Experimental and numerical simulation study on the influence mechanism of flue gas displacing coalbed methane under stress conditions

Recently, the application of flue gas (N2 and CO2 mixture) in coalbed methane (CBM) replacement has been widely studied. However, the effect of CO2 and N2 mixture on the displacement process is still unclear. Based on the actual composition of flue gas, this paper systematically studied the effects of gas composition, injection pressures and triaxial stresses on the CH4 displacement process. The results showed that with the triaxial stress increasing the CH4 displacement quantities by pure N2, 5 % CO2 mixture (95 % N2), 15 % CO2 mixture (85 % N2), pure CO2 injection decreased, but when the triaxial stress was 2 MPa the CH4 displacement quantities by pure CO2 injection were the largest (1131 mL), when they were 3.5 and 5 MPa the CH4 displacement quantities by 15 % CO2 mixture injection were the largest (1038 and 890 mL). These results demonstrate that when the permeability of coal seam is slightly great, the great adsorption characteristics of gas on coal sample plays a more crucial role, while the permeability is greatly reduced, both the great permeability and adsorption characteristics of gas determine the CH4 displacement efficiency, which can provide the great theoretical guidance for the gas selection in practical applications of displacement technology.