Effect mechanism of wettability on CO2 replacement brine in nanopores

Abstract

The mechanism of using CO2 to replace brine in nanopores is still unclear, and the influence of the wettability of pore walls on the interfacial effect needs to be improved. In this study, the effect of wettability in tight sandstone reservoirs on CO2 replacement brine in nanopores was probed through molecular dynamics simulations. The results showed that the replacement front of CO2 molecules was meniscus shaped in hydrophilic and amphipathic systems, while it advanced as a whole in lipophilic systems. When CO2 replaced brine in nanopores, the interaction energies of the H2O and CO2 molecules near the pore walls with the pore walls were much greater than those of molecules in the centers of the pore channels·H2O molecules had strong interactions with the hydroxyl groups on the pore walls but weak interactions with the methyl groups on the pore walls, while CO2 molecules had strong interactions with both hydroxyl and methyl groups. Water films were always present on the hydrophilic pore walls to separate CO2 molecules from the pore walls, while the H2O molecules could be basically replaced in the lipophilic pore channels. CO2 molecules could easily strip the H2O molecules around the hydrophobic groups on the amphipathic pore walls, but they could not replace the H2O molecules around the hydrophilic groups on the amphipathic pore walls. During the process of CO2 replacement brine in nanopores, the interaction energies of the H2O molecules with pore walls dominated in hydrophilic and amphipathic systems, which were much greater than the energies of the CO2 molecules with H2O molecules and the CO2 molecules with pore walls. However, in the lipophilic system, the difference in interaction energy between different components was less. The results of this research could serve as a reference for understanding the mechanism of CO2 storage in tight sandstone reservoirs.