Crack Propagation Characteristics in Shale under Cyclic In Situ Methane Detonation Impact Fracturing
Chengzheng Cai, Jiacheng Li, Cheng Zhai, B. Wang, Junjie Xue, Zhen Dong, Zhengchao Zhu
Abstract
Methane in situ detonation fracturing enhances reservoir permeability through controlled detonation, which generates transient high pressure to create complex fracture networks. A numerical model based on PFC2D was developed to simulate this process, incorporating a double-exponential function to represent detonation pressure evolution. The results indicate that crack density increases nonlinearly with peak pressure; for instance, a 37.4% rise is observed as pressure increases from 60 to 160 MPa. For pressure rise time, when it is increased from 0.5 to 1.5 ms, short cracks near the wellbore are reduced by 23.4% while the propagation of long cracks is facilitated. When the pressure decay time is extended from 4 to 6 ms, the number of long cracks is enhanced by 66.7% and the total crack count increases by 19.4%. Fracture growth is suppressed by higher in situ stress, with a 12.6% decrease in crack density observed when stress increases from 8 to 16 MPa. An incremental pressure mode with increments of at least 20 MPa/cycle outperforms the decremental mode, resulting in a 26.7% increase in long cracks after three cycles and an expansion of the crushed zone after four cycles. Based on these findings, a cyclic fracturing strategy of detonation cycles limited to three or fewer and increments of at least 20 MPa/cycle is proposed. This study provides theoretical guidance for optimizing detonation fracturing parameters in shale reservoirs.