Coupling Mechanism of THMC Fields in Crushed Soft Coal with Low Permeability after CO₂ Injection and Its Application in CO₂-ECBM Technology

During the CO2-ECBM process in crushed soft coal with low permeability (CSLP-Coal), the analysis of the fully coupled mechanism of the thermo-hydro-mechanical-chemical (THMC) fields is conducive to engineering promotion of CO2-ECBM technology. In this study, the CSLP-Coal in Huainan and Huaibei coalfields was taken as the research object; the fully coupled mathematical models of THMC fields were established; the effects of injection pressure, temperature, and initial permeability were studied; the evolution connotation of reservoir permeability was discussed; and the reaction mechanism of the chemical field was analyzed. The evaluation system of the CO2-ECBM project in CSLP-Coal was further established. The results show that the mathematical models of THMC fields are fully coupled during the CO2-ECBM process. Injection pressure, temperature, and initial permeability have great influence on the CO2-ECBM technology. The variation of reservoir permeability is mainly influenced by the effective stress effect and competitive adsorption effect of CH4/CO2. The formation of a geochemical reaction system with CH4/CO2-H2O-coal is conducive to the engineering popularization of the CO2-ECBM technology. Based on the three indexes, the evaluation system for the practical effectiveness of CO2-ECBM engineering in CSLP-Coal in Huainan and Huaibei coalfields can be constructed. This study has important theoretical and practical significance for improving the recovery efficiency of CH4 and enhancing the ability of CO2 storage in CSLP-Coal.