Experimental evaluation of mixed fluid reactions between supercritical carbon dioxide and NaCl brine: Relevance to the integrity of a geologic carbon repository

Geochemical reactions in a carbon repository extend beyond pH decrease and carbonate mineral precipitation. Rock-dominated reaction systems yield to acid-dominated and related reactions controlled by mixed fluid equilibria (i.e., a fluid-dominated system). Escape of CO2 or migration of brine from the repository into overlying aquifers may cause silica super-saturation and increased alkalinity due to mixed fluid phase equilibria. These geochemical changes could be monitored in aquifers as indicators of repository integrity. Return of silica super-saturated brine to a rock-dominated reaction system buffered to neutral pH conditions may enhance precipitation of quartz, chalcedony, or amorphous silica. In addition to the potential effects (beneficial or deleterious) that silica super-saturation and precipitation may hold for repository performance, an understanding of the effects of multi-phase equilibrium relationships between supercritical CO2 and dissolved silica in aquifer-brine systems also raises new questions for a variety of geologic systems. Multi-phase fluid equilibria may, for example, account for quartz cements in some sedimentary basin sandstones and quartz vein mineralization in some ore districts.