Forsterite dissolution in saline water at elevated temperature and high CO₂ pressure

The rates and mechanisms of magnesium silicate dissolution can control the aqueous chemistry in ways that influence carbonate mineral precipitation during geologic carbon sequestration (GCS). A series of batch experiments was performed with forsterite (Mg_1.81Fe_0.19SiO₄) powder to determine the effects of pressure (10-100 bar CO₂), temperature (25-100 C), and salinity (0-50,000 mg/L NaCl) on its dissolution rate at conditions relevant to GCS. Dissolution rates and products were determined by analysis of the aqueous phase, equilibrium and reaction path modeling, and solid phase characterization by scanning electron microscopy and X-ray diffraction. After an initially rapid dissolution period, the dissolution rate declined significantly, an effect that is attributed to the formation of a silica-rich layer at the forsterite surface. The initial dissolution rate increased with increasing temperature and increasing CO₂ pressure; the effect of CO₂ was through its influence on the pH. The dissolution rate was enhanced by NaCl, which may have been due to its inhibition of the formation of a silica-rich surface layer. The experimental results provide information about magnesium silicate dissolution at conditions that will be encountered during GCS that can be used to predict the fate of CO₂ and the evolution of subsurface geochemistry following CO₂ injection.