Thermodynamics analysis of the adsorption of CH4 and CO2 on montmorillonite

The greater adsorption ability of shale to CO 2 offers an alternative method to enhance shale gas recovery, where a potential solution for CO 2 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 CH 4 and CO 2 on montmorillonite at 288 K, 308 K and 328 K were obtained using gravimetric apparatus. The selectivity factor of CO 2 over CH 4 ( S CO2/CH4 ) and Henry's constants ( K H ) of CH 4 and CO 2 were calculated. More importantly, the thermodynamics parameters of CH 4 and CO 2 , including Gibbs free energy change (Δ G ), surface potential ( Ω ), isosteric heat of adsorption ( Q st ) and entropy change (Δ S ), were thoroughly analysed. The results indicate that all S CO2/CH4 values are obviously greater than one, and injecting CO 2 into a clay-rich shale reservoir to enhance the shale gas recovery is feasible. CH 4 has smaller K H than CO 2 , which shows that CH 4 has a weaker affinity on the montmorillonite surface. Low temperature helps to enlarge K H and promote the adsorption of CH 4 and CO 2 . Increasing the pressure causes the increase in Δ G and Ω of CH 4 and CO 2 . The higher Δ G and Ω of CO 2 compared to CH 4 reveal that the CO 2 adsorption on montmorillonite is more favourable and spontaneous. When the loading increases, the Q st values of CH 4 and CO 2 reduce. CH 4 has a smaller Q st than CO 2 , which suggests the stronger interaction of CO 2 with montmorillonite. CH 4 and CO 2 have decreasing Δ 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 ΔS of CO 2 illustrates that CO 2 molecules constitute a much more stable rearrangement than CH 4 molecules. The temperature has a slight effect on the Δ S of CH 4 and CO 2 .