[S-CG49] Innovation through the Integration of Solid Earth Science and Materials Science

[SCG49-03] A Molecular Dynamics Study on Contact angle and Water Film in CO₂-Water-Clay Mineral Systems

*Masashige Shiga¹, Tetsuya Morishita², Masaatsu Aichi³, Naoki Nishiyama¹, Masao Sorai¹ (1.Research Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology, 2.CD-FMat, National Institute of Advanced Industrial Science and Technology, 3.Graduate School of Frontier Sciences, The University of Tokyo)

Keywords:CO2 geological storage, Wettability, Molecular dynamics simulation, Water film

The wettability of mineral surfaces is a crucial parameter for fluid behavior in geological formations in Carbon Dioxide Geological Storage (CGS), with the contact angle being one of the commonly used evaluation metrics. It is known from experiments that on hydrophilic mineral surfaces, such as quartz and clay minerals which are predominant in the target geological formations, adsorbed water films of nanometer-scale thickness form between the non-wetting phase (i.e., CO₂) and the mineral phase. In the fields of petroleum engineering and CGS, the potential energy of water films is modeled by using the augmented Young-Laplace equation to study critical factors influencing wettability changes. However, the formulation of hydration forces on mineral surfaces and the parameters are empirically determined, making the interpretation of their physicochemical meaning challenging. Hydration forces are effective within a region of 1-2 nm from the surface, necessitating atomic-scale studies for their understanding. In this context, numerous studies using Molecular Dynamics (MD) have been reported, focusing on evaluating contact angles and the thickness of water films, yet the correlation between the energy of water films and contact angles has not been verified at the atomic level. Therefore, in this study, MD simulations of the CO₂-water-clay mineral system were conducted. Using clay minerals with different layer charges, we examined the differences in contact angles and water film energies. The results showed a trend of decreasing contact angle and increasing water film thickness with an increase in negative layer charge. Calculating the free energy change in the process of CO₂ molecules passing through the water film to approach the mineral surface revealed a tendency for the amount of work required for CO₂ molecules to approach the surface to increase with an increase in negative layer charge, consistent with the correlation between contact angle and water film energy in the above-mentioned interfacial thermodynamics model. This is the first report demonstrating this correlation at the atomic level. Additionally, analysis of the structure and dynamics of the adsorbed water provided new insights from an atomic-scale perspective, which are unattainable through conventional continuum models. In particular, the connectivity of the adsorbed water molecules on the surface allows us to interpret the effects of the layer charge on the contact angle and film energetics.