The reactivity of caprock minerals in H2O-bearing supercritical CO2 fluids (WBSF) has received significant attention due to its relevance to the chemistry of carbon sequestration. In particular, divalent cation-rich silicate minerals such as olivines have a high potential for undergoing carbonation reactions and phyllosilicate minerals such as clays can incorporate CO2 in their interlayer space. Both phenomena can strongly influence the long-term storage of carbon. In this presentation, molecular simulations aimed at shedding light on the chemistry of both systems in the presence of WBSF will be reviewed. Studies published to date on olivine carbonation in WBSF have highlighted the strong dependence of the extent and rate of carbonation on the water content of the WBSF. Therefore, a detailed understanding of WBSF-olivine interfaces as a function of water content is critical to elucidating the mechanisms of carbonation reactions in conditions relevant to geologic carbon sequestration. A suite of computational techniques including static electronic structure calculations, ab initio molecular dynamics (MD) simulations, and classical MD simulations were employed to evaluate the energetics of CO2/H2O competitive adsorption at the surface of a number of olivine minerals (M2SiO4, M=Ca, Mg, Fe, Mn, etc.) as a function of surface coverage and quantify the partition of water between the WBSF and a model Mg2SiO4 surface as a function of water content. Clay minerals, including montmorillonites, are important components of shale caprocks. Upon contact with WBSF, montmorillonite clays could shrink or swell to, respectively, open or close mineral fractures in the caprock. Therefore, a molecular-level understanding of the CO2/H2O competitive intercalation in clay interlayer space, including as a function of the nature of the interlayer cations, is key to predicting their behavior in realistic carbon sequestration scenarios. Classical MD simulations were performed to determine the structure and energetics of WBSF in montmorillonite interlayer space for several interlayer cation types (e.g. Na+, Cs+, and NH4+). Calculated interlayer spacings were compared to experimental data obtained from X-ray diffraction.

Molecular simulations of the reactivity of H2O-bearing supercritical CO2 fluids with silicate minerals

253rd National Meeting (2017)

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