Effect of Aquifer Heterogeneity on CO2 Sequestration

Abstract This paper investigates experimentally the effect of heterogeneity on buoyancy-driven flow in CO2 storage. The application of this work is for the storage of CO2 in aquifers, where the degree of heterogeneity affects its trapping capacity. The experimental set up idealizes the post-injection period when the migration of CO2 plume in an aquifer is mainly due to the buoyancy drive that results from the density contrast between the injected CO2 and formation brine. A column of air was trapped beneath a brine-saturated sand pack at room temperature and the whole arrangement was placed in an Amott cell. The Amott cell was filled with brine to account for background regional groundwater support. The amount of air beneath the sand pack and the cumulative air production were measured throughout the experiments to determine the sand pack air saturation. As the air plume migrates up the sand pack, the buoyancy drive decreases. This causes some regions in the sand pack that once permitted the upward migration of the air plume to serve as capillary barriers to the trailing air bubbles. In all the studies, the uniformity of the air front depends on the degree of heterogeneity in the sand pack. However, air migration through a homogeneous sand pack provided a more uniform front than the heterogeneous sand pack. The presence of a high-permeability streak in the sand pack reduced the trapping capacity of the entire pack. The presence of a low-permeability layer in the sand pack increased its trapping capacity. Irrespective of the degree of heterogeneity, a substantial amount of air was trapped in and below the sand packs (about 40% of the initial air volume). The results also showed that better consolidated sands trapped more air than poorly consolidated sands. Pulsing flows (fluctuation of air saturation with time) occurred in some experiments after air breakthrough.