Convective mixing influenced by the capillary transition zone

Convective mixing of dissolved carbon dioxide (CO2) with formation brine has been shown to be a significant factor for the rate of dissolution of CO2 and thus for determining the viability of geological CO2 storage sites. In most previous convection investigations, a no-flow boundary condition was used to represent the interface between an upper region with CO2 and brine and the single-phase brine region beneath. However, due to interfacial tension between the phases, the water phase is partly mobile in the upper region and advection may occur. Based on linear stability analysis and numerical simulations, we show that advection across the interface leads to considerable destabilization of the system. In particular, the time of onset of instability is reduced by a factor of two and the rate of dissolution is enhanced by a factor of two for three of four formations we consider, and by 40 % for the fourth formation. It is found that exponential decay of the relative permeability away from the interface provides a useful approximation to the real system. In addition, the exponential decay also simplifies the linear stability analysis. Interestingly, formations with large absolute permeability and small porosity have the largest impact from the transition zone, despite the fact M. T. Elenius ( ) Uni CIPR, P.O. Box 7810, 5020 Bergen, Norway e-mail: [email protected] J. M. Nordbotten · H. Kalisch Department of Mathematics, University of Bergen, P.O. Box 7800, 5020 Bergen, Norway Present Address: M. T. Elenius Tufts University, 113 Andersen Hall, 200 College Ave, Medford, MA 02155, USA that the relative permeability decays quickly above the interface in these formations. This is because the length-scale of instability is smallest in these formations.