Supercritical CO2 Effects on Calcite Wettability: A Molecular Perspective

The wettability behavior of reservoir rocks plays a vital role in determining CO2 storage capacity and containment security. Several experimental studies characterized the wettability of CO2/brine/rock systems for a wide range of realistic conditions. To develop a fundamental understanding of the molecular mechanisms responsible for such observations, the results of molecular dynamics simulations, conducted at atomistic resolution, are reported here for representative systems in a wide range of pressure and temperature conditions. Several force fields are considered, achieving good agreement with experimental data for the structure of interfacial water but only partial agreement in terms of contact angles. In general, the results suggest that, at the conditions chosen, water strongly wet calcite, resulting in water contact angles either too low to be determined accurately with the algorithms implemented here or up to similar to 46 degrees, depending on the force field implemented. These values are in agreement with some, but not all experimental data available in the literature, some of which report contact angles as high as 90 degrees. One supercritical CO2 droplet was simulated in proximity of the wet calcite surface. The results show pronounced effects due to salinity, which are also dependent on the force field implemented to describe the solid substrate. When the force field predicts complete water wettability, increasing NaCl salinity seems to slightly increase the calcite affinity for CO2, monotonically as the NaCl concentration increases, because of the preferential adsorption of salt ions at the water- rock interface. When the other force field was implemented, it was not possible to quantify salt effects, but the simulations suggested strong interactions between the supercritical CO2 droplet and the second hydration layer on calcite. The results presented could be relevant for predicting the longevity of CO2 sequestration in geological repositories.