Carbon dioxide reaction processes in a model brine aquifer at 200 °C and 200 bars: implications for geologic sequestration of carbon

The reactive behavior of supercritical CO2 under conditions relevant to geologic storage and sequestration of C is largely unknown. Experiments were conducted in a flexible cell hydrothermal apparatus to determine the extent of fluid–rock reactions, in addition to carbonate mineral precipitation, that may occur in a brine aquifer–aquitard system that simulates a saline aquifer storage scenario. The system was held at 200 °C and 200 bars for 59 days (1413 h) to approach steady state, then injected with CO2 and allowed to react for another 80 days (1924 h). In addition to magnesite precipitation, silicate minerals (quartz, plagioclase, microcline and biotite) in the aquifer and the aquitard display textures (etch pits, mineralization) indicating significant reaction. Changes in elemental abundances in the brine following addition of CO2 include pH decrease and enrichment in Cl−, partly due to supercritical CO2 desiccation of the brine. Geologic sequestration systems have potential for geochemical reactions that extend beyond simple aqueous dissolution of CO2 and precipitation of carbonate. These reactions may produce geochemical and geotechnical consequences for sequestration and provide important characteristics for monitoring and evaluation of stored CO2. An understanding of multi-phase equilibrium relationships between supercritical CO2 and aquifer–brine systems also raises new questions for a variety of geologic systems. Multi-phase fluid equilibria may, for example, account for the large amounts and heterogeneous distributions of calcite cement in a wide variety of geologic systems, particularly in sedimentary basin sandstones.