Study on the evolution mechanism of permeability and displacement efficiency in CO₂-ECBM in low-permeability coal...

Unmineable coal seams and residual coal resources in closed mines provide ideal carriers for CO₂ sequestration. However, the evolution mechanisms of permeability and displacement efficiency during CO₂-ECBM in low-permeability coal seams remain unclear, which restricts the engineering application efficiency of this technology. In this study, the No. 15 coal seam of the Lutaishan Coal Mine was taken as the research object. A fully coupled thermal-hydraulic-mechanical model of CO₂-ECBM was constructed and validated using COMSOL Multiphysics software, and a quantitative evaluation method for key influencing factors was proposed based on coal deformation effects. The results indicate that injection pressure is the dominant factor affecting permeability, and its influence range expands with increasing pressure and time. In the near-injection well region, coal matrix swelling induced by CO₂ adsorption is stronger than matrix shrinkage caused by CH₄ desorption, leading to a decrease in coal reservoir permeability; the higher the injection pressure, the more significant the permeability reduction. In the near-production well region, CH₄ desorption causes matrix shrinkage, resulting in increased permeability. The intermediate zone remains relatively stable. The influence of injection temperature is weak: in the near-injection well area, thermal expansion reduces permeability; in the adjacent zone, reduced gas adsorption capacity leads to a recovery in permeability; the peripheral zone is stable, and the near-production well area remains dominated by CH₄ desorption. Increasing injection pressure accelerates displacement, promoting CH₄ recovery and CO₂ storage, while increasing temperature slightly reduces recovery efficiency but has a minor impact. Under all scenarios, CO₂ storage exceeds CH₄ production. This study aims to provide guidance for injection strategies of CO₂-ECBM in low-permeability coal seams, and its findings help optimize displacement parameters, thereby improving engineering implementation efficiency.