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.