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

The main purpose of this exploratory project is to develop a more fundamental understanding of the long-term sequestration of CO2 via mineral carbonation reactions involving the common Mg-silicates in serpentinite and basalt mineral assemblages. Past experimental studies have shown that these reactions are kinetically limited, so we are exploring ways to enhance their kinetics, including the use of activators such as organic acids and catalysts such as natural metalloenzymes that enhance the rate of conversion of hydrated CO2 to bicarbonate ions. During the past year, we have combined surface science studies of (1) the interaction of CO2 and H2O with Mg-oxides and Mg-silicates, (2) hydrothermal experimental studies of the rates of Mg-silicate carbonation reactions, (3) reactive transport modeling of these carbonation reactions in initially porous Mgsilicate rocks, and (4) field-based studies of natural analogs of serpentinite carbonation reactions. We have carried out first-of-their-kind in situ ambient pressure photoemission spectroscopy (APPES) studies of the interaction of CO2 and CO2 + H2O with model Mgoxide surfaces, initiated experimental hydrothermal studies of the rate of interaction of CO2-rich aqueous solutions with Mg-olivine and other Mg-silicates, completed preliminary simulations of these and other reactions of CO2-rich aqueous solutions with Mg-silicates using the reactive transport code CrunchFlow, and are beginning field studies of natural analogs with a focus on identifying all carbonate phases. Our in situ APPES studies have shown that the interaction of 0.2 Torr of CO2 with a model MgO (100) surface results in rapid formation of MgCO3 and that subsequent exposure to 0.1 Torr of H2O enhances the reaction. In contrast, when water is reacted with MgO(100) first, followed by reaction with CO2, the carbonation reaction is inhibited. This observation may help explain enhanced carbonation of Mg-silicates upon preheating. Our numerical reactive transport simulations of the reaction of CO2-rich fluids with Mgsilicates have shown that under the conditions relevant for subsurface CO2 injection, primary mineral dissolution, coupled with the spatial distribution of reaction fronts along the flow path, allow for efficient precipitation of Mg-carbonate without complete destruction of the initial porosity. This finding contrasts with previous closed-system batch reactor models that suggested the positive volume change during Mg-silicate carbonation would result in complete filling of the pore space. Over the range of initial