Numerical Investigation on the Hydraulic Fracture Evolution of Jointed Shale

Joints are a common structure of heterogeneous shale rock masses, and in situ stress is the environment in which heterogeneous rock masses can be found. The existence of joint plane and confining pressure difference influences the physical properties of shale and propagation of fractures. In this study, jointed shale specimens were simulated under different confining pressures to explore the failure patterns and fracture propagation behavior of hydraulic fracturing. Different from the common research of hydraulic fracturing on signal parallel joint rock mass, the simulations in this study considered three points (parallel joint, multi-dip angle joint, and no-joint points). The effects of the single-dip angle joint, multi-dip angle joint, and confining pressure difference on the hydraulic fracture evolution and stress evolution of the jointed shale were studied comprehensively. The confining pressure difference coefficient proposed in this study was used to accurately describe the confining pressure difference. Results indicate that the larger is the confining pressure difference, the stronger is the control of the maximum principal stress on fracture evolution; by contrast, the smaller is the confining pressure difference, the stronger is the control of the joint plane on fracture evolution. Under the same confining pressure difference, the hydraulic fracture propagates more easily along the small dip angle joint plane. As the value of the confining pressure difference coefficient moves closer to zero, the hydraulic fracture propagates randomly, the tensile stress region around the fracture tip widens, and the joint planes fractured by tensile increase. This study can offer valuable guidance to the design of unconventional reservoir reconstruction.