Topological characteristics and channel properties of porous media that underpin anomalous transport

Abstract:

Understanding how flow and transport through porous media are regulated by structural features of the pore space is a problem of central concern for many environmental matters. Nevertheless, predictions of flow behavior in heterogeneous porous media from measurable structural properties remain a challenge, given that the relationship between structure and function is tenuously understood. The work herein presented identifies key relationships between pore structure and intermittent flow, which give rise to anomalous transport in porous media. To do this, flow is first determined in transparent porous structures of varying heterogeneity with a 3D Particle Tracking Velocimetry method, from which Lagrangian measurements are retrieved. Second, the structural features of the pore space are characterized from X-ray micro Computed Tomography images analyzed in a network framework. Such an approach permits detailed extraction of the topology and channel properties of the structure. Lastly, a physically parameterized Continuous Time Random Walk model is developed to evaluate the identified relationships between structural features and flow. Analyses of flow dynamics reveal distinguishing features of intermittency for all media (e.g., intermittent Lagrangian velocity and acceleration, heavy tailing of velocity PDFs, and sub-ballistic displacement), with structures of greater complexity exhibiting more intense effects. Structural analyses demonstrate that heterogeneity of the pore space is manifested at a variety of scales that do not necessarily influence anomalous transport. We show that classic metrics on porosity and pore (or grain) size distribution are insufficient to explain the observed features of the flow. Rather, we propose that flow intermittency –and therefore anomalous transport— is governed by the characteristic mean node degree of the system that describes how pores are interconnected, and that the effect is intensified by the presence of long but wide channels that act as expeditious conduits for flow.