Fracture toughness and failure mode of Wufeng−Longmaxi shale under explosion load impact

The explosive fracturing is a waterless fracturing technology used to stimulate shale gas reservoirs for improving gas production. The investigation of the fracture toughness and failure modes of shale under explosive loads is a key for the industrial application of methane-explosion fracturing technology. Fracture toughness is widely known as a critical factor for evaluating the effectiveness of fracturing technique. The quantitative characterization of the complexity of crack networks could provide a way to quantify the failure mode of shale after explosive impact. Some shale samples were collected from the Wufeng−Longmaxi Formation in Changning, Sichuan Province, China. Experimental studies have been carried out with a self-made methane-explosion fracturing setup and the SHPB system. According to the rock fracture theory, a method has been determined to calculate the dynamic fracture toughness of shale for investigating the evolution of dynamic fracture toughness and damage patterns of shale samples. The results indicate that the dynamic fracture toughness of the samples increases linearly with the increased bedding angle under different explosion loading rates. It is much larger than the static fracture toughness. The maximum fracture toughness of the 90° bedding shale is about 4.98 MPa·m^1/2. As the loading rate increases, the dynamic fracture toughness increases significantly. When a loading rate of 157.57 GPa·m^1/2/s is used for the shale with a 0° bedding angle, the corresponding fracture toughness is about 4.93 MPa·m^1/2, while the fracture toughness for a loading rate of 35.43 GPa·m^1/2/s is 1.72 MPa·m^1/2, reducing about 2.9 times. Meanwhile, due to the influence of shale bedding plane, the initial cracking direction is tended to deflect away from, extend along and cut through the bedding plane. It could cause an increase in the complexity of the crack network. All initial crack angles of the tested samples are located within the range of 0°−70.51°, which indicates a mixed I−II fracture type according to the fracture theory. As the explosion pressure increases from 20−25 MPa to 66−71 MPa, the distribution range of initial cracking angles increases by 178%. When the explosion pressure passes over 50 MPa, more cracks and connected cracking patterns are generated. The appearance of shear-slip deformation in brittle shales can create tree-like cracks, which can easily create fracture networks in shale. The laboratory results prove that the explosive impact has an advantage in producing complex fracture networks in shale reservoirs, which provides a basic support to the theoretical work of explosive fracturing.