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

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.