Displacement Effect of CO₂/N₂-ECBM Process with Different Water Saturations Based on THMC Coupling

Understanding the impact of gas injection and water saturation on displacement efficiency is essential for facilitating the application of CO2/N2-ECBM. This research focuses on the Panyidong mine located in the Huainan mining district. Initially, comprehensive thermo-hydro-mechanical-chemical mathematical models were developed. Subsequently, the geological geometries were established. Then, the influence of the injection strategies and water saturation on permeability and displacement efficiency was examined. Furthermore, the underlying principles of the continuous fluid process during the CO2/N2-ECBM process, the evolution of reservoir permeability under various gas injection methods, and the impact of water saturation on gas migration were explored. Finally, the new technology of CO2 injection in the CBM production well group was proposed. The results show that the progression of CO2/N2 primarily follows macroscopic fractures, transitioning to microscopic fractures, and ultimately reaching matrix pores. The CO2/N2 injection method, the distance between injection and production wells, reservoir permeability, and water saturation directly influence the gas migration radius. Changes in reservoir permeability are significantly affected by reservoir effective stresses, the competitive adsorption of gases, and water saturation. Injection of N2 leads to a gradual increase in reservoir permeability, while CO2 injection results in a gradual decrease. The adsorption/desorption strain, as well as the injection and production rates and volumes of CO2, N2, and CH4, is substantially impacted by the gas injection strategy, type, ratio, and water saturation. The technique for efficient CBM flooding through gas injection and extraction encompasses 6 stages. The research findings hold significant theoretical importance for managing and reversing the declining trend of permeability, as well as reducing the adverse effects of water saturation in the exploration of CO2/N2-ECBM-related engineering practice.