Permeability-driven optimization of carbon dioxide injection timing in enhanced coalbed methane recovery

Reducing greenhouse gas emissions and improving unconventional gas recovery are pressing challenges worldwide. This study investigates the leading role of coal permeability in CO2-enhanced coalbed methane recovery (CO2-ECBM) using an improved thermo-hydro-mechanical (THM) coupling model. The model is validated and applied to the simulation of CO2-ECBM process under varying permeability. The direct positive relationship between both cumulative CH4 production and cumulative CO2 injection with coal permeability, and operating duration of CO2-ECBM is negatively correlated with coal permeability. The delay in the injection start time will extend the operating duration of CO2-ECBM and overcome the problem of early CO2 breakthrough. An increase in both delayed start time for injection and coal permeability progressively boosts CH4 production, while diminishing CO2 sequestration. The optimal start timing for CO2 injection is contingent upon primary objectives, such as CO2 sequestration or CH4 production. For the primary constraint of CO2 sequestration, the optimal starting time should be before the peak gas production. For the primary constraint of CH4 production, the optimal starting time should be after the peak gas production. In analyzed instances, the optimal timing for CO2 injection exhibits an inverse correlation with coal permeability. The results provide practical insights for CO2 injection optimization and field applications.