Abstract
Solute transport in unsaturated porous materials is a complex process, which exhibits some distinct features differentiating it from transport under saturated conditions. These features emerge mostly due to the different transport time scales at different regions of the flow network, which can be classified into flowing and stagnant regions, predominantly controlled by advection and diffusion, respectively. Under unsaturated conditions, the solute breakthrough curves show early arrivals and very long tails, and this type of transport is usually referred to as non-Fickian. This study directly characterizes transport through an unsaturated porous medium in three spatial dimensions at the resolution of 3.25 μm and the time resolution of 6 s. Using advanced high-speed, high-spatial resolution, synchrotron-based X-ray computed microtomography (sCT) we obtained detailed information on solute transport through a glass bead packing at different saturations. A large experimental dataset (>50 TB) was produced, while imaging the evolution of the solute concentration with time at any given point within the field of view. We show that the fluids' topology has a critical signature on the non-Fickian transport, which yet needs to be included in the Darcy-scale solute transport models. The three-dimensional (3D) results show that the fully mixing assumption at the pore scale is not valid, and even after injection of several pore volumes the concentration field at the pore scale is not uniform. Additionally, results demonstrate that dispersivity is changing with saturation, being twofold larger at the saturation of 0.52 compared to that at the fully saturated domain.
Highlights
Erogeneous porous media, since the available pore space does not homogeneously contribute to flow; some regions are dead ends or very slow in transport due to poor connectivity to the main flow pathways
This research has two key impacts: 1) we demonstrate a workflow for direct characterization of solute transport in any porous material, which allows better design of fabricated porous materials and detailed characterization of natural porous materials, and 2) we evaluate the underlying assumptions used in the existing theories and identify the major gaps in theories that need to be addressed for better predictive modeling
In an effort to improve our understanding of the fundamental physical processes occurring during two-phase flow and transport, this paper has three main objectives which have been achieved and are reported: 1) an imaging technique has been established to evaluate the time-resolved three-dimensional (3D) concentration fields in a partially saturated porous medium, 2) a unique dataset has been generated that can be used for validation of the existing theories of transport in unsaturated porous media, and 3) insights into the effect of the pore-scale fluids topology on the non-Fickian transport in a 3D system are provided
Summary
Erogeneous (fractured) porous media, since the available pore space does not homogeneously contribute to flow; some regions are dead ends or very slow in transport due to poor connectivity to the main flow pathways. On the Darcy scale, where pore space properties (e.g., porosity, saturation, and permeability) are averaged, advective transport leads to solute dispersion as a consequence of tortuous and spatially variable flow pathways in porous media This becomes even more complex under two-phase flow conditions, as well as in het-. In an effort to improve our understanding of the fundamental physical processes occurring during two-phase flow and transport, this paper has three main objectives which have been achieved and are reported: 1) an imaging technique has been established to evaluate the time-resolved three-dimensional (3D) concentration fields in a partially saturated porous medium, 2) a unique dataset has been generated that can be used for validation of the existing theories of transport in unsaturated porous media, and 3) insights into the effect of the pore-scale fluids topology on the non-Fickian transport in a 3D system are provided
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