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

In CO2-enhanced coalbed methane recovery (i.e., CO2-ECBM), the selection of CO2 injection timing is pivotal for maximizing methane (i.e., CH4) recovery while preventing premature CO2 breakthrough. This study constructs fully coupled thermal–hydrological–mechanical–chemical (i.e., THMC) models to assess the impacts of different CO2 injection timings on the CO2-ECBM process, using the Panyi Coal Mine (Huainan, China) as a case study. Primary recovery and CO2-ECBM recovery are compared, and the evolution of reservoir permeability under different CH4 extraction strategies is emphasized. Results show that CO2 injection markedly increases CH4 recovery, elevating the peak daily CH4 production from 167.73 m3/d to 921.35 m3/d. The optimal CO2 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 CH4 production and CO2 storage volume display a linear correlation with the injection timing. Initial reservoir water saturation restricts CH4/CO2 migration, while CH4 desorption and elevated injection pressure improve permeability contrasting with the permeability reduction induced by CO2 adsorption and associated thermal release. Permeability evolution is driven by strains from gas adsorption and desorption, thermal expansion, volumetric changes, and gas pressure. CO2 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 CO2 adsorption must first offset the shrinkage strain from CH4 desorption before the porosity declines. These insights contribute to optimizing CO2-ECBM operations and enhancing CO2 sequestration efficiency.