Numerical Study of Optimal Injected Gas Mixture Proportions for Enhancing Coalbed Methane Recovery

A comprehensive thermo-hydro-mechanical numerical model for gas mixture-enhanced coalbed methane recovery was developed, in combination with coal deformation, competitive adsorption, ternary gas seepage, gas-water migration, and heat transfer. The model, implemented using COMSOL Multiphysics, investigates optimal gas injection proportions under varying conditions of injection temperature, initial water saturation, and initial permeability. The results demonstrate that higher injection temperatures and initial permeability, with lower initial water saturation, significantly enhance methane production. Specifically, at an injection temperature of 340 K and a permeability of 1.028 mD, optimal CO₂ concentrations are 50 and 70%, resulting in cumulative methane productions of 6.3 × 10⁶ and 7.2 × 10⁶ m³, respectively. In contrast, at an initial water saturation of 0.8, a CO₂ concentration of 30% proves to be the most effective, yielding a cumulative methane production of 5.8 × 10⁶ m³. These findings offer critical insights into optimizing the balance between CO₂ and N₂ proportions, thereby maximizing methane recovery while minimizing the risks of premature breakthrough and permeability reduction caused by coal matrix swelling.