CO₂ dissolution in a background hydrological flow
Unwin, H Juliette T
Journal of Fluid Mechanics
Cambridge University Press
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Unwin, H. J. T., Wells, G., & Woods, A. (2015). CO₂ dissolution in a background hydrological flow. Journal of Fluid Mechanics, 789 768-784. https://doi.org/10.1017/jfm.2015.752
During CO₂ sequestration into a deep saline aquifer of finite vertical extent, CO₂ will tend to accumulate in structural highs such as offered by an anticline. Over times of tens to thousands of years, some of the CO₂ will dissolve into the underlying groundwater to produce a region of relatively dense, saturated water directly below the plume of CO₂. Continued dissolution then requires the supply of unsaturated aquifer water. In an aquifer of finite vertical extent, this may be provided by a background hydrological flow, or a laterally-spreading buoyancy-driven flow caused by the greater density of the CO₂ saturated water relative to the original aquifer water. We investigate the long time steady-state dissolution in the presence of a background hydrological flow. In steady-state, the distribution of CO₂ in the groundwater upstream of the aquifer involves a balance between three competing effects: (i) the buoyancy-driven flow of CO₂ saturated water; (ii) the diffusion of CO₂ from saturated to under-saturated water; and (iii) the advection associated with the oncoming background flow. This leads to three limiting regimes. In the limit of very slow diffusion, a nearly static intrusion of dense fluid may extend a finite distance upstream, balanced by the pressure gradient associated with the oncoming background flow. In the limit of fast diffusion relative to the flow, a gradient zone may become established in which the along aquifer diffusive flux balances the advection associated with the background flow. However, if the buoyancy-driven flow speed exceeds the background hydrological flow speed, then a third, intermediate regime may become established. In this regime, a convective recirculation develops upstream of the anticline involving the vertical diffusion of CO₂ from an upstream propagating flow of dense CO₂ saturated water into the downstream propagating flow of CO₂ unsaturated water. For each limiting case, we find analytical solutions for the distribution of CO₂ upstream of the anticline, and test our analysis with full numerical simulations. A key result is that, although there may be very different controls on the distribution and extent of CO₂ bearing water upstream of the anticline, in each case the dissolution rate is given by the product of the background volume flux and the difference in concentration between the CO₂ saturated water and the original aquifer water upstream.
HJTU was funded by an EPSRC Doctoral Training Partnership scheme (grant EP/J500380/1). Data relating to this publication is available in Unwin & Wells (2015).
External DOI: https://doi.org/10.1017/jfm.2015.752
This record's URL: https://www.repository.cam.ac.uk/handle/1810/253113