A theoretical study of gas adsorption on α-quartz (001) 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 a-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 N-2), 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.