Shale Gas Stimulation Technology: Large-Scale Triaxial Physical Simulation Tests on Longmaxi Formation Shale

Large-scale triaxial physical simulation tests were conducted on Longmaxi Formation shale to identify the fracture network formation mechanism. The extension of the hydraulic fractures was analyzed using a red tracer in the fracturing fluid. The fracturing curve and its relationship with the crack propagation law were determined, and the initiation and expansion of the hydraulic fractures were deduced. The factors that influence the hydraulic fracture propagation law are discussed and the following conclusions were obtained. The hydraulic fractures initiated from the water outlet. During expansion, the hydraulic fractures were subjected to fracture arrest, divarication, penetration or turning, and eventually evolved into a fracture network. The criterion of crack initiation and expansion shows good agreement with the experimental results. The fracture toughness and tensile strength of the shale bedding plane were 49.39% and 31.40% of the shale matrix, respectively, which implies that the bedding planes were sufficiently weak to permit the fractures to more easily steer and divaricate. A fracture network therefore only forms if there are sufficient bedding planes with mechanical properties weaker than those of the shale matrix.