CO2 Replacing CH4 Behaviors under Sub- and Supercritical Conditions

Injecting CO2 into deep coal seams is key to successful CO2 sequestration and enhancing coalbed methane production; however, various underground conditions influence the process. In this study, experiments were conducted to determine optimum conditions for replacing CH4 with CO2 injection at different temperatures (20, 30, and 40 degrees C), pressures (2, 3, and 4 MPa), and supercritical conditions (35, 45, and 55 degrees C). The results show that the proportion of adsorbed CH4 decreased with decreasing desorption pressure but increased with increasing temperature. Low temperatures were relatively more conducive to replacing CO2 with CH4. With an increase in the original CH4 adsorption equilibrium pressure, there was an increase in CH4 adsorbed proportion but a decrease in CO2 adsorbed concentration. Desorption rates showed that the replacement effect was superior under low original reservoir pressures. Under supercritical conditions, the maximum Gibbs adsorption volume for CO2 was obtained, and the absolute adsorption volume conformed to classical Langmuir curves. During the replacement process, the percentage of adsorbed CO2 increased with increasing temperature, while the concentration of CH4 decreased. The CH4 desorption rate was higher, and the displacement effect was superior when temperature and pressure were close to the critical condition within the supercritical range.