Influence Mechanism of N₂-CO₂ Mixtures on the CH₄ Displacement Behavior from Different Coal Samples under Stress...

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.