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

The adsorption of gas molecules (CO2, CH4, H2O, H2S and N-2) on calcite (104) has been studied by means of Density Functional Theory for molecular characterization of Enhanced Gas Recovery (EGR) CO2 injection. Results show that the geometry of both the surface and gas were not affected by the adsorption, highlighting the physisorption nature of the process. On average, water adsorbs the strongest followed by hydrogen sulfide, because of hydrogen bonding, and carbon dioxide comes next. Finally, 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 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 the issue of water impurity, since this aspect could dramatically hinder the application of the whole technique. Coverage studies of methane and carbon dioxide further highlight the affinity of the latter to the carbonate surface.