Effect of capillary heterogeneity on buoyant plumes: A new local trapping mechanism

CO2 can be immobilized in an aquifer as a residual phase trapped by capillary forces. The “inject low and let rise” strategy of storing CO2 in d eep saline aquifers is one method to maximize residual trapping. The more uniform the saturation front during upward movement of CO2, the greater the amount of CO2 trapped in this form. Previous studies suggest that capillary pressure i s a secondary influence on the CO2 plume, and that the permeability field governs the major features of the rising plume. In this work we show that sp atially varying capillary entry pressure can cause qualitatively different plume behavior. To illustrate the behavior, we si mulate buoyancy-driven migration when supercritical CO2 is placed below brine in a heterogeneous domain. After generating geostatistical realizations of permeability, we apply the Leverett J-function so that each grid block has a drainage curve (pc vs Sw) physically consistent with its permeability. The behavior of the displacement front depends strongly on the correlation structure of the heterogeneity and upon the magnitude of entry pressure. In a relatively homogeneous domain, capillar ity is a second-order effect. It damps the tendency toward channeling of the rising CO2 front and increases the uniformity of the plume. As the heterogeneity of the aquifer increases, capillarity begins to dominate buoyancy. Regions with smaller permeability, through which single-phase flow would readily occur, can completely block rising CO2, simply because the capillary entry pressure in these regions is somewhat larger than in neighboring regions. These local capillary barriers prevent CO2 from rising and cause it to mov e laterally. The disruption can be so extreme that above-residual saturations of CO2 accumulate below these barriers. These local accumulations can remain undisturbed when the top seal of the aquifer is breached. Thus we distinguish them as a new mode of CO2 trapping, dubbed “local capillary trapping”. Overall, the displacement front is much less uniform in heterog eneous domains. Despite the nonuniformity, the extent of dissolution trapping remains significant, much more than would be expected considering the ramified structure of the displacement front. T he reduction in residual phase trapping is balanced by an increase in local capillary trapping. Thus the extent of immobilization is similar to that found in previous works.