Influence Mechanism of N ₂ –CO ₂ Mixtures on the CH ₄ Displacement Behavior from Different Coal Samples under Stress Condition

Recently, CH4 displacement from coal seams through flue gas injection has attracted increasing attention. However, the influence mechanism of different gases on the CH4 displacement behavior remains a matter of debate. This paper systematically studied the basic properties of coal, their adsorption behaviors toward gases, permeability, and the CH4 displacement behaviors through different gas injections under triaxial stress. The results showed that the CH4 displacement quantities through pure N2, N2–CO2 mixtures (85%:15% and 95%:5%), and pure CO2 injection from the tested bituminous coal were 798.3, 825.7, 868.7, and 875 mL at 2 MPa triaxial stress and 620.7, 711.4, 750.1, and 704.5 mL at 6 MPa triaxial stress, respectively. This indicated that pure CO2 injection achieved good displacement efficiency for the coal sample with a well-developed mesoporous structure under lower triaxial stress; however, but under higher triaxial stress, N2–CO2 mixture injection achieved greater displacement efficiency owing to the favorable permeability of N2 and the promotion of adsorbed CH4 desorption by coexisting CO2. Furthermore, as for lignite and anthracite with poor mesoporous structures, the displacement efficiency by pure CO2 injection decreased, respectively, from 705.2 and 1378.7 mL at 2 MPa triaxial stress to 513.2 and 783.8 mL at 6 MPa triaxial stress because of their poor migration ability, even being worse than that through pure N2 injection (527.4 and 1136.8 mL at 6 MPa triaxial stress). These results can provide a theoretical basis for selecting the appropriate gas when applying gas injection technology to different coal seams and confirm the feasibility and necessity of the N2–CO2 mixture injection.