Influence of liquid CO2 phase transition blasting on hydraulic fracturing in combined fracturing conditions

Enhancing the permeability of coal seams is crucial for improving coalbed methane extraction. To better understand the effects of liquid CO2 phase transition blasting on hydraulic fracturing within gas–liquid combined permeability enhancement technology, a multifunctional fracturing tube was developed, capable of achieving combined gas–liquid fracturing. Experiments were conducted on dense artificial samples under simulated in situ stress conditions. The results indicate that the fracture morphology and distribution induced by liquid CO2 phase transition blasting are strongly influenced by the number and arrangement of fracture holes at the energy release end of the multifunctional fracturing tube. Specifically, a single-hole energy release structure promotes the formation of directional long fractures. The hydraulic fracturing process can be divided into four distinct stages: filling, cyclic energy storage and fracturing, drainage, and pump stopping. Notably, the initiation and propagation of hydraulic fractures primarily occur during the cyclic energy storage and fracturing stage, where a strong correlation between injection pressure and acoustic emission (AE) energy signals was observed. The relationship between breakdown pressure and in situ stress shows that a reduction in maximum principal stress (σ1) increases the sample's breakdown pressure, while intermediate (σ2) and minimum (σ3) principal stresses exhibit the opposite effect. After liquid CO2 phase transition blasting, the influence of blasting-induced fractures on hydraulic fractures is reflected in three key aspects: shortening the duration of the cyclic energy storage and fracturing stage, reducing breakdown pressure, and altering the fracture propagation direction. These findings have important guiding significance for improving the mining efficiency of coalbed methane in low permeability coal seams. In addition, the gas–liquid combined fracturing technology proposed in this study provides a new perspective and inspiration for the study of the interaction between gas–liquid two-phase fluids in coal seams in the future.