Experimental study on CO2 adsorption and displacement of methane incoals: An NMR relaxation method

The essence of CO2 injection for enhancing coalbed methane recovery (CO2-ECBM) is the transformation of adsorbed methane to free phased due to the CO2-CH4 competitive adsorption characterizations in coals. The investigation of the realtime dynamic evolution of native adsorbed and rejected nonadsorbed methane during the process of CO2-ECBM has a dual desirable benefit both in theoretical and engineering research. The authors first determined the nuclear magmatic resonance (NMR) calibration coefficient of free methane based on a series of free methane NMR experiments, without coals in the sample cell. After that, two contrasting long flame coal and anthracite collected from the Southern Junggar Basin and Qinshui Basin, respectively, were applied for exploring the characterizations of methane desorption and CO2 displacement under different CO2 injection pressures or different experimental temperatures using the selfdesigned NMRbased CO2 displace methane equipment. The NMR calibration coefficients of adsorbed methane were determined and further used for quantitatively characterizing the dynamic process of CO2 adsorption and displacement in coals. Results show that the NMR transverse relaxation time (T2) distributions of methane adsorption in coals exhibit three peaks, labeled from left to right as P1, P2, and P3, corresponding to the NMR relaxation of adsorbed methane, free methane in interparticles, and free methane occurs at headspace of the sample cell. With the increase of CO2 injection pressure, the NMR amplitude of adsorbed methane presents a significant decrease trend. Meanwhile, the NMR amplitude of free methane (P2 and P3 peak) decreases with the increase of experimental temperature. In the process of methane desorption and CO2 displacement, the maximum methane adsorption capacity following increase CO2 injection pressure occurs in two stages: a rapid initial decline under low gas pressure stages, followed by a slower tail in high gas pressure stages. Additionally, the CO2 displacement capacity decreases with the increase of coal reservoir temperature.