Adsorption Characteristics and Thermodynamics of CH4, CO2, and N2 on Shale at Different Temperatures

To study shale adsorption characteristics during the process of CO2 and N2 enhanced shale gas recovery, a high-pressure gas adsorption instrument was used to transfer different adsorbed gases (CH4, CO2, and N2) in the adsorption test on shale at different temperature conditions. The Langmuir–Freundlich (L–F) model considering the effect of pressure and temperature was introduced. Further, a modified model for calculating the isosteric enthalpy of adsorption (ΔH) was established. The results showed that the adsorption capacity of CO2 on the shale surface was the strongest and that of N2 was the weakest. The L–F model can better characterize the adsorption behavior of shale under the combination of temperature and pressure. The modified model for calculating ΔH could better characterize the relationship between the ΔH and adsorption amounts. As the adsorbed amount increased, the ΔH at different temperatures showed a trend of decreasing slowly and then decreasing rapidly; the ΔH of CO2 was the largest, and that of N2 was smallest. As the temperature increased, the ΔH revealed a downward trend. With an increase in CO2 pressure and in the mole fraction of CO2 free phase, the adsorption selectivity factor of CO2 over CH4 increased gradually, while the adsorption selectivity factor of N2 over CH4 decreased with the increase in N2 pressure and the decrease in mole fraction of N2 free phase. Temperature will inhibit the preferential adsorption of shale to CO2 and N2.