Determining the Starting Time of CO₂ Injection and Its Implications for the CO₂-ECBM Process

Determining the starting time of the CO₂ injection is the key factor affecting the effect of CO₂ injection and CH₄ production. This study uses the Panyidong mine in the Huainan mining area as an example. First, fully coupled mathematical models are established. Second, the evolution law of the reservoir parameters under different starting times of CO₂ injection is visualized. Then, the starting time of the CO₂ injection is determined, and the transformation process of CH₄ displaced by CO₂ is discussed. Finally, the connotation of the continuous fluid process during the CO₂-ECBM process is analyzed, and constructive suggestions for CO₂ injection are presented. The results show that during the CO₂-ECBM process, the reservoir pressure and gas content greatly change near the CO₂ injection well, and gradually decrease near the CH₄ production well. The competitive adsorption of the gas is exothermic, and the reservoir temperature gradually increases. The competitive adsorption of gas causes matrix expansion, which results in a decrease in permeability. The CH₄ production rate can be increased by about 2 times, and the evolution law of the CO₂ injection rate shows little difference. If the purpose of CO₂ injection is to store CO₂, the start time of CO₂ injection may be set as the 2000th day. The CO₂-ECBM process can be divided into three stages to characterize the importance of displacement and replacement effects in each stage. Determining the starting time of CO₂ injection and giving more desorption time to CH₄ is conducive to the expansion of pore and fracture spaces, and then gives more competitive adsorption time to gas. This study can promote the engineering implementation of the CO₂-ECBM technology.