Development characteristics and quantitative analysis of shale fracture network under methane explosion load

The methane in-situ explosion fracturing technology is a revolutionary technology that uses methane gas obtained in-situ in shale reservoirs and artificially injected combustion promoters to form an effective fracture network for methane gas transport. The self-developed explosion system was used to obtain the characteristics of methane-oxygen detonation pressure curve. The development characteristics of the effective fracture network around the deep shale perforation channel under methane-oxygen detonation pressure load were studied by theoretical model and numerical simulation method. The results show that the stress distribution state of the rock mass is significantly impacted by the initial stress conditions, and a higher stress concentration occurs with a larger initial stress deviation. The average boost of methane-oxygen detonation pressure is 85 μs. Under the condition of isotropic initial stress, the fracture development is uniform and circular radial. When initial stress is anisotropic, the fracture tends to grow in the direction of higher initial stress, but the tendency degree is weaker than that of explosive fracturing. The area of the explosion shattering area can be effectively reduced by the explosive fracturing technology, which is more conducive to the formation of more and longer radial fractures. The increase of peak detonation pressure promotes the development of circumferential fracture at the end of radial fractures, making it more conducive to connecting natural fractures in the reservoir to construct complex fracture networks. Under methane-oxygen detonation load, the shale rupture (effective fracture) and shattering damage threshold D_s were selected as 0.15 and 0.70, respectively. The mathematical expression between the fracture development rate P_s and the damage threshold D_s was approximated as an exponential function. The effective fracture development rates under the E−10, E−40 and E−80 conditions were 18.92%, 14.11% and 8.85%, respectively. The increase of the effective fracture development rate and the formation of a highly complex fracture network can be significantly promoted by increasing the peak pressure of detonation.