Buoyancy-Dominated Multiphase Flow and Its Impact on Geological Sequestration of CO2

Abstract

Abstract 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. Introduction Storing CO2 in deep, saline aquifers will be a key technology if society elects to limit the amount of greenhouse gases entering the atmosphere. The volumes to be stored would be prodigious, on the order of 109 tons per year [1]. In terms of volumetric flow rates through wellbores, this rate of storage is the same magnitude as the current global rate of oil production. Thus inexpensive, reliable methods of ensuring that stored CO2 remains in place will be essential. CO2 can be stored in an aquifer in four modes: as a bulk phase within a structural trap, as a residual phase trapped by capillary forces, as aqueous species dissolved in brine, and as a precipitated mineral. The latter three forms of storage are "permanent" in the sense that the CO2 will remain in the aquifer at least as long as the residence time of water in the aquifer. On the other hand CO2 held in a structural trap at large saturations (above residual) is "potentially mobile", in that it will remain trapped only as long as the seal remains intact. Storage methods that reduce the amount of potentially mobile CO2 correspondingly reduce the risk of leakage over the long term. The "inject low and let rise" strategy is one such method [2, 3]. Under typical storage conditions, CO2 is less dense than brine. If CO2 is injected only into the lower part of an aquifer, then after injection ends, buoyancy will cause CO2 to rise into the upper part of the aquifer. As it rises it will leave behind a residual phase trapped by capillary forces. By choosing the volume injected, one can prevent the CO2 from reaching the top of the aquifer. The distance that the CO2 rises depends on the uniformity of the displacement front and the saturation of CO2 behind the front. In this paper we discuss factors that control the former feature. We report on the latter in upcoming publications.