A theoretical study of gas adsorption on α-quartz (0 0 1) for CO₂ enhanced natural gas recovery

The adsorption of gas molecules (CO₂, CH₄, H₂O, H₂S and N₂) on the “dense” (0 0 1) surface of α-quartz has been investigated by means of Density Functional Theory (DFT) for the molecular characterization of Enhanced Gas Recovery (EGR) processes by CO₂ injection. Several configurations have been studied for the different compounds. Overall, the five molecules are weakly physisorbed on the surface; no charge transfer takes place and no new bonds are formed. The potential surface is quite flat, with all the different values falling into a range of less than 0.15 eV. Consequently, all the molecules can easily move from a configuration to the other. As for the order of preference, hydrogen sulfide adsorbs the most with a maximum adsorption energy of −0.23 eV, followed by carbon dioxide with −0.21 eV. For the other three molecules (H₂O, CH₄ and N₂), there is no net sequence but values lower than that of CO₂ are inferred. Coverage analysis of CO₂ and CH₄ show that carbon dioxide can replace methane on the surface improving its extraction, but high CO₂ concentration could weaken its adsorption, hindering the whole process. These results could prove to be useful in designing the EGR process based on CO₂ injection.