleness of shale. (3) According to the fracture mechanism of rock, it can be divided into tensile fracture and shearing fracture, and straight fracture and tortuous fracture according to the fracture tortuosity. When θ < 45°, the fracture propagation of shale is affected by the loading direction and bedding plane, and it is easy to form complex fracture network. On the contrary, a single straight fracture is formed along the direction of bedding. (4) There is a good linear correlation between fracture work and fracture toughness of shale, indicating that shale fracture performance can affect the selection of fracture propagation paths. (5) Selecting a large-angle intersecting bedding direction to implement perforation or pre-slicing, so that the fracturing fractures and propagation along this direction, can maximize the creation of a hydraulic fracture network and improve the efficiency of shale reservoir fracturing.