Damage evolution law and permeability enhancement effect of coal subjected to liquid nitrogen fracturing in coalbed methane mining

Liquid nitrogen (LN2) fracturing has great potential to stimulate the cracking of low-permeability coal seams and thus promote the efficient development of coalbed methane (CBM) by leveraging its cryogenic (–196 °C) effects. To investigate the damage evolution law of coal during LN2 fracturing, a thermo–hydro–mechanical coupling damage model is developed that explicitly incorporates LN2-induced frost-heaving effects. Subsequent reservoir simulations quantified permeability enhancement in stimulated coal seams. A comparative analysis of hydraulic (H2O), nitrogen (N2), and LN2 fracturing revealed that LN2 fracturing achieved the greatest improvement in permeability, followed by N2 fracturing, with H2O fracturing exhibiting the lowest efficacy. LN2 fracturing outperforms conventional methods, with a 297% increase in microcrack density, a 32.9% greater fracture radius, and a 38.0% higher number of damaged units. Post-LN2 fracturing achieves a gas production rate (GPR) enhancement factor of 3.11. A LN2–N2 costimulation protocol is subsequently proposed based on economic feasibility. The scheme can be optimized to maximize the permeability enhancement of the reservoir by adjusting the LN2 injection time. Optimizing the LN2 duration increased the costimulation GPR enhancement factor to 5.285 with cumulative CBM production increasing by 75.9%. These findings provide critical insights for advancing CBM extraction efficiency in low-permeability reservoirs through cryogenic-mechanical synergies.