Critical Water Coverage during Forsterite Carbonation in Thin Water Films: Activating Dissolution and Mass Transport

In geologic carbon sequestration, CO₂ is injected into geologic reservoirs as a supercritical fluid (scCO₂). The carbonation of divalent silicates exposed to humidified scCO₂ occurs in angstroms to nanometers thick adsorbed H₂O films. A threshold H₂O film thickness is required for carbonate precipitation, but a mechanistic understanding is lacking. In this study, we investigated carbonation of forsterite (Mg₂SiO₄) in humidified scCO₂ (50 °C and 90 bar), which serves as a model system for understanding subsurface divalent silicate carbonation reactivity. Attenuated total reflection infrared spectroscopy pinpointed that magnesium carbonate precipitation begins at 1.5 monolayers of adsorbed H₂O. At about this same H₂O coverage, transmission infrared spectroscopy showed that forsterite dissolution begins and electrical impedance spectroscopy demonstrated that diffusive transport accelerates. Molecular dynamics simulations indicated that the onset of diffusion is due to an abrupt decrease in the free-energy barriers for lateral mobility of outer-spherically adsorbed Mg²⁺. The dissolution and mass transport controls on divalent silicate reactivity in wet scCO₂ could be advantageous for maximizing permeability near the wellbore and minimize leakage through the caprock.