A Molecular Dynamics Study on CO₂ Diffusion Coefficient in Saline Water Under a Wide Range of Temperatures, Pressures, and Salinity Concentrations: Implications to CO2 Geological Storage

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Carbon dioxide (CO₂) sequestration in saline aquifers has been introduced as one of the most practical, long-term, and safe solutions to tackle a growing threat originating from the emission of CO₂. Successfully executing and planning the process necessitates a comprehensive understanding of CO₂ transport properties -- particularly the diffusion coefficient, influencing the behavior of CO₂ dissolution in water/brine regarding the shape of viscous fingers, the onset of instabilities, etc. In this research, Molecular Dynamics (MD) simulation was employed to compute the CO₂ diffusion coefficient in various NaCl saline water concentrations under the broad spectrum of temperatures and pressures to acquire a data-set. The NaCl concentration increase gives rise to a decrease in the CO₂ diffusion coefficient, by which the reduction is most notably at higher temperatures. In addition, the rise in the CO₂ diffusion at elevated temperatures can be explained by the cation's hydration shell size reduction with temperature increment due to intensifying repulsive forces among water molecules. A new precise correlation is proposed for estimating CO₂ diffusion coefficients. Regarding the pressure variation effects, no tangible changes are observed with pressure increase. Furthermore, the variability of the CO₂ diffusion coefficient in the presence of other salts, namely MgCl2, CaCl2, KCl, and Na2SO4, were computed separately. Comparing the influence of various salts, CaCl2 and KCl have the highest and lowest effect on the CO₂ diffusion coefficient, respectively. Finally, a set of direct numerical simulations was conducted to study the impact of the CO₂ diffusion coefficient on the CO₂ dissolution process. The results shed light on the importance of CO₂ diffusion coefficient changes under the saline water condition in predicting dissolution process behavior and further calculations.