Injection and Recovery Effects during CO 2 -ECBM Process under Different CO 2 Injection Times and Its Influence on Permeability

In CO 2 -enhanced coalbed methane recovery (i.e., CO 2 -ECBM), the selection of CO 2 injection timing is pivotal for maximizing methane (i.e., CH 4 ) recovery while preventing premature CO 2 breakthrough. This study constructs fully coupled thermal–hydrological–mechanical–chemical (i.e., THMC) models to assess the impacts of different CO 2 injection timings on the CO 2 -ECBM process, using the Panyi Coal Mine (Huainan, China) as a case study. Primary recovery and CO 2 -ECBM recovery are compared, and the evolution of reservoir permeability under different CH 4 extraction strategies is emphasized. Results show that CO 2 injection markedly increases CH 4 recovery, elevating the peak daily CH 4 production from 167.73 m 3 /d to 921.35 m 3 /d. The optimal CO 2 injection initiation occurs after the drainage and depressurization phases, which circumvents inefficient early injection and yields higher injection rates compared to immediate injection. Both cumulative CH 4 production and CO 2 storage volume display a linear correlation with the injection timing. Initial reservoir water saturation restricts CH 4 /CO 2 migration, while CH 4 desorption and elevated injection pressure improve permeability contrasting with the permeability reduction induced by CO 2 adsorption and associated thermal release. Permeability evolution is driven by strains from gas adsorption and desorption, thermal expansion, volumetric changes, and gas pressure. CO 2 injection shortens the pore pressure-dominated phase and intensifies the role of adsorption/desorption strains in porosity variation. For delayed injection, the swelling strain from CO 2 adsorption must first offset the shrinkage strain from CH 4 desorption before the porosity declines. These insights contribute to optimizing CO 2 -ECBM operations and enhancing CO 2 sequestration efficiency.