Role of convective mixing in the long-term storage of carbon dioxide in deep saline formations

Storage of carbon dioxide in deep formations is being actively considered for the reduction of greenhouse gas emissions. Relevant experience in the petroleum industry comes from natural gas storage and enhanced recovery using carbon dioxide, but this experience is over a time scale less than the hundreds or thousands of years required for carbon dioxide storage. On these long time scales, different mechanisms need to be considered. In the long term, the dominant mechanism for dissolution of carbon dioxide in formation water is convective mixing rather than pure diffusion. This arises because the density of formation water increases upon dissolution of carbon dioxide, creating a density instability. Linear stability analysis has been used to estimate the time required for this instability to occur in anisotropic systems. For sufficiently thick formations with moderate vertical permeability, this time ranges from less than a year up to a few hundred years. Further Approximate analysis shows that the time needed for the injected gas to dissolve completely is typically much longer, on the order of hundreds of years to tens of thousands of years, depending on the vertical permeability. This theoretical analysis is compared with the results of numerical simulations.