Mechanism of breaking and fracture expansion of liquid CO2 phase change jet fracturing in low-permeability coal seam

Liquid CO2 phase change jet fracturing technology has been widely used and studied as an effective low per-meability coal seam enhanced gas extraction method. However,due to the lack of research on the mechanism of coal breaking and crack formation,the further development and application are affected. In this study,based on the theoreti-cal basis of thermal engineering,elastic mechanics and fracture mechanics,a liquid CO2 phase change gas jet pressure model was established. The mechanical mechanism of liquid CO2 phase change jet coal breaking and fracture expan-sion under in-situ stress conditions was analyzed. Then,the PFC2D discrete element particle flow analysis software was used to carry out a numerical simulation. The distribution characteristics of rock-cracking fractures by liquid CO2 phase change jet under different in-situ stress and jet pressure conditions were analyzed. Finally,on the basis of above re-search,the field test was carried out in Baijiao Coal Mine. The research shows that the technology can effectively im-prove the gas drainage efficiency of low permeability coal seam.