Adsorption Characteristics of CO2/CH4/H2S Mixtures in Calcite Nanopores with the Implications for CO2 Sequestration

Injecting CO₂ into reservoirs for storage and enhanced oil recovery (EOR) is a practical and cost-effective strategy for reducing carbon emissions. Commonly, CO₂-rich industrial waste gas is used as the CO₂ source, whereas contaminants such as H₂S may severely impact carbon storage and EOR via competitive adsorption. Hence, the adsorption behavior of CH₄, CO₂, and H₂S in calcite (CaCO₃) micropores and the impact of H₂S on CO₂ sequestration and methane recovery are specifically investigated. The Grand Canonical Monte Carlo (GCMC) simulations were applied to study the adsorption characteristics of pure CO₂, CH₄, and H₂S, and their multicomponent mixtures were also investigated in CaCO₃ nanopores to reveal the impact of H₂S on CO₂ storage. The effects of pressure (0–20 MPa), temperature (293.15–383.15 K), pore width, buried depth, and gas mole fraction on the adsorption behaviors are simulated. Molecular dynamics (MD) simulations were performed to explore the diffusion characteristics of the three gases and their mixes. The amount of adsorbed CH₄, CO₂, and H₂S enhances with rising pressure and declines with rising temperature. The order of adsorption quantity in CaCO₃ nanopores is H₂S > CO₂ > CH₄ based on the adsorption isotherm. At 10 MPa and 323.15 K, the interaction energies of CaCO₃ with CO₂, H₂S, and CH₄ are −2166.40 kcal/mol, −2076.93 kcal/mol, and −174.57 kcal/mol, respectively, which implies that the order of adsorption strength between the three gases and CaCO₃ is CO₂ > H₂S > CH₄. The CH₄-CaCO₃ and H₂S-CaCO₃ interaction energies are determined by van der Waals energy, whereas electrostatic energy predominates in the CO₂-CaCO₃ system. The adsorption loading of CH₄ and CO₂ are lowered by approximately 59.47% and 24.82% when the mole fraction of H₂S is 20% at 323.15 K, reflecting the weakening of CH₄ and CO₂ adsorption by H₂S due to competitive adsorption. The diffusivities of three pure gases in CaCO₃ nanopore are listed in the following order: CH₄ > H₂S ≈ CO₂. The presence of H₂S in the ternary mixtures will limit diffusion and outflow of the system and each single gas, with CH₄ being the gas most affected by H₂S. Concerning carbon storage in CaCO₃ nanopores, the CO₂/CH₄ binary mixture is suitable for burial in shallower formations (around 1000 m) to maximize the storage amount, while the CO₂/CH₄/H₂S ternary mixture should be buried as deep as possible to minimize the adverse effects of H₂S. The effects of H₂S on CO₂ sequestration and CH₄ recovery in CaCO₃ nanopores are clarified, which provides theoretical assistance for CO₂ storage and EOR projects in carbonate formation.