The problem of CO2 sequestration in geologic formations is analyzed from a fundamental perspective. The mechanisms of trapping, dissolution and chemical reactions are not accounted for in order to clearly understand the first order behavior of the system. The analysis is concerned with the post injection period when the CO2 plume rises due to buoyancy. Characteristics of the plume for a one-dimensional problem show that a pair of shocks moving in opposite directions is produced at the top end. The downward moving shock interacts with the bottom end of the plume resulting in a decrease in the maximum value of the CO2 saturation. High accuracy numerical simulations are employed to understand the two-dimensional mechanisms of plume evolution in terms of the viscosity ratio and the capillary number. Two-dimensional results show that the plume rises to significantly lower height, in shorter times, as compared to the 1-D problem. This behavior is governed by the two-dimensional velocity field around the plume that additionally leads to spanwise wave interactions and results in a faster decrease of the maximum CO2 saturation. The initial dimensions of the plume have a strong influence on the time scales of the wave interactions. The maximum upward velocity that is generated due to buoyancy is closely related to the maximum saturation and decays rapidly to very small values with a decrease in saturation. In the case where the viscosity of CO2 is a tenth of the viscosity of the surrounding fluid, the plume rises up about 500 m in 700 yrs. Our results provide an upper bound on the maximum rise distance and the sequestration time for the problem involving trapping and dissolution. Comparison with experimental results show that the buoyancy velocity obtained from our results is of the same order as observed in the experiments.

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