Thermal-Hydraulic-Mechanical-Damage-Coupled Investigation of CH4 Displacement by N2 and CO2 Injection in Low-Permeability Coal

Gas injection is an effective strategy for CBM recovery in low-permeability coal reservoirs. However, the coupled evolution of gas migration, coal deformation-damage behavior, and the relative contributions of replacement and sweeping mechanisms during different gas injection processes remain insufficiently understood. In this study, a THMD-coupled numerical model was developed and validated against experimental data to investigate methane displacement behavior in low-permeability coal under N2 and CO2 injection conditions with different injection pressures. To quantitatively distinguish the displacement mechanisms, the Replacement-Sweeping Consumption Ratio (RSCR) was proposed, defined as the ratio of the amount of injected gas consumed through competitive adsorption for CH4 replacement to that transported in the free-gas state for CH4 sweeping. The results indicate that both N2 and CO2 injection significantly enhance CH4 recovery compared with natural drainage. N2 and CO2 injection increased cumulative CH4 production by up to 113% and 97%, respectively, with N2 exhibiting higher recovery performance under identical injection conditions. The two displacement gases differ markedly in permeability evolution: N2 injection undergoes a stable-rapid decline-gradual recovery three-stage process and ultimately outperforms natural drainage, whereas CO2 injection yields a continuous permeability drop via strong adsorption-driven matrix swelling with scarce local damage. Mechanism analysis revealed that N2 displacement is dominated by rapid diffusion and sweeping effects, which continuously promote CH4 desorption and migration. By contrast, CO2 displacement is primarily controlled by competitive adsorption replacement. Accordingly, CO2 exhibited substantially higher RSCR values than N2, indicating a greater relative contribution of adsorption-induced replacement, whereas N2 relied more strongly on free-gas sweeping. The combined effects of slow migration and permeability deterioration fostered high-concentration CH4 accumulation zones, forming an inhibition mechanism of rapid replacement but slow migration. The findings provide mechanistic insights into gas selection and injection pressure optimization for enhanced methane recovery in low-permeability coal reservoirs.