A theoretical study of gas adsorption on calcite for CO2 enhanced natural gas recovery

Abstract

The adsorption of gas molecules (CO2, CH4, H2O, H2S and N2) on calcite (104) has been studied by means of Density Functional Theory, in order to investigate the molecular characterization of Enhanced Gas Recovery (EGR) processes by CO2 injection. Results show that the geometries of both the surface and gas were not affected by the adsorption, highlighting the physisorption nature of the process. Water and hydrogen sulphide adsorb the strongest due to hydrogen bonding, while carbon dioxide follows next. Regarding the other gases, nitrogen adsorption is stronger in average when compared to methane, with a certain degree of complexity. In general, all the configurations' energies can be found in a range of less than 0.4 eV for each adsorbate. Nevertheless, the larger affinity of CO2 in comparison with methane confirms the suitability of CO2 injection for methane release in EGR operations. The stronger water adsorption compared to carbon dioxide (-0.91 eV versus -0.38 eV) gives a quantitative estimate of the impact of water as impurity. Further investigations need to address this issue, since this aspect could dramatically hinder the application of the whole technique. Coverage studies on methane and carbon dioxide further highlights the affinity of the latter to the carbonate surface.