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

N₂/CO₂ flooding coalbed methane storage technology is an effective technology to improve coalbed methane recovery and CO₂ 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 N₂, CO₂ and CH₄ ternary gas displacement adsorption is established, and the reliability of the model is verified. The results show that CO₂ is the main reason to strengthen CH₄ production. Simple CO₂ injection has a significant adsorption and expansion effect on coal body, and high-pressure N₂ gas is needed to drive CO₂ to migrate to the far field of coal body. However, too high N₂ gas injection ratio is more likely to lead to production well breakthrough, which reduces the recovery rate of CH₄. The higher the proportion of CO₂ injection, the greater the total adsorption heat released, and the larger the range of coal temperature rise. When the proportion of CO₂ 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 CO₂ 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 CO₂ injection ratio, the farther away from the injection well, the lower the permeability of the coal body. However, with the increase of CO₂ injection ratio, the permeability of the coal body shows a trend of increasing first and then decreasing. When the CO₂ 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 N₂ to drive CO₂ to migrate to the far field coal body. Considering the recovery rate of CH₄ and the storage effect of CO₂, 60% CO₂ + 40% N₂ is the best injection ratio. This study provides a theoretical basis for optimizing the N₂/CO₂ mixed injection ratio to balance CH₄ recovery and CO₂ storage in coalbed methane reservoirs, supporting sustainable energy development and greenhouse gas emission reduction.