We have previously proposed the "inject low and let rise" strategy of storing CO2 in deep saline aquifers. The idea is to maximize the amount of CO2 stored in immobile forms by letting CO2 rise toward the top seal of the aquifer but not reach it. The distance that the CO2 rises depends on the uniformity of the displacement front. In this paper, we address the question of whether the intrinsic instability of a buoyancy-driven immiscible displacement leads to fingering. Fingers could reach the top seal of the aquifer, leading to an accumulation of CO2 at large saturations. We study the mechanisms governing this type of displacement in a series of very fine-grid numerical simulations. Each simulation begins with a finite volume of CO2 placed at large saturation at the bottom of a two-dimensional aquifer. Boundaries are closed, so that CO2 rises and brine falls as the simulation proceeds. Several fine-scale geostatistical realizations of permeability are considered, and the effects of capillary pressure, anisotropy and dip angle are examined. In these simulations, buoyant instability has very little effect on the uniformity of the displacement front. Instead, the CO2 rises along preferential flow paths that are the consequence of spatially heterogeneous rock properties (permeability, drainage capillary pressure curve, and anisotropy). Capillary pressure broadens the lateral extent of the flow paths. If the formation beds are not horizontal, capillary pressure and anisotropy can cause the CO2 to move predominantly along the bedding plane, rather than vertically. Accurate assessment of CO2 migration after injection ends will therefore require accurate characterization of the spatial correlation of permeability in the target formation, and of the capillary pressure and relative permeability curves.

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