Geological Sequestration of CO2: Mechanisms and Kinetics of CO2 Reactions in Mafic and Ultramafic Rock Formations

We investigated the interactions between CO2-rich fluids and Mg-silicate minerals using experiments to provide information on reaction rates and to identify potential catalysts for the reactions, and studies of natural analogues to assess the conditions that lead to massive carbonation of ultramafic rocks and thus the potential for field-scale implementation. Key findings from our exploratory work include the following: (1) Mineral transitions that occur during alteration of basalt and serpentinites are efficient and could result in complete removal of CO2 from circulating fluids under the correct conditions. However, these reactions also result in extremely high net volume changes and dramatic reductions in porosity and permeability that prevent the continual removal of CO2 unless the dissolution and precipitation reactions occurred in different zones and the precipitation reactions were accompanied by substantial hydrofracturing. This emplacement mechanism is consistent with our observations from Red Mountain, CA Magnesite District and the findings of Boschi et al. (2009); (2) The use of organic acids (e.g., salicylic acid) as activators caused the rate of olivine dissolution to increase by a factor of 3, while the rate of MgCO3 precipitation increased by an order of magnitude at the highest concentration of salicylic acid (1 g/L) and decreased by an order of magnitude (relative to pure water) at lower concentrations of salicylic acid (0.1 g/L). The presence of organic acids in natural systems is also indicated by the carbon isotopic composition of the magensite from Red Mountain. The negative carbon isotope signature (δC = -11.6 ± 2.4 ‰) of the magnesite (MgCO3) is consistent with CO2 derived from degradation of organic matter and/or decarboxylation reactions and thus indicates that organic acids were likely present during the conversion of Mg-silicates to magnesite. While the exact role of organic acids has not yet been precisely determined, the effects of aqueous organic ligands on the dissolution rate, solubility, and stability of Mg-silicates relative to carbonate minerals are likely to be of first-order importance in optimizing the industrial utility of geologic sequestration of CO2 in ultramafic and mafic rock formations. Our experimental work and characterization studies of natural analogues inspired us to expand our research using the techniques that we developed during our exploratory research to submit a full GCEP proposal focused on the reactivity of CO2 in both ultramafic/mafic rocks and in saline aquifers.