Sensitivity Analysis of Geological–Engineering Parameters and Injection Optimization for CO2-ECBM in Coal Seams Based on Numerical Simulation

CO2-enhanced coalbed methane recovery and storage (CO2-ECBM) is a promising approach for improving methane recovery and increasing CO2 storage in low-permeability coal seams. However, limited injectivity and insufficient criteria for injection parameter optimization remain major constraints. Taking the No. 11-2 coal seam of the Zhangji Coal Mine in the Huainan mining area as the study object, this study established a thermo–hydro–mechanical coupled model that considers CO2/CH4 competitive adsorption, matrix diffusion, fracture seepage, gas–water two-phase flow, coal deformation, and porosity–permeability evolution. A 10-year numerical simulation was conducted to evaluate the effects of initial porosity, initial permeability, elastic modulus, CO2 injection pressure, and injection scheme on CO2-ECBM performance. The comprehensive sensitivity results show that initial porosity, CO2 injection pressure, and initial permeability are the dominant controlling factors, whereas elastic modulus has a relatively weak influence. Initial porosity mainly determines reservoir storage space and CO2 sequestration potential; permeability controls pressure propagation and gas migration; and injection pressure directly affects CH4 displacement intensity, CO2 storage capacity, and reservoir safety margin. Multi-objective evaluation indicates that the injection pressure should be controlled within 8.0–9.0 MPa, with 8.0–8.5 MPa recommended for long-term stable operation. When the engineering objective prioritizes CO2 storage or CH4 recovery and sufficient safety margin is confirmed, the injection pressure may be increased to approximately 9.0 MPa. Continuous constant-pressure injection favors cumulative CH4 production and CO2 storage, whereas stepwise pressurization reduces early pressure disturbance and improves later-stage injectivity. Therefore, an injection strategy combining early-stage stepwise pressurization with middle- and late-stage constant-pressure injection is recommended. These results provide a reference for injection parameter optimization in similar low-permeability coal reservoirs.