Sharp transition to strongly anomalous transport in unsaturated porous media

TL;DR

This paper presents a theoretical framework explaining how partial saturation in porous media causes a sharp transition to strongly anomalous transport due to flow reorganization and heterogeneity, with a predictive transport model.

Contribution

The study introduces a new theoretical model linking phase heterogeneity to flow and transport dynamics in unsaturated porous media, capturing the transition to anomalous transport.

Findings

Flow heterogeneity increases with gas presence in pores.

Velocity distribution shifts, indicating anomalous transport.

The model accurately predicts transport across saturation levels.

Abstract

The simultaneous presence of liquid and gas in porous media increases flow heterogeneity compared to saturated flows. However, so far the impact of saturation on flow statistics and transport dynamics remained unclear. Here, we develop a theoretical framework that captures the impact of flow reorganization on the statistics of pore-scale fluid velocities, due to the presence of gas in the pore space, which leads to the development of a highly-structured flow field. Preferential flow is distributed spatially through the denoted backbone and flow recirculation occurs in dead-end regions branching from it. This induces a marked change in the scaling of the velocity PDF compared to the saturated case, and a sharp transition to strongly anomalous transport. We develop a transport model based on the continuous time random walk theory that successfully predicts advective transport dynamics for all saturation degrees. Our results provide a new modelling framework linking phase heterogeneity to flow heterogeneity and to transport in unsaturated media.