Confinement-Driven fracture behavior transition in sandstone

Hydraulic fracturing experiments were performed on hollow double-wing crack specimens of tight sandstone from the Xujiahe Formation under simulated in-situ confining pressures corresponding to burial depths of 0, 3263, 4078, 4894 and 5710 m (0–140 MPa). The sandstone consistently developed straight main fractures along pre-existing fissures across this entire pressure range, reflecting its homogeneous microstructure and uniform interparticle stress transfer network—behavior that contrasts sharply with the bedding-controlled fracture modes observed in shale. Fracture pressure increased linearly with confining pressure, whereas net pressure rose quadratically, reaching five times atmospheric pressure at 140 MPa and thus explaining the steep rise in energy consumption during deep fracturing. Furthermore, sandstone’s fracture toughness and fracture energy exhibited far greater pressure sensitivity (400 % and 2394 % increases, respectively) compared to shale. Microscopic analysis revealed that elevated confining pressure enhances intergranular contact forces, driving a shift from transgranular to intergranular microcracking that governs the evolution of macroscopic fracture parameters. These results provide essential data and optimization strategies for designing more efficient fracturing treatments in deep tight-sandstone gas reservoirs.