Research on CO₂-enhanced coalbed methane extraction effect based on different reservoir parameters and injection timing

The CO₂ injection enhanced coalbed methane extraction technology (CO₂-ECBM) can not only significantly improve the efficiency of coalbed methane extraction, but also reduce greenhouse gas emissions, which is crucial for alleviating the global energy crisis and the greenhouse effect. To realistically restore the CH₄ recovery situation in coal seams, a thermal-hydro-mechanical coupling model was constructed, considering coal deformation, gas-water transport, and thermal expansion. Utilizing numerical simulation, the coalbed methane production performance of conventional extraction and CO₂-enhanced extraction was compared and studied. The impacts of injection temperature, injection pressure, and initial water saturation on CO₂-ECBM engineering were analyzed, and the optimal starting time for CO₂ injection was determined. The results show that the enhancement factor of 3^# coal seam in Qinshui Basin is 1.37 after 4000 days of extraction, which indicates the feasibility of CO₂-enhanced coalbed methane recovery; due to the competitive effects of pore pressure, gas adsorption/desorption and thermoelastic expansion strain, the permeability shows a decreasing - increasing - decreasing trend; increasing the injection temperature and pressure will raise the temperature of the coal reservoir, promote the production of CH₄ and the storage of CO₂, and increase the decline in permeability; the greater the initial water saturation, the less favorable it is for gas production and storage, and the smaller the decline in permeability; Delaying the injection time helps to increase the recovery rate of CH₄ and the well closure threshold, but it will have a negative impact on the average production rate of CH₄; when the start time of CO₂ injection was 2 500 days, the average output rate was 1 142.10 m³/d, and the economic benefits of this demonstration project reached the optimum. The research results have enriched the stimulation mechanism of CO₂-ECBM under different reservoir parameters and provided a new theoretical perspective for the optimization of its injection timing.