Simulation study on nonlinear permeability characteristics of coalbed methane mining driven by mixed N2/CO2 pressure

N 2 /CO 2 flooding coalbed methane storage technology is an effective technology to improve coalbed methane recovery and CO 2 geological storage at the same time. In this paper, considering the influence factors such as heat conduction and coal deformation, the characterization volume element method is used to describe the adsorption, desorption and seepage of ternary gas in coal and the related changes caused by coal matrix. The coupling model of N 2 , CO 2 and CH 4 ternary gas displacement adsorption is established, and the reliability of the model is verified. The results show that CO 2 is the main reason to strengthen CH 4 production. Simple CO 2 injection has a significant adsorption and expansion effect on coal body, and high-pressure N 2 gas is needed to drive CO 2 to migrate to the far field of coal body. However, too high N 2 gas injection ratio is more likely to lead to production well breakthrough, which reduces the recovery rate of CH 4 . The higher the proportion of CO 2 injection, the greater the total adsorption heat released, and the larger the range of coal temperature rise. When the proportion of CO 2 injection is too low, the temperature near the injection well first decreases slightly and then increases rapidly due to the combined effects of gas expansion cooling and CO 2 adsorption heat release. The permeability ratio on the route from the injection well to the production well increases rapidly and then decreases slowly. The lower the CO 2 injection ratio, the farther away from the injection well, the lower the permeability of the coal body. However, with the increase of CO 2 injection ratio, the permeability of the coal body shows a trend of increasing first and then decreasing. When the CO 2 injection ratio increases to 60%, the change trend of permeability ratio at different injection time is highly similar. After that, it is necessary to increase the injection ratio of N 2 to drive CO 2 to migrate to the far field coal body. Considering the recovery rate of CH 4 and the storage effect of CO 2 , 60% CO 2 + 40% N 2 is the best injection ratio. This study provides a theoretical basis for optimizing the N 2 /CO 2 mixed injection ratio to balance CH 4 recovery and CO 2 storage in coalbed methane reservoirs, supporting sustainable energy development and greenhouse gas emission reduction.