Fracture characteristics and pore structure evolution law of shale under different methane explosive fracturing loads

Methane in-situ explosive fracturing technology is a revolutionary waterless fracturing technology. The fracturing characteristics and pore structure modification mechanisms of shale in near-wellbore region under varied explosive loads remain unclear, and some key parameters are unable to be determined for technological applications. In this study, a methane explosion tube with the maximum pressure resistance of 100 MPa was built and the explosion fracturing experiments were conducted on the ϕ55 mm×55 mm sieve tube completion shale samples. The methane explosion pressure-time curve in the pipeline was collected during the experiment. The morphology evolution of fractures with increasing loads was characterized by collecting fracture specimens. Large-scale fracturing experiments on the ϕ1000 mm×1000 mm shale analogs elucidated the controlling factors of reservoir fracturing by explosive loading. Mercury intrusion porosimetry revealed the evolutionary patterns of shale pore-fracture structures. The main findings are as follows: ① Increasing the initial pressure of methene-oxygen mixture gas can significantly improve peak blasting loads and pressure rise multiples; ② With the increase of the explosive loading, the near-wellbore region transits from a single dominant fracture to multiple radial fractures that initiate collaborated cracking. However, excessive loads may cause crushed zones near the wellbore, with the optimal fracturing pressure being 71 MPa in this study; ③ Deflagration shock waves and detonation product gas wedge effects are the controlling mechanisms for fracture initiation and propagation. Deflagration fracturing helps to construct stimulated zones with small crushed regions but extensive fracture networks; ④ The volumes of shale macropores and microcracks remarkably increase with rising deflagration loads, exhibiting a decreasing then increasing trend for micropores, jointly controlled by dynamic impacts and thermal effects. This study elucidates the control mechanisms of deflagration loads on reservoir fracturing behaviors and pore-fracture structure modifications through laboratory and large-scale experiments.