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Abstract
CO2 sequestration in deep formations is being actively considered for the reduction of greenhouse gas emissions. Saline aquifers are considered as one of the most favorable options for this purpose. It has been observed that dissolution of CO2 into brine causes increased density of the mixture. If the corresponding Rayleigh number of the porous medium is high enough to initiate convection flows, density-driven-convection happens and the rate of dissolution increases. Early time dissolution of CO2 in brine is mainly dominated by molecular diffusion while it will be accelerated by density-driven-convection. More contribution of dissolution mechanism for trapping of CO2 decreases the risk of leakage.

Density-driven-convection mechanism was investigated in a Hele-Shaw cell with colored-brine and fresh-water instead of CO2-diffusive layer and brine. A convective instability is created by colored-brine diffusing onto the surface of a fresh-water layer. For this configuration, density-driven-convection flow enhances the mass transfer rate of high density fluid into the low density one. The analysis is also done numerically with Eclipse reservoir simulation software. With this analysis, the effects of density-driven-convection on accelerating the rate of dissolution are investigated. Although the horizontal wavelength of the initial instability is small, an increase in the horizontal wavelength of the convective flow with time and depth is observed as the resulting two-dimensional convection develops. Effects of density of fluids and also dip of the systems on convection flows are studied here. Also the changes in geometry of the convection streams with depth and time are investigated. The results have important implications in dissolution trapping of CO2 in brine aquifers.

Introduction
If the emissions of CO2 from the use of fossil energy continue on the current scale, then it has been predicted that significant changes in the global climate will occur in the next hundred years (Houghton et al. 2001). One of the most promising solutions for the purpose of reducing greenhouse gas emissions is disposal of CO2 in geological formations. The geological formations such as coal beds, depleted oil and gas reservoirs and deep brine aquifers are widely available with large capacities. In particular, the brine aquifers have an estimated capacity of 320 to 10,000 Gt (1 Gt = 1012 kg) of CO2 worldwide and they can be considered as one of the major types of geological formations for CO2 storage (Bachu 2002).