Recently, in the context of reducing emissions of combustion-generated carbon dioxide, there has been considerable interest in geologic storage of liquid CO2, both below the land's surface, and in cavities and porous formations below the seafloor. Since the cost of drilling and injection increase substantially with seafloor depth, there is considerable interest in sub-seafloor storage at locations where the seafloor depth (and pressure) are such that liquid CO2 is less dense than water.
Risk analysis for such storage locations requires assessment of the fate of liquid CO2 that escapes from storage as a result of loss during injection, passage though natural fissures, and seismic activity. Key issues in assessing the fate of escaped CO2 drops include the rate at which they rise through and dissolve in seawater, and how those rates depend on drop size.
In this video we show results from numerical simulations of rising CO2 drops performed with a volume-of-fluid approach. In the simulations the full incompressible Navier-Stokes equations are solved, and mass transfer is neglected. The deformable interface is assumed clean, fully mobile, and a constant interfacial tension is used.
At typical depths, the density of the liquid CO2 is slightly lower than the density of the seawater, and the viscosity of the CO2 is approximately one-tenth that of the seawater. An interesting characteristic is that for this combination of physical properties, the flow internal to the drop is significantly more complex than the flow external to the drop.
The video shows a 2 mm, 4 mm, and 5 mm liquid CO2 drop rising in seawater. For the smallest drop, the drop rises steadily, and short finger-like structures form in the drop. The 4 mm drop experiences significant net horizontal motion, and exhibits a "sloshing" - like flow internal to the drop and long finger-like structures emanating from the drop in the wake. The 5 mm drop rises essentially in a rectilinear trajectory, and the flow internal to the drop resembles that of the transitional flow associated with a free vortex ring. The complex flow internal to the drop is expected to enhance mass transfer and the dissolution of CO2 into the seawater.