(573u) Surface Wettability and Pore Size Effect on CO2 Diffusivity in Water-Saturated Kaolinite Nanopores

Geological CO₂ sequestration (GCS) is a promising method to alleviate CO₂ emission and induced dire consequences. Evaluating CO₂ potential leakage would be of great importance as the ultimate goal of GCS is to permanently store CO₂ in geological locations. Diffusive loss through caprock is identified to be one of the possible ways for CO₂ leakage, which is closely associated with CO₂ diffusivity. Caprocks typically consist of clay minerals and contain a considerable number of nano-scale pores. Besides, caprocks are saturated with formation water. Even though CO₂ self-diffusivity and diffusivity in water/brine are widely studied, its diffusivity in water-saturated nanopores remains less understood.

In this work, we use molecular dynamics (MD) simulations to study CO₂ diffusivity in water-saturated nanopores under in-situ GCS condition. Kaolinite, a common type of clay minerals in caprocks, is used as substrate. It has surfaces of different wettability and pore sizes are also considered in our simulation set-up. We find that CO₂ diffusivity is higher in water-saturated hydrophobic confinements than in hydrophilic ones. It is further reduced by stronger surface hydrophilicity due to increasing content of H₂O. Moreover, smaller pore size can inhibit diffusion of CO₂ as the result of stronger surface-CO₂ interactions. But there is a sudden drop for CO₂ diffusivity in all types of confinements when the pore width is smaller than 1.0 nm as all the molecules are adsorbed on the surface. Our study should provide fundamental understanding into the effect of surface wettability and pore size on CO₂ diffusion in caprock, which may help evaluate the CO₂ leakage in GCS projects.