CO2 injection in coal: Advantages and influences of temperature and pressure

Coalbed methane (CBM) recovery and CO2 storage have always been important topics of study for clean energy utilization and environmental governance. To enhance CBM recovery, CO2/N2 injection is applied in coal mining, especially CO2 injection enhanced coalbed methane (CO2-ECBM). CO2 injection not only improves the production of CBM but also helps to reduce the greenhouse effect. As there are differences between CO2 injection and N2 injection, the advantages of CO2 should be studied in depth. The adsorbed methane accounts for more than 80% of total methane, and the amount of variation adsorbed methane is the key to CO2/N2 injection. In this paper, the desorption rate of adsorbed methane, the replacement ratio and the residual mixed gas content are discussed to evaluate the effect of CO2/N2 injection. After CO2/N2 injection, the desorption rate of adsorbed methane could be enhanced. The results also show that CO2 injection is better than N2 injection for enhanced coalbed methane (ECBM) recovery; however, the residual mixed gas content could be increased after CO2 injection. Moreover, anthracite (QC) could be more suitable for CO2 injection because of the value of replacement ratio. The influences of temperature and injection pressure are also empirically studied. Increasing both temperature and pressure could not significantly enhance the desorption rate of adsorbed methane significant. Increasing the temperature could reduce the residual mixed gas content; however, it is difficult to induce ECBM recovery with increasing temperatures. Increasing the injection pressure could cause a slight increase of the desorption rate of the adsorbed methane; however, higher injection pressure corresponds to greater economic cost. Thus, the most suitable injection pressure is twice as much as the initial pressure of methane.