Mechanism and field application of CO₂ phase‑change fracturing for gas extraction in low‑permeability coal seams

To address the inefficient gas extraction problem of high‑gas low‑permeability coal seams in Guizhou Province, this study investigates the permeability‑enhancing mechanism and field performance of liquid CO₂ phase‑change fracturing, taking local anthracite and bituminous coal as research objects. Relying on the special thermophysical properties of liquid CO₂, phase‑change shock loading creates multi‑zone fracturing in coal mass; meanwhile, CO₂ can displace adsorbed methane to improve seam permeability. Energy equivalence calculation indicates that 1.4 kg liquid CO₂ releases phase‑transition energy equivalent to 243 g TNT, proving good safety and controllability of this technology. Field tests show that gas production of fractured boreholes follows negative‑exponential attenuation with fitting R² of 0.85‑0.99. The effective gas drainage radius expands rapidly within 10‑30 days after fracturing and finally increases by 1.9‑2.5 times compared with original coal seams. Under consistent fracturing construction parameters, despite inherent disparities in baseline geological properties between the two test coal seams, bituminous coal achieves better pressure‑relief and permeability‑enhancement performance than anthracite. Further analysis reveals the coal‑rank‑dependent differential response mechanism: dense‑matrix anthracite dissipates most impact energy near boreholes and limits fracture propagation, whereas well‑cleated bituminous coal develops long‑range connected fracture networks for efficient gas seepage. This study clarifies the lithology‑controlled differential mechanism of liquid CO₂ phase‑change fracturing and forms targeted gas‑control technology for Guizhou coal mines. The findings provide theoretical support and engineering references for efficient coalbed methane exploitation in high‑gas low‑permeability coal seams of Southwest China.