Interface instability of two-phase flow in a three-dimensional porous medium

TL;DR

This study investigates how the interface between two immiscible fluids behaves in a 3D porous medium, revealing how flow rate, medium disorder, and forces influence stability and morphology.

Contribution

It introduces a new stability criterion for fluid interfaces in 3D porous media, incorporating permeability and pressure dynamics, and highlights the role of medium crystallinity.

Findings

Interface morphology varies from unstable fingers to stable sheets with flow rate changes.

The stability criterion involves permeability and pressure derivative analysis.

Crystalline regions in the medium influence interface stability.

Abstract

We present an experimental study of immiscible, two-phase fluid flow through a three-dimensional porous medium consisting of randomly-packed, monodisperse glass spheres. Our experiments combine refractive-index matching and laser-induced fluorescence imaging to resolve the morphology and stability of the moving interface resulting from the injection of one fluid into another. The imposed injection rate sets a balance between gravitational and viscous forces, producing interface morphologies which range from unstable collections of tangled fingers at low rates to stable sheets at high rates. The image data are complemented by time-resolved pressure measurements. We develop a stability criterion for the fluid interface based on the analysis of the 3D images and the pressure data. This criterion involves the Darcy permeability in each of the two phases and the time derivative of the pressure drop across the medium. We observe that the relative permeability encountered by the invading fluid is modified by the imposed flow rate in our experiment, which impacts the two-phase flow dynamics. We show that, in addition to the balance between the relevant forces driving the dynamics, local regions of crystalline order in the beadpack (crystallites) affect the stability of the invading front. This work provides insights into how disorder on multiple length scales in porous media can interact with viscous, capillary, and gravitational forces to determine the stability and dynamics of immiscible fluid interfaces.