Experimental Study on the Displacement Difference of CH₄ in Coal by N₂ and Air Under Different Injection Pressures

Coal seam gas injection is an effective way to improve coalbed methane recovery. In this study, the seepage and diffusion of CH 4 in the displacement process were investigated by physical experiments when N 2 and air were injected at different injection pressures. The key parameters analyzed include CH 4 concentration and flow rate, injection duration, cumulative CH 4 displacement rate, and injection–production ratio. By analyzing the changes of the data, it is found that both gas injection methods show that the higher the gas injection pressure, the better the replacement effect, and the overall effect of N 2 injection is better. However, with the increase of gas injection pressure, the disadvantage of air is gradually weakened, and sometimes even better than N 2 . Specifically, with the increase of gas injection pressure, the difference of gas injection duration between the two is decreasing, the CH 4 flow rate reaches the peak faster, the replacement efficiency gradually increases, the injection–production ratio is smaller, and the energy consumption is lower. Therefore, considering the cost of gas injection, air can be considered a viable gas source for the gas injection technology in low‐permeability and coal seams.