Mechanisms of Methane Displacement by CO2/N2 Injection in Tectonic Coal under Different Gas-Driving Modes

During underground gas injection in coal seams to displace coalbed methane, the injected gas spread toward coal fractures and macropores under pressure difference while also diffusing on micropore surfaces relying on concentration difference. This paper investigates how CH4 is displaced in tectonic coal under various gas-driving modes. The CH4 displacement tests were conducted by injecting isobaric and high-pressure CO2/N-2 into an anthracite sample at initial adsorption pressures of 0.50-2.50 MPa. The results show that after isobaric diffusion, the CH4 displacing amount was influenced by CH4 partial pressure change and multi-gas competitive adsorption. The CH4 desorption quantity increased with initial adsorption pressure but the CH4 desorption rate and the displacing efficiency decreased gradually. Comparatively, the system's total pressure decreased after CO2 diffusion equilibrium whereas it increased after N-2 diffusion equilibrium. When injecting high-pressure CO2/N-2, the adsorbed CH4 is primarily driven by multi-gas competitive adsorption, while CH4 partial pressure change inhibits its desorption. The total pressure would be enlarged after injection especially in a high-pressure N-2 injection system. The CH4 desorption rate by CO2/N-2 injection differs by approximately 20% between the two gas-driving modes, and the displacing effect by CO2 injection is generally more significant than that by N-2 injection due to CO2 holding a vital interaction with the anthracite molecule.