Competitive sorption and diffusion of methane and carbon dioxide mixture in Carboniferous-Permian anthracite of south Qinshui Basin, China

A comprehensive understanding of the migration and competitive adsorption of methane and carbon dioxide in coal is crucial for the injection optimization design and injection displacement evaluation of the CO2 enhanced coalbed methane recovery (ECBM) project. In this paper, adsorption-desorption tests of three gaseous mixtures (75% CH4 + 25% CO2, 50% CH4 + 50% CO2, and 25% CH4 + 75% CO2) and pure CH4 and CO2 were performed on the anthracite from the southern Qinshui Basin. The results show that (1) there was an apparent hysteresis in sorption isotherms, and the size of hysteresis loops was related to the proportion of CO2 in the mixture. There was also a non-negligible error in using the extended Langmuir model to predict mixture sorption. (2) Separation factor (SCO2-CH4) for the sorption of CH4 and CO2 mixture varied with the equilibrium pressure and the equilibrium concentration of gas components. Its value was usually between 4 and 20. During the adsorption process, the SCO2-CH4 at low pressure was generally greater than that at high pressure. Moreover, SCO2-CH4 in the desorption process was generally higher than the adsorption process. (3) Bidisperse diffusion model could better describe the diffusion process of the mixture. In general, the macropore effective diffusivity varied as a power function with pressure. Under the same pressure, the macropore effective diffusivity increased with the increasing CO2 content. Further, the macropore effective diffusivity in the desorption process was larger than that in the adsorption process. The micropore effective diffusivity and the ratio of macropore uptake to micropore uptake had no apparent relationship with the gas component concentration and pressure. The results will help to comprehensively understand the sorption processes of the CH4 and CO2 mixture and provide basic numerical simulation data for the CO2-ECBM project in the southern Qinshui Basin.