Adsorption behavior of CO2/H2S mixtures in calcite slit nanopores for CO2 storage: An insight from molecular perspective

It is acknowledged that injecting CO₂ into oil reservoirs and saline aquifers for storage is a practical and affordable method for CO₂ sequestration. Most CO₂ produced from industrial exhaust contains impurity gases such as H₂S that might impact CO₂ sequestration due to competitive adsorption. This study makes a commendable effort to explore the adsorption behavior of CO₂/H₂S mixtures in calcite slit nanopores. Grand Canonical Monte Carlo (GCMC) simulation is employed to reveal the adsorption of CO₂, H₂S as well as their binary mixtures in calcite nanopores. Results show that the increase in pressure and temperature can promote and inhibit the adsorption capacity of CO₂ and H₂S in calcite nanopores, respectively. CO₂ exhibits stronger adsorption on calcite surface than H₂S. Electrostatic energy plays the dominating role in the adsorption behavior. Electrostatic energy accounts for 97.11% of the CO₂-calcite interaction energy and 56.33% of the H₂S-calcite interaction energy at 10 MPa and 323.15 K. The presence of H₂S inhibits the CO₂ adsorption in calcite nanopores due to competitive adsorption, and a higher mole fraction of H₂S leads to less CO₂ adsorption. The quantity of CO₂ adsorbed is lessened by approximately 33% when the mole fraction of H₂S reaches 0.25. CO₂ molecules preferentially occupy the regions near the pore wall and H₂S molecules tend to reside at the center of nanopore even when the molar ratio of CO₂ is low, indicating that CO₂ has an adsorption priority on the calcite surface over H₂S. In addition, moisture can weaken the adsorption of both CO₂ and H₂S, while CO₂ is more affected. More interestingly, we find that pure CO₂ is more suitable to be sequestrated in the shallower formations, i.e., 500–1500 m, whereas CO₂ with H₂S impurity should be settled in the deeper reservoirs.