Thermodynamics analysis of the adsorption of CH4 and CO2 on montmorillonite

The greater adsorption ability of shale to CO2 offers an alternative method to enhance shale gas recovery, where a potential solution for CO2 sequestration into a deep shale reservoir can also be realized. Clay minerals are the important storage space for shale gas. In this work, the isotherm adsorption curves of CH4 and CO2 on montmorillonite at 288 K, 308 K and 328 K were obtained using gravimetric apparatus. The selectivity factor of CO2 over CH4 (S-CO2/CH4) and Henry's constants (K-H) of CH4 and CO2 were calculated. More importantly, the thermodynamics parameters of CH4 and CO2, including Gibbs free energy change (Delta G), surface potential (Omega), isosteric heat of adsorption (Q(st)) and entropy change (Delta S), were thoroughly analysed. The results indicate that all S-CO2/CH4 values are obviously greater than one, and injecting CO2 into a clay-rich shale reservoir to enhance the shale gas recovery is feasible. CH4 has smaller K-H than CO2, which shows that CH4 has a weaker affinity on the montmorillonite surface. Low temperature helps to enlarge K-H and promote the adsorption of CH4 and CO2. Increasing the pressure causes the increase in Delta G and Omega of CH4 and CO2. The higher Delta G and Omega of CO2 compared to CH4 reveal that the CO2 adsorption on montmorillonite is more favourable and spontaneous. When the loading increases, the Q(st) values of CH4 and CO2 reduce. CH4 has a smaller Q(st) than CO2, which suggests the stronger interaction of CO2 with montmorillonite. CH4 and CO2 have decreasing Delta S with the loading. Under larger surface coverage conditions, the disorder of the adsorbed molecules is larger, and most of the injected molecules are trapped as a free state in the pore space. The higher Delta S of CO2 illustrates that CO2 molecules constitute a much more stable rearrangement than CH4 molecules. The temperature has a slight effect on the Delta S of CH4 and CO2.