要約

According to the conventional theory of hydraulic fracturing, breakdown pressure decreases with an increase in initial pore pressure. However, in the practice of hydraulic fracturing for gassy coal seams, the higher the gas pressure, the higher the pumping water pressure required to inject into the coal seam. The initial pore gas pressure leads to an increase in water injection pressure. How does the initial pore pressure affect the hydraulic fracturing of unsaturated rock and its breakdown pressure? Experiments on hydraulic fracturing for unsaturated cement mortar samples (300 mm × 300 mm × 300 mm) with five degrees of initial pore pressures were conducted. The influencing law of initial pore pressure on breakdown pressure of borehole radial fracture in hydraulic fracturing was studied with a large-sized true triaxial hydraulic fracturing experimental system. The experimental results show that the hydraulic fracturing of rock is primarily controlled by the stress field, initial pore pressure, and water injection pressure. A critical pore pressure gradient exists for the initiation and propagation of hydraulic fracture. When the pore pressure gradient reaches a critical point, the fracture in rock will occur. With an increase in initial pore pressure, the breakdown pressure of rock increases. The subsequent propagation pressure of hydraulic fracture also increases with an increase in initial pore pressure.