Dynamics of Air Flow in Partially Water-Saturated Porous Media

Dynamics of flowing air in partially water-saturated, porous geological formations are governed by a wide range of forces and parameters. These dynamics are reviewed in the contexts of flow patterns that arise and the corresponding applicability of diverse modeling approaches. The importance of reliable gas-liquid flow models draws from the key role gases play in earth systems, and the various engineering practices involving air injection into geological formations. Here, we focus on air flow in water-wet porous media. We survey the factors that affect flow patterns and phase configurations, and the measures that quantify them. For single-phase flow in saturated media (i.e., air flow in dry media or water flow in water-saturated media), the continuum approach (Darcy's law) is generally applicable and offers a good interpretive tool. However, the coupled two-phase flow continuum approach appears appropriate only for phase-saturation degrees that allow both phases to be continuous in the flow domain. Furthermore, air flow in wet media is highly unstable. As a result, air commonly flows in preferential pathways or in the form of bubbles and ganglia, which are not amenable to continuum modeling. On the other hand, pore-scale models that account for the complex geometries and interfaces between the fluids and the media require extreme computational efforts, and generally inaccessible details on medium characteristics. Other stochastically-based representations, such as percolation theory, have value in the conceptualization of complex flow problems but demonstrate limited success in interpreting phase configurations, saturation degrees, and relative permeabilities.