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

The reactive behavior of a mixed fluid (supercritical CO2 and brine) under physical–chemical conditions relevant to geologic storage and sequestration in a carbon repository is largely unknown. Experiments were conducted in a flexible cell hydrothermal apparatus to evaluate fluid–rock (aquifer plus aquitard) reactions that may adversely impact the integrity of the repository. A 5.5 molal NaCl brine–rock system was held at 200 °C and 200 bars (20 MPa) for 32 days (772 h) to approach steady state, then injected with CO2 and allowed to react for an additional 45 days (1079 h). In a separate experiment at 200 °C and 200 bars, the system was allowed to react for 77 days (1845 h) without injection of CO2. Corroded magnesite and euhedral siderite crystallized in a paragenetic sequence after CO2 injection. Nucleation and growth of siderite on shale suggests the aquitard is a reactive component in the system. Changes in elemental abundances in the brine following addition of CO2 include pH decrease and depletion of sodium due to accelerated growth of analcime. A pH increase follows pressure and temperature decrease and loss of saturated CO2 from acidic brine. Silica concentrations and dissolution rates are enhanced and silica precipitation inhibited in the acidic brine. mical 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.