Gas drainage enhancement by multi-cycle deep displacement of binary gases

Conventional gas drainage in low-permeability coal seams is limited by low extraction efficiency and a rapid decline in gas concentration. Injecting CO₂ or N₂ to displace CH₄ can improve methane desorption and recovery, but the changing roles of pressure-driven drainage and desorption enhancement at different methane-content stages remain unclear. In this study, multi-cycle displacement and adsorption experiments were conducted on gas-bearing anthracite particles using CO₂ and N₂. Gas chromatography and the SRK real-gas equation of state were used to analyze tail-gas composition, CH₄ release behavior, and the occurrence state of the injected gases. The results show that after twelve displacement cycles, the CH₄ volume fraction in the tail gas decreased from 69.8% to 11.3% in the CO₂ group and from 64.5% to 13.4% in the N₂ group, with both groups showing clear staged declines. Because N₂ has weak adsorption affinity, it mainly remains in a free state and helps maintain relatively high pore pressure, resulting in stronger pressure-driven drainage. As a result, the final residual CH₄ proportion in the coal samples decreased to 52.0%, and the initial CH₄ release volume was 2.7 times that of the CO₂ group. In contrast, CO₂ is more strongly adsorbed, allowing it to enter and remain in coal matrix micropores. This led to a higher residual CH₄ proportion of 63.2% but also increased the total adsorbed gas content in the coal. Based on these findings, a staged gas-injection strategy is proposed. N₂ injection should be used in the early stage to accelerate gas drainage and maintain coal permeability, while CO₂ injection should be applied in the later stage to displace micropore-adsorbed CH₄ more deeply and support geological CO₂ storage.