Experimental Study of Fingering Flow in Porous Hele-Shaw Cells 

Fereidoun Rezanezhad, Hans-Jörg Vogel, Kurt Roth

Background

Method

Image Processing

Observations

Publications

Background: The fluid flow through preferential paths is important in the hydrological processes of infiltration through the soil profile. Unstable fingering flow of water during infiltration into unsaturated porous media belongs to the class of preferential flow. Fingered flow in porous media was studied for many years, both in the petroleum industry to better understand oil recovery and in the environmental sciences as an instance of preferential flow of water through soil. We have a quite complete description of the general phenomena.  Fingered flow is important for some practical issues, e.g., for rapid contaminant transport with irrigation in many arid regions. However, it also illuminates our very understanding of the physics of multiphase flow in porous media. Indeed, the current theory, Richards' equation ,S cannot explain the existence of such fingers.

With the aim of studying the physical process concerning of the fingering phenomena in two dimensions, experiments of vertical infiltration through multi-layered sand were carried out in the laboratory using Hele-Shaw cells. Wetting front instability, or unstable gravity-driven fingering, can occur during vertical infiltration through initially dry, coarse-textured, and uniform porous media when the infiltration rate is below the saturated hydraulic conductivity (Ks) and gravitational influences on the imbibing solution dominate the forces of capillary. We established such conditions in a Hele-Shaw cell , 160 × 60 × 0.3 cm, where a layer of fine sand was placed on top of a coarse sand. We study the dynamics of water saturation in fingering flow for different flow conditions. High-resolution measurements of the water saturation are obtained from imaging technique, Light Transmission Method (LTM) that have been calibrated by X-ray transmission.

The film (left) shows a typical experiment in a 3-layer system with a fine sand on top, a homogeneous coarse sand in the middle and a structured sand at the bottom. The infiltration into the fine sand is homogeneous and breaks into fingers (bright areas) at the transition to the homogeneous coarse sand. The fingers are destroyed as soon as the material becomes structured (bottom). Additionally, after stabilization of the fingered flow pattern, we applied a dye tracer to visualize the velocity field within flow fingers and in order to distinguish mobile and immobile water fractions.

The overall objectives of the this project has been to advance:

1) Infiltration experiments in a large Hele-Shaw cell to observe fingering flow patterns during redistribution.

2) Our understanding of the physical processes of the special and interesting case of water infiltration through porous media (fingered flow).

3) Realistically describing this type of flow using the relevant physical processes.

4) Understanding of two crucial phenomena for fingering flow (i) the observed saturation overshoot which initializes a finger and (ii) the hysteresis of the soil-water characteristic which stabilizes it by dramatically reducing lateral flow.

5) Application of a visualization and monitoring method (LTM) with high spatial and temporal resolution to characterize multiphase and transient flow in porous media.