EXPERIMENTAL EVALUATION OF MIXED FLUID REACTIONS BETWEEN SUPERCRITICAL CARBON DIOXIDE AND A NACL BRINE: RELEVANCE TO GEOLOGIC AQUIFER CARBON SEQUESTRATION

The reactive behavior of supercritical carbon dioxide under physical-chemical conditions relevant to geologic storage and sequestration of carbon 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 simulated as a saline aquifer storage scenario. The aquifer is a synthetic arkose (microcline (Or91-97)+oligoclase (An17-21)+quartz+biotite), the aquitard is argillaceous shale, and the fluid is 5.5 molal NaCl brine. The system was held at 200 C and 200 bars for 32 days (772 hours) to approach steady state, then injected with carbon dioxide and allowed to react for an additional 45 days (1079 hours). In a separate experiment at 200 C and 200 bars, the system was allowed to react for 77 days (1845 hours) without injection of carbon dioxide.

Brine-rock reaction decreases pH from 8 to ~5.9 in both experiments. In the brine-rock-carbon dioxide experiment, injection of carbon dioxide produces a pH drop (possibly below 4, based on calculation) followed by rebound to ~6.6 as the minerals react with carbon dioxide-charged brine. Concentrations of Si and Mg in the brine increase following injection of carbon dioxide. In addition to carbonate mineral precipitation, silicate minerals (quartz, oligoclase, microcline and biotite) in the aquifer and aquitard display textures (etch pits, mineralization) indicating significant reactions. A pressure decrease of 23 bars occurred in the experimental cell over a 3-day period following carbon dioxide injection. Pressure was stable afterwards. The pressure decrease is interpreted as consumption of supercritical carbon dioxide by dissolution in brine and subsequent precipitation as carbonate mineral. The experimental reactions provide initial constraints on reactions and reaction rates that can impact the containment interface in moderate temperature brine aquifer systems with potential for carbon sequestration.