Synopsis: Convection Speeds Up on a Slant

Convective mixing of fluids goes faster when the container is tilted—a finding that may impact the choosing of geological sites for carbon dioxide storage.

A potential strategy for reducing greenhouse gas emissions is to capture carbon dioxide (${\text{CO}}_{2}$) and store it underground. One of the most promising storage options for carbon capture and sequestration (CCS) is offered by deep saline aquifers, which are brine-filled reservoirs covered by a cap of impermeable rocks. A new experimental study reported in a Rapid Communication in Physical Review E looks at how convection in the ${\text{CO}}_{2}$-brine mixture can help trap the greenhouse gas. In particular, the authors find that a sloping saline formation could promote convection and thus provide better storage than a horizontally level one.

Deep saline formations offer a large storage capacity. Ideally, the carbon dioxide will dissolve and get trapped into the brine. However, ${\text{CO}}_{2}$—being less dense than brine—will rise upward and may form a separate top layer that restricts dissolution, except at the boundary between the two fluids. In this situation, convection could provide much-needed stirring. Previous studies have shown that, under certain conditions, ${\text{CO}}_{2}$-rich brine at the boundary will sink downwards in convective plumes that force fresh brine upward into the ${\text{CO}}_{2}$.

Peichun Amy Tsai and her colleagues from Princeton University, New Jersey, investigated convection in a laboratory model of a saline formation. Unlike previous work, they considered the effect of slanted boundaries, as found at some potential CCS sites. The team poured water and propylene glycol (as stand-ins for ${\text{CO}}_{2}$ and brine) into a pore-filled container with transparent walls. They filmed the formation of convective plumes and found that tilting the container by $20$ degrees increased the rate of dissolution by $20$ percent. This enhancement is analogous to the faster sedimentation of suspended particles in a tilted tube (known as the Boycott effect). – Michael Schirber

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