Effects of pressure and solution composition on mineral weathering rates as applied to geologic storage of carbon dioxide

CO₂-mediated weathering of silicate minerals and subsequent carbonate mineral precipitation may allow permanent trapping of carbon dioxide stored in deep saline aquifers. The time-scales and extents of the relevant reactions, however, are incompletely understood for receptor reservoir conditions. To address current shortcomings, experiments were conducted to investigate the effects of pressure, pH, and dissolved inorganic carbon (C) concentration on rates and mechanisms of silicate mineral dissolution. A 500 cm³ high-pressure stirred flow-through reactor was used to contact 53-106 mm size-fraction forsteritic olivine ((Mg_0.89Fe_0.11)₂SiO₄) with C-rich and C-free aqueous solutions. The system allowed monitoring and control of temperature (40° C), total pressure (10⁵ and 10⁷ Pa), pH (3.1 and 7.1), flow rate (0.03 and 0.13 cm³ s^-1), and dissolved inorganic carbon concentration. Effluent samples were analyzed using inductively coupled plasma spectrometry to determine total aqueous magnesium, iron, and silicon concentrations for inference of quasi-steady state mineral dissolution rates. Mineral solids were characterized both pre- and post-dissolution using N₂-adsorption and scanning electron microscopy with energy dispersive X-ray analysis. Mean forsteritic olivine dissolution rates derived from aqueous silicon concentrations show strong dependence on pH (3.0 x 10^-12 mol cm^-2 s^-1 at pH 3.1 and 1.0 x 10^-13 mol cm^-2 s^-1 at pH 7.1) and are consistent with previously published values at ambient conditions. No effect of pressure on dissolution rate was observed in the absence of dissolved inorganic carbon, suggesting ambient pressure measurements may be used to characterize deep subsurface mineral dissolution rates. However, preliminary analyses indicate a possible olivine dissolution rate enhancement due to the presence of inorganic carbon. Minor morphological alteration with no apparent chemical modification was observed in post-dissolution olivine grains; however, discrete Fe-carbonate precipitates appear to have formed during pH 7.1, C-rich experiments.