Disentangling the Complex Pore-Scale Dispersion Process in Natural Porous Media by Means of DNS

Abstract:

Over the recent past, important advances have been made in the area of micro-CT scanning of natural porous media. The acquired pore-space geometries enable detailed investigations of flow and transport via pore-scale direct numerical simulation (DNS). In this work, we utilize pore-scale DNS to investigate single-phase pore-scale dispersion. We focus on data stemming from beadpacks, Bentheimer sandstone, and Ketton and Estaillades carbonates. Our DNS results clearly show the transition from ballistic dispersion to the asymptotic Fickian regime (see figure a) at decimeter or meter scale depending on the medium type. We outline a universal Lagrangian model for tracer dispersion that is based on a low-dimensional parametrization of the complex three-dimensional motion of tracer particles (see figure b). We relate the process parameters to certain pore-geometry characteristics such as the tortuosity. Our model accurately captures the wide range of flow and transport dynamics observed in the samples considered. We establish the accuracy of the model by validating its limiting dispersion behavior, the resulting velocity statistics, and also, most challenging, snapshots of tracer plumes at travel times encompassing both ballistic and Fickian behavior.