Molecular Insights on Competitive Adsorption Characteristics of CO2/CH4/H2S Multicomponent Mixtures in Kaolinite Nanopores

Summary The sequestration of CO2 in shale formations has dual benefits of reducing carbon emissions and enhanced oil recovery (EOR). CO2 purified from industrial waste gas is the main gas source for CO2 injection into oil reservoirs. Impurity gases in industrial waste gas, such as H2S, may impair carbon storage and gas recovery via competitive adsorption. Consequently, it is worthwhile to evaluate the competitive adsorption characteristics of CO2, CH4, and H2S, and their multicomponent mixtures within kaolinite nanopores, and the implications on carbon sequestration and CH4 recovery. Drawing upon the kaolinite nanopores model, the impact of various surfaces on the adsorption characteristics of CO2, CH4, H2S, and their mixes was examined using the grand canonical Monte Carlo (GCMC) method. Various factors, including temperature (293.15–383.15 K), pressure (0–20 MPa), pore width, and gas mole fraction, are investigated in the adsorption behaviors of the mixtures in the nano-confined space. The quantity of three molecules adsorbed enhances as the pressure grows and decreases with rising temperature. The rank of the absolute adsorbed amount is H2S > CO2 >CH4. The interaction energies of kaolinite with CO2, CH4, and H2S at 20 MPa and 293.15 K are –1,421.1138,–479.0825, and −2,528.245 kcal/mol, respectively, and the van der Waals energy is the main component of the interaction energy. Compared with hydrophobic pores, the loading of H2S, CO2, and CH4 in hydrophilic pores under 323.15 K and 20 MPa is 22.76%, 13.17%, and 11.57% greater, respectively. Three molecules show better adsorption stability on hydrophilic walls. The adsorption capacity of three molecules in binary/ternary mixtures is H2S>CO2>CH4. Gas components are crucial factors affecting the adsorption capacity of each component in multicomponent mixtures. In CO2/H2S and CH4/H2S binary mixtures, the adsorption of CO2 and CH4 is negatively impacted by H2S, and the adsorbed CO2 and CH4 are reduced by roughly 59.66% and 87.11%, respectively, at 25% (molar fraction) H2S. In CO2/H2S/CH4 ternary mixtures, CO2 and H2S exhibit strong competitive adsorption in the vicinity of the pore wall, whereas CH4 is predominantly distributed in the central region of the pore. An increase in H2S content leads to a transition of the three molecules toward lower energy states, facilitating their adsorption in a more stable configuration within the kaolinite nanopores. The nanoscale mechanism of gas adsorption in the clay mineral was disclosed from a molecular perspective, which theoretically supports the application of CO2 storage and EOR projects in shale reservoirs.