Effect of Carbon Dioxide’s Injection Concentration on the Diffusion Coefficient of Methane

The process of displacing methane (CH4) using carbon dioxide (CO2) in coal seams involves the adsorption and diffusion of a binary gas mixture in coal. Diffusion serves as a bridge linking adsorption, desorption, and seepage of the two gases, thereby affecting the production of coal-bed methane (CBM) and the injection of CO2. In the present paper, the adsorption, desorption, and diffusion behaviors of both gases were investigated during the displacement of CH4 using CO2 in coal seams. The results indicate that the CO2 injection process to enhance CH4 drainage from coal seams can be divided into three stages: a stage where both replacement and displacement effects occur simultaneously; a stage dominated by replacement effects; and a steady state. Based on the fitting relationships of the diffusion amounts of CH4 and CO2 gases with experimental time, computational models for the diffusion coefficients of both gases were established. The calculation results indicate that the diffusion coefficients for both gases decrease rapidly with an increase in experimental time. Furthermore, as the injection concentration of CO2 increases, the diffusion coefficient of CH4 shows an upward trend, while the diffusion coefficient of CO2 exhibits a downward trend.