Liquid CO 2 phase transition blasting effect and optimization strategy under multi-hole arrangements

Liquid CO2 phase transition fracturing is a green coalbed methane recovery technique. The layout of empty holes in multi-hole blasting significantly influences the fracturing effectiveness, thereby affecting the gas extraction rate. However, systematic research on the crack propagation under different empty hole layouts remains insufficient. The study employed numerical simulation and field testing to investigate the limitations of conventional rectangular and plum-blossom-shaped borehole layouts under double-empty-hole conditions and further examined the damage pattern of cracks under various empty hole configurations. The results demonstrated that the incorporation of empty holes transformed the double-hole blasting from non-penetrating to fully penetrating. The plum-blossom-shaped layout showed better performance than the rectangular pattern due to its staggered arrangement, resulting in 0.79% and 7.25% increases in fracture ratio and area, respectively. However, blasting blind zones still existed in central region due to limited guidance by empty holes in non-horizontal directions. Fracture statistics and fractal dimension analysis identified the optimized layout with a double-empty-hole spacing of 120 cm as optimal solution, achieving 3.63% and 31.24% improvements in fracture ratio and area, respectively, compared to the conventional plum-blossom-shaped pattern. Field tests show that the optimized scheme effectively enhances CO2 fracturing effectiveness, increasing gas extraction flow by 12.37–16.31%.