Anorthite Dissolution under Conditions Relevant to Subsurface CO 2 Injection: Effects of Na + , Ca 2+ , and Al 3+

Supercritical CO 2 is injected into subsurface environments during geologic CO 2 sequestration and CO 2 -enhanced oil recovery. In these processes, the CO 2 -induced dissolution of formation rocks, which contain plagioclase, can affect the safety and efficiency of the subsurface operation. In subsurface brines, Na + and Ca 2+ are naturally abundant, and Al 3+ concentration increases due to acidification by injected CO 2 . However, our current understanding of cation effects on plagioclase dissolution does not provide sufficiently accurate prediction of plagioclase dissolution at such high salinities. This study investigated the effects of up to 4 M Na +, 1 M Ca 2+, and 200 μM Al 3+ on anorthite (as a representative mineral of Ca-containing plagioclase) dissolution under conditions closely relevant to subsurface CO 2 injection. For the first time, we elucidated the inhibition effects of Al 3+ on anorthite dissolution in far-from-equilibrium systems, and found that the Al 3+ effects were enhanced at elevated temperature. Interestingly, Na + inhibited anorthite dissolution as well, and the effects of Na + were 50% stronger at 35 °C than at 60 °C. Ca 2+ had similar effects to those of Na +, and the Ca 2+ effects did not suppress Na + effects when they coexisted. These findings can contribute to better predicting plagioclase dissolution in geologic formations and will also be helpful in improving designs for subsurface CO 2 injection.