Anorthite Dissolution under Conditions Relevant to Subsurface CO2 Injection: Effects of Na+, Ca2+, and Al3.

Abstract

Supercritical CO2 is injected into subsurface environments during geologic CO2 sequestration and CO2-enhanced oil recovery. In these processes, the CO2-induced dissolution of formation rocks, which contain plagioclase, can affect the safety and efficiency of the subsurface operation. In subsurface brines, Na+ and Ca2+ are naturally abundant, and Al3+ concentration increases due to acidification by injected CO2. 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 Ca2+, and 200 μM Al3+ on anorthite (as a representative mineral of Ca-containing plagioclase) dissolution under conditions closely relevant to subsurface CO2 injection. For the first time, we elucidated the inhibition effects of Al3+ on anorthite dissolution in far-from-equilibrium systems, and found that the Al3+ 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. Ca2+ had similar effects to those of Na+, and the Ca2+ 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 CO2 injection.