Stress wave attenuation and damage patterns of shale under methane/oxygen explosion

Methane in-situ explosion fracturing of shale is a revolutionary technology that utilizes the methane analyzed from shale reservoirs and the input combustion aids to carry out explosion reactions, thereby forming a three-dimensional fracture network in in-situ fracturing of shale reservoirs. This study conducted explosion experiments to investigate the variation patterns of key parameters of explosion (i.e. explosion pressure, explosion speed, rise time of peak pressure). We obtained the attenuation patterns of stress wave in shale reservoir through dimensionless analysis. We used the finite element software ANSYS/LS-DYNA to establish models of different working conditions for analyzing the number of cracks and damage evolution patterns of shale caused by explosion. Results show that: 1) The pressure during the methane/oxygen explosion is about 30 times the initial pressure. The rise time of peak pressure is 85 μs. 2) Peak stress of the stress wave and the specific distance in the shale reservoir under the explosion load are exponentially related. 3) The increase in loading rate could form multiple cracks around the explosion hole. 4) Compared with open hole completion, the presence of casing reduces the pressure near the wellbore wall, causing type Ⅰ cracks to extend along the initial fracture direction. The perforation length after extension increasesby 47 % compared to the original length.