Investigation of hydraulic fracturing mechanism by using a coupled continuous-discontinuous hydromechanical model

Fluid-induced fracture nucleation, propagation, and interaction are the essentials for a better understanding of the hydraulic fracturing process in unconventional reservoirs. In this study, a coupled continuous-discontinuous hydromechanical model was established to investigate the hydraulic fracturing propagation under varied conditions. The interactions between induced fractures and natural fractures are investigated and discussed by a series of hydraulic fracturing simulations. Also considered are the influences of bedding joints, in-situ stress ratios, and fluid injection rates on the patterns of hydraulic fractures and the stress field. It was found that hydraulic fracture propagation is controlled by both in-situ stress state and strength anisotropy of the reservoir rock. The simulations indicate that an increase in the fluid injection rate is favorable to the formation of a complex fracture network. More hydraulic fractures were developed when fracture fluids were injected into rock specimens with a faster pressurization rate than a quasi-static pressurization rate. However, higher fluid injection rates could result in higher breakdown pressures for fracture initiation and propagation. In addition, hydraulic fractures tend to extend along the direction of the maximum principal stress or approach this preferred path. Bedding joints are preferred locations and orientations for fracture initiation and propagation in laminated shale reservoirs.