Experimental Study on the Kinetics of Adsorption of CO₂ and CH₄ in Gas-Bearing Shale Reservoirs

Abstract

Knowledge of the kinetics of adsorption of CO2 and CH4 on shales is essential for the optimization of the technique of enhanced gas recovery by CO2 injection. In this paper, the experiments of adsorption kinetics of CO2 and CH4 on marine Wufeng (WF) shale and continental Yanchang (YC) shale at 318, 338, and 358 K over the pressure range of 3.0–8.0 MPa were conducted by a volumetric method. Results show that the adsorption processes of CH4 and CO2 on the shales can be described as an initial rapid adsorption stage and a slow sorption stage. The unipore model can simulate the kinetics data of CO2 and CH4 well. The diffusion coefficient (D) of gas increases with the increase of pressure. High temperatures will help to improve the diffusivities of CO2 and CH4. The order of magnitude of D of CO2 and CH4 on the two samples is 10–11 m2/s. The diffusion activation energies (Ea) of CO2 and CH4 on YC shale are 3.37 and 10.15 kJ/mol, respectively, and Ea of CO2 and CH4 on WF shale are 1.33 and 3.56 kJ/mol, respectively. The Ea value of CH4 greater than that of CO2 indicates that CH4 diffusion in shale formation is more difficult and needs to jump a higher potential barrier. There is a positive correlation of pressure with the adsorption rates of CO2 and CH4. As the temperature enhances, the gas adsorption rate reduces. The ratios of D of CO2, CH4, and N2 are 1.42:1.15:1.00 on YC shale and 1.72:1.38:1.00 on WF shale. The adsorption rates of CO2, CH4, and N2 on the shales exhibit a decreasing trend.