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Abstract

We examine buoyancy-driven multiphase flow when the less dense phase is placed below the other phase 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 (P_c vs S_w) 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 the mean entry pressure. This behavior is of particular interest for assessing the degree of immobilization of anthropogenic CO₂ injected into an aquifer.

In a relatively homogeneous domain, capillarity is a second-order effect. It damps the instability of the rising CO₂ front and smooths the shape of the plume. As the heterogeneity of the aquifer increases, capillarity begins to dominate buoyancy. Regions with smaller permeability that would readily conduct single-phase flow 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 move laterally. The disruption can be so extreme that above-residual saturations of CO₂ become trapped below these barriers. These local accumulations respond differently when the top seal of the aquifer is breached. Thus we distinguish them as a new mode of CO₂ trapping, dubbed “capillary trapping.” Overall, in some regions the CO₂ follows preferential flow paths determined by the spatial correlation of permeability, while in others capillarity determines the flow path. Though the displacement front is much less uniform, the extent of dissolution trapping remains significant.

Introduction

Consumption of fossil fuel (natural gas, petroleum, and coal) is increasing the total load of carbon dioxide in the atmosphere. Although the long-term consequences of these changes are still debated, one likely outcome is the alteration of global climate as greenhouse gases trap heat at the earth's surface. One possible response is to capture gasses after combustion and inject them into subsurface formations where they will be retained for geological periods of time. Reliable methods that ensure stored CO₂ remains in place are therefore desirable.

Immobilization of CO₂ as a residual disconnected phase trapped by capillary forces, and as aqueous species dissolved in brine are two modes of sequestration that decrease the risk of leakage. Previous studies have studied the effectiveness of residual phase trapping (Kumar et al. 2004; Ozah et al. 2005; Mo and Akervoll 2005; Hesse et al. 2006). One way to maximize residual trapping is the “inject low and let rise” strategy (Kumar et al. 2004). When CO₂ is injected in deeper parts of aquifer, buoyancy forces drive the injected CO₂ upward, since CO₂ is less dense than brine under typical storage conditions. As it rises, a residual phase trapped by capillary forces remains in the rock previously occupied by large CO₂ saturation. The intrinsically unstable character of buoyancy-driven flow does not govern the displacement; instead, the CO₂ follows preferential flow paths determined by the spatial correlation of permeability in the aquifer (Bryant et al. 2006). The behavior is then better referred to as channeling, not fingering.

Simulations of the “inject low and let rise” scenario using a single P_c-S_w curve in a heterogeneous permeability field indicate that capillary pressure always smooths a rising CO₂ front. A uniform displacement front allows a large volume of CO₂ to rise but not reach the top of the formation, thereby maximizing the amount of trapped CO₂. In this situation, capillarity increases the security of CO₂ storage.

However, capillary entry pressure of a rock is correlated with its permeability (Leverett 1941), and the use of a single drainage curve in a heterogeneous formation is physically inconsistent. Moreover, heterogeneity that leads to a small decrease in permeability in the vertical direction would cause the capillary entry pressure to increase in that direction. This can be sufficient to block the rising CO₂, which must instead accumulate or move laterally.