Surface chemical insights into CO2, N2, H2O and CH4 adsorption on manganese carbonate for flue-gas-assisted shale gas recovery

Abstract

Shale gas reservoirs represent important energy resources, and the injection of unseparated flue gas has been proposed as a potential strategy for coupling geological carbon storage with enhanced shale gas recovery. In this study, density functional theory calculations were employed to systematically investigate facet-dependent adsorption behaviors and underlying surface chemistry of CO2, N2, H2O, and CH4 on two representative MnCO3 crystal facets, (1 0 4) and (1 1 0). The gas–solid interfacial interactions were elucidated through adsorption configurations, adsorption energies, charge density redistribution, and electronic structure characteristics. The results show that CO2 and N2 exhibit substantially stronger interactions with MnCO3 than CH4, particularly at exposed Mn sites on the (1 1 0) surface, accompanied by pronounced charge redistribution and orbital hybridization. The (1 1 0) surface also exhibits lower stability (smaller absolute cohesive energy: −1.09 vs −1.51) and correspondingly higher surface reactivity, consistent with its stronger gas–surface interactions. Thermodynamic analysis further indicates that CO2, N2, and H2O maintain more favorable adsorption tendencies over a wider temperature range than CH4. These results reveal a pronounced facet-dependent adsorption preference for flue-gas components over CH4 at the atomistic level. Based on independent single-molecule adsorption calculations, these findings establish a molecular-level framework for understanding the selective interaction of flue-gas constituents with MnCO3 surfaces and their potential role in CH4 displacement. They further underscore the critical role of surface structure in regulating gas–mineral interactions and provide a mechanistic foundation for future multicomponent and multiscale studies of flue-gas-assisted shale gas recovery under realistic reservoir conditions.