Crossover from viscous fingering to fracturing in cohesive wet granular media: a photoporomechanics study

TL;DR

This study investigates the transition from viscous fingering to fracturing in cohesive wet granular media using photoporomechanics, revealing grain-scale stress patterns and developing a model to explain the crossover driven by fluid and cohesion forces.

Contribution

It introduces a novel experimental approach combining photoelasticity with granular media to visualize stress fields and explains the viscous fingering to fracturing transition with a two-phase poroelastic model.

Findings

Visualization of tensile and compressive force chains during fluid invasion.

Identification of hoop effective stress region behind the invasion front.

A model explaining the crossover based on force competition.

Abstract

We study fluid-induced deformation and fracture of cohesive granular media, and apply photoporomechanics to uncover the underpinning grain-scale mechanics. We fabricate photoelastic spherical particles of diameter d=2mm, and make a monolayer granular pack with tunable intergranular cohesion in a circular Hele-Shaw cell that is initially filled with viscous silicone oil. We inject water into the oil-filled photoelastic granular pack, varying the injection flow rate, defending-fluid viscosity, and intergranular cohesion. We find two different modes of fluid invasion: viscous fingering, and fracturing with leak-off of the injection fluid. We directly visualize the evolving effective stress field through the particles' photoelastic response, and discover a hoop effective stress region behind the water invasion front, where we observe tensile force chains in the circumferential direction. Outside the invasion front, we observe compressive force chains aligning in the radial direction. We conceptualize the system's behavior by means of a two-phase poroelastic continuum model. The model captures granular pack dilation and compaction with the boundary delineated by the invasion front, which explains the observed distinct alignments of the force chains. Finally, we rationalize the crossover from viscous fingering to fracturing by comparing the competing forces behind the process: viscous force from fluid injection that drives fractures, and intergranular cohesion and friction that resist fractures.