Experimental Assessment of Basalt Dissolution Rates under Shallow Aqueous Carbon Dioxide Sequestration PCO2 and Temperatures

Sequestration by CO2 mineralization in basalt formations is a greenhouse gas emission mitigation strategy. An advantage of this strategy is that CO2 injected as a free phase or aqueous phase reacts with the basalt immobilizing CO2 as carbonate minerals. Free phase injections are limited to pressures greater than the critical pressure of CO2 whereas aqueous injections allow for shallower depths limited by CO2 solubility under formation temperatures, pressures, and geochemical conditions. Pilot/field-scale demonstrations of injected supercritical CO2 (Wallula Basalt Pilot Project, SE Washington, USA) or aqueous CO2 (CARBFIX, Iceland) show that CO2 rapidly reacts with basalts to form carbonate minerals over relatively short time frames.
We are planning to conduct a aqueous CO2 injection into the basalts of the Eastern Snake River Plain (ESRP), Idaho USA. In preparation, we conducted twelve, 60-plus-day long batch laboratory water-rock-CO2 experiments at temperatures of 21 and 40°C, PCO2 of 15 and 45 bars (1.5 to 5.5 wt.% CO2), and crushed basalt with effective gain diameters of 0.22 and 0.57 mm to assess the dissolution rates of ESRP basalt. Initial silica release rates were calculated from sampling-corrected experimental concentration vs. time profiles. Essentially no difference in rates were observed between experiments conducted under the same P-T conditions but with different grains sizes. This result is consistent with measured B.E.T. surface areas which differed from each other by less than 4%. Rates at 40°C and 45 bars were 1.8±0.11 and 1.3±0.06 times faster than for 21°C and 15 bars experiments, respectively. Using B.E.T. surface area normalized initial silica release rates, an empirical rate law of the form r=A·e‑Ea/RT·(mCO2)n was derived where Ea is the activation energy (42.5±5.0 kJ/mol) and n is the reaction order with respect to CO2 molality. This rate law predicts that at constant mCO2 the silica release rate is 2.9 times faster at 40 compared to 21°C and reflects the opposite effects of increasing temperature on basalt reaction rates (increasing) and CO2 solubilities (decreasing). On going activities include assessment of release rates for other elements and considerations of the observed time dependency on rates.