Effect of Heterogeneous Capillary Pressure on Buoyancy-Driven CO2 Migration

摘要

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.