Reactive precipitation during overlaying CO2 dissolution into brine: The role of porous structure

Abstract

After sequestration of CO₂ into subsurface saline aquifer, CO₂ cap forms at the top of a stratum. As overlaying CO₂ dissolves into brine, precipitation reactions between CO₂ and in-situ ions emerge. The role of reactive precipitation during this process has long been under debate due to the lack of direct observation. Here we conduct visualized experiments on high-pressure CO₂ convective dissolution into Ca(OH)₂ solution, where CaCO₃ precipitation forms. We show that the presence of a porous structure largely shapes the dissolution pattern. In absence of porous structure, sharp and flat reactive front is observed, with cloudy particle suspension chaotically flowing; however, in presence of a porous structure, symmetric fingers are observed above the reactive front, and precipitates locally deposit at the pore-scale without participating in convection. We theoretically rationalize these observations and discuss their impacts on CO₂ dissolution kinetics. Inspired by these experimental observations, we propose several major simplifications for numerical modeling. This work also provides a benchmark for future experimental and numerical studies of CO₂ convective dissolution with reactive precipitation during CO₂ sequestration.