Scaling of capillary trapping in unstable two-phase flow: Application to CO[subscript 2] sequestration in deep saline aquifers

Abstract

The effect of flow instabilities on capillary trapping mechanisms is a major source of uncertainty in CO2 sequestration in deep saline aquifers. Standard macroscopic models of multiphase flow in porous media are unable to explain and quantitatively predict the onset and structure of viscous-unstable flows, such as the displacement of brine by the injected CO2. We present the first step of a research effort aimed at the experimental characterization and mathematical (continuum) modeling of such flows. Existing continuum models of multiphase flow are unable to explain why preferential flow (fingering) occurs during infiltration into homogeneous, dry soil. We present a macroscopic model that reproduces the experimentally observed features of fingered flows. The proposed model is derived using a phase-field methodology and does not introduce new independent parameters. From a linear stability analysis, we predict that finger velocity and finger width both increase with infiltration rate, and the predictions are in quantitative agreement with experiments.