CUED Publications database

CO<inf>2</inf>dissolution in a background hydrological flow

Unwin, HJT and Wells, GN and Woods, AW (2016) CO<inf>2</inf>dissolution in a background hydrological flow. Journal of Fluid Mechanics, 789. pp. 768-784. ISSN 0022-1120

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Abstract

© 2016 Cambridge University Press. During CO 2 sequestration into a deep saline aquifer of finite vertical extent, CO 2 will tend to accumulate in structural highs such as offered by an anticline. Over times of tens to thousands of years, some of the CO 2 will dissolve into the underlying groundwater to produce a region of relatively dense, saturated water directly below the plume of CO 2 . 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 CO 2 saturated water relative to the original aquifer water. We investigate long time steady-state dissolution in the presence of a background hydrological flow. In steady state, the distribution of CO 2 in the groundwater upstream of the aquifer involves a balance between three competing effects: (i) the buoyancy-driven flow of CO 2 saturated water; (ii) the diffusion of CO 2 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 CO 2 from an upstream propagating flow of dense CO 2 saturated water into the downstream propagating flow of CO 2 unsaturated water. For each limiting case, we find analytical solutions for the distribution of CO 2 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 CO 2 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 CO 2 saturated water and the original aquifer water upstream.

Item Type: Article
Subjects: UNSPECIFIED
Divisions: Div C > Applied Mechanics
Depositing User: Cron Job
Date Deposited: 17 Jul 2017 19:40
Last Modified: 17 Nov 2017 20:18
DOI: