Fracture Evolution of Stress-Constrained Bedding Coal under Explosive Loads

In situ methane explosion fracturing (MISEF) technology generates multiple radial fractures around the wellbore by detonating coalbed methane (CBM), thereby enhancing the permeability of coal seams. To reveal the fracture propagation mechanism in bedded coal under the combined loading of in situ stress and methane explosive load, this study developed an explosive fracturing experimental system with a pseudo-triaxial stress coal holder. Bedding coal samples are subjected to the hoop stress of 12 MPa and the axial stress of 8 MPa. CH4–O2 mixture gas (volume ratio = 1:2) at initial pressures (P0) of 0.6–2.4 MPa are detonated to generate different explosive loads. μ-CT scanning, 3D digital core reconstruction, and LS-DYNA numerical simulation method are employed to analyze the regulatory effects of the explosive load and bedding on fracture propagation. The results reveal that explosive loads exhibit oscillatory attenuation behavior. Elevating P0 increases the explosion overpressure from 10.269 to 83.821 MPa, the pressure rise rate from 27.864 to 559.965 MPa/ms, and the maximum temperature from 308.1 to 667.3 °C. Bedding samples developed 3–5 radial fractures near the borehole. With the 2D fracture rate, fracture volume, and fractal dimension peak at the overpressure of 73.824 MPa, the influence of bedding diminishes. Increasing the explosive load initially increases fracture density and complexity and then shifts to enlarging fracture apertures and pulverized zones. Permeability simulations demonstrate exponential increases with explosive load, with vertical bedding fractures surpassing parallel fractures in permeability. Numerical analysis confirms that bedding fractures first initiate, leading to the vertical concentration of tensile stress, while higher explosive loads significantly raise the effective stress and increase radial fractures. Based on these findings, multibranch well methane explosion fracturing is proposed to enhance the fracture range while maintaining wellbore stability, addressing limitations of dynamic impact methods. This research provides theoretical support for applying MISEF to improve gas extraction efficiency in CBM reservoirs.