Olivine dissolution and carbonation under conditions relevant for in situ carbon storage

•We reacted olivine (Fo92) with H2O and scCO2 in a batch reactor and formed magnesite and amorphous silica.•Olivine dissolution rates are constant with pH (4.5 < pH < 5.5) and time (10-70 days).•Elevated [SiO2(aq)] decreases the rate of olivine dissolution, 0.5 M NaCl increases it.•Iron is incorporated in precipitated magnesite; the precipitation rate is higher than expected.

In order to evaluate the chemistry and kinetics of mineral carbonation reactions under conditions relevant to subsurface injection and storage of CO2, olivine alteration was studied at 60 °C and 100 bar CO2 pressure, including olivine dissolution and the formation of carbonate minerals. Batch experiments were performed with olivine (Fo92), water, CO2, and NaCl inside gold cells contained within rocking autoclaves. Two reproducible experiments yielded an initial (1 h) dissolution rate of 9.50 ± 0.10 × 10− 11 and a long-term (10-70 days) rate of 1.69 ± 0.23 × 10− 12 mol cm− 2 s− 1. The long-term rate is consistent with previously published rate laws at 4.5 < pH < 5.5. The dissolution rates presented here are constant with increasing pH in the same range, suggesting a pH-independent dissolution mechanism at elevated CO2(aq) and SiO2(aq) at 60 °C. A Si-rich surface layer forms on olivine grains within 2 days of reaction and appears to slow dissolution by passivating the surface. The olivine dissolution rate decreases by 2 orders of magnitude over 4 days as the system approaches amorphous silica saturation but remains constant thereafter based on a linear increase in Mg concentrations. Secondary phases consist of amorphous silica and magnesite, with up to 7 mol% olivine converted to Mg-carbonate over 94 days of reaction. Magnesite precipitation rates could not be precisely quantified due to experimental limitations. However, our minimum estimate of 1.40 × 10− 13 mol cm− 2 s− 1 suggests that the precipitation rate is several orders of magnitude greater than predicted by previous studies. Finally, the presence of 0.5 M NaCl resulted in a decrease in olivine dissolution rate at reaction times of less than 4 days, but a significant enhancement of the reaction rate at reaction times greater than 4 days relative to electrolyte-free experiments. Our results suggest that geochemical models developed to predict the behavior of subsurface CO2 storage systems in mafic and ultramafic rocks should incorporate the effects of dissolved species, including SiO2 and NaCl.