Experimental Investigation of Methane Explosion-Induced Fracture Propagation and Load Characteristics within Reservoir Perforation

Methane in situ explosion fracturing (MISEF) technology constructs complex fractures by igniting methane adsorbed from unconventional gas reservoirs (UCGRs). The effects of fracture propagation and explosion load characteristics of MISEF technology under high-pressure within perforation remain crucial areas of investigation. In this study, a high-pressure MISEF experimental setup is established to conduct fracturing experiments by igniting methane-oxygen mixtures at initial pressures of 0.9–2.1 MPa. Fracturing experiments are conducted on coal-like materials and shale samples. Pressure–time (p–t) curves at 3 perforation locations are monitored. The explosion loads are numerically applied to the perforation in the numerical model to simulate the fracture propagation behavior under in situ stress. The results reveal that with increasing explosive load, the crushed zone radius, the fracture numbers, average tortuosity, mass of fracturing fragments, and the fractal dimension all increase. Compared with coal-like materials, shale samples generate simpler fractures. Microscopic analysis unveiled a shift in fracture mode with rising explosion load, along with a significant presence of iron oxide nanoparticles on fracture surfaces. The explosion loads exhibit distinct reflective amplification effects and oscillatory attenuation characteristics. With increasing initial pressure, the explosion generated higher overpressure, balanced pressure, and basic frequency. Peak pressures, loading rate and oscillation frequencies are significantly influenced by the position of the perforation. A mathematical expression for the p–t curve, incorporating oscillation friction, has been developed. This expression accurately matches the experimental results. Numerical simulations show that the pulsed oscillation characteristics of methane explosion loads benefit both the repeated initiation and propagation of fractures, as well as overcoming the negative influence of in situ stress on fracture propagation.