Abstract:

Deep oil and gas resources have considerable development potential. However, reservoir fracturing is affected by high confining pressure. The fracture tip exhibits pronounced nonlinear mechanical behavior, which hinders hydraulic fracture propagation. To clarify the nonlinear fracture characteristics of rocks under confining pressure, three-point bending fracture experiments were conducted on standard single-edge notched beam specimens of yellow sandstone. The evolution of the fracture process zone (FPZ) was monitored using an acoustic emission system. The results show that rock exhibits significant nonlinear fracture characteristics under confining pressure, and that higher confining pressure prolongs the entire pre-peak evolution of the FPZ. At high confining pressures, the microscopic fracture mechanism in rock shifts from intergranular to transgranular fracture. This transition in the microscopic fracture mode is the primary reason for the smoother fracture surface and the increase in fracture energy. Based on the cohesive zone model (CZM), an analytical model was established to predict the FPZ length. As confining pressure increases, the FPZ length decreases, whereas the fracture energy increases. This indicates that confining pressure not only limits the extent of FPZ development but also significantly enhances the energy dissipation intensity per unit area within the zone. Macroscopically, this manifests as a requirement for a higher stress level to drive further FPZ development and fracture propagation. This phenomenon reflects the strengthening effect of confining pressure on the nonlinear fracturing behavior of rock. The research results provide theoretical support for stimulation and modification of deep reservoirs.

Key words: confining pressure, fracture mechanics, three-point bending experiment, nonlinear fracture, fracture process zone