Dynamics of Air Flow in Partially Water‐Saturated Porous Media

Abstract

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.The manner in which air passes through soils depends on its velocity, the flow direction, the amount of resident water in the soil, and the soil characteristics such as grain shapes and void (pore) connectivity. Air flow in soil and its spatial distribution affect many physical, biological, and chemical processes. At the same time, air flow patterns and air distribution in soil indicate the applicability of different modeling approaches. In this contribution, we review the different types of physics governing air flow at the pore scale (microns) and at a larger scale of a few centimeters (commonly termed the Darcy scale). We further present and discuss different modeling approaches at different scales and their limitations in the context of air flow in wet soils. In general, capturing the complex structure of air‐water interfaces and media geometry is restricted by the overwhelming demand of computational resources, while small‐scale models are also limited by the ability to upscale them to a larger (usually more practical) scale. Larger scale models, on the other hand, are limited by poor representation of averaged soil parameters and by the need to conceptualize the air phase as a continuum, especially for upward air flow in wet soils. Capillary, viscous, gravity, and inertial forces, together with medium geometry, control flow patterns, and fluid phase configurations Pore network models are useful in quantifying relative permeabilities but rely on accurate representation of phase connectivity The continuum approach, despite its poor description of air flow, remains the sole method for modeling large‐scale, two‐phase flow