Numerical simulation of enhanced permeability in coal seams by liquid CO₂ phase-transition fracturing

To investigate the fracture evolution in coal seams during liquid CO₂ phase-transition fracturing and to clarify the dominant mechanisms of permeability enhancement under phase-transition blasting loads, this study establishes a control model for liquid CO₂ phase-transition permeability enhancement. Combined with the geological and stress conditions of the Daning Coal Mine, numerical simulations of phase-transition blasting were conducted using the LS-DYNA finite element software, and the damage evolution of coal around the borehole, the stress distribution characteristics, and the crack propagation laws were analyzed. The results show that liquid CO₂ phase-transition blasting produces a significant borehole enlargement effect, forming distinct crushed and fractured zones around the borehole, with the borehole diameter increasing by 196%. The propagation direction of cracks around the borehole is significantly controlled by the deviatoric stress coefficient; as the deviatoric stress coefficient increases, cracks tend to propagate toward the direction of the maximum principal stress. The in-situ stress level inhibits the propagation distance of blast-induced cracks, but has little effect on the distribution of the crushed zone. The initiation load mainly affects the extent of the crushed zone around the borehole. Increasing the initiation pressure can further improve permeability, but due to the limitation of the total energy of phase-transition CO₂, the increase in crack propagation range is limited. Liquid CO₂ phase-transition fracturing can significantly improve the fracture structure of coal seams, and its permeability enhancement effect is synergistically controlled by in-situ stress and initiation parameters. These findings can provide a theoretical basis for the design and construction parameter optimization of liquid CO₂ phase-transition blasting for permeability enhancement in deep coal seams.