Experimental evaluation of in situ CO2-water-rock reactions during CO2 injection in basaltic rocks:: Implications for geological CO2 sequestration

[1] Deep aquifers are potential long-term storage sites for anthropogenic CO2 emissions. The retention time and environmental safety of the injected CO2 depend on geologic and physical factors and on the chemical reactions between the CO2, the aquifer water, and the host rocks. The pH buffer capacity of the aquifer water and the acid neutralization potential of the host rocks are important factors for the permanent stabilization of the injected CO2. Mafic rocks, such as basalt, which primarily consists of Ca, Mg silicate minerals, have a high acid neutralization capacity by providing alkaline earth elements that form stable carbonate minerals. The carbonate minerals formed thus sequester CO2 in a chemically stable and environmentally benign form. In this study, we present results from a small-scale CO2 injection test in mafic and metasedimentary rocks. The injection test was conducted using a single-well push-pull test strategy. CO2 saturated water (pH = 3.5) was injected into a hydraulically isolated and permeable aquifer interval to study the acid neutralization capacity of Ca, Mg silicate rocks and to estimate in situ cation release rates. Release rates for Ca, Mg, and Na were calculated by use of solute compositions of water samples retrieved after the CO2 injection, the incubation time of the injected solution within the aquifer, and geometric estimates of the reactive surface area of the host rocks. Our results confirm rapid acid neutralization rates and water-rock reactions sufficient for safe and permanent storage of CO2. Carbonic acid was neutralized within hours of injection into a permeable mafic aquifer by two processes: mixing between the injected solution and the aquifer water, and water-rock reactions. Calculated cation release rates decrease with increasing pH that is confirmed by laboratory-based experiments. Large differences between release rates obtained from the field and laboratory experiments may be mainly due to uncertainties in the estimation of the reactive surface area in the field experiment and in hydrological and geological factors. Our results underscore the importance of defining bulk rock dissolution rates under in situ conditions in order to evaluate target formations for permanent mineral sequestration of carbon dioxide.