Synergistic effects of CO2 density and salinity on the wetting behavior of formation water on sandstone surfaces: Molecular dynamics simulation

Reservoir wettability is very important to predict the structure and residual capture capacity, potential CO2 leakage accidents and improve the risk assessment and sequestration safety during CO2 geological sequestration. The wetting behavior of fluid on reservoir rock surface is actually an interface phenomenon caused by the interaction of different fluid molecules on rock surface. At present, it is difficult for laboratory experiments to observe the microscopic interface phenomena at the molecular scale under reservoir conditions, while molecular dynamics simulations can characterize the microscopic changes in wetting behavior and analyze the interaction between the fluid and the rock surface. In this study, molecular dynamics simulations are used to study the effects of different CO2 densities and different salinities on the wetting behavior of formation water on the sandstone surface during CO2 geological sequestration. The results show that both salinity and CO2 density have significant effects on the wettability of quartz surfaces. However, the increase in salinity can only weaken the wettability, while the increase in CO2 density can transform the wettability of the quartz surface. Both CO2 molecules and H2O molecules form two adsorption layers at 3.5 Å and 6.9 Å away from the quartz surface. CO2 molecules will replace the positions originally occupied by water molecules on the quartz surface, reducing the contact area between water droplets and the quartz surface. The CO2 density mainly affects the interface behavior between the components in the system, while the NaCl concentration mainly affects the interaction between H2O molecules and ions in the system. The research results can further improve the understanding of the wettability of fluid on the pore surface of reservoir rocks during CO2 geological sequestration.