The reactive behavior of a mixed fluid (supercritical CO 2 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 CO 2 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 CO 2. Corroded magnesite and euhedral siderite crystallized in a paragenetic sequence after CO 2 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 CO 2 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 CO 2 from acidic brine. Silica concentrations and dissolution rates are enhanced and silica precipitation inhibited in the acidic brine. 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 CO 2 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 CO 2 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.