Single fiber transport by a fluid flow in a fracture with rough walls: influence of the fluid rheology

TL;DR

This study experimentally investigates fiber transport in fractured models with rough walls, highlighting how fluid rheology, especially shear thinning fluids, enhances fiber mobility and reduces wall friction.

Contribution

It demonstrates the influence of fluid rheology on fiber transport in rough fractures, showing shear thinning fluids improve fiber mobility and enable transport in narrower regions.

Findings

Fibers move faster with shear thinning fluids.

Transport occurs mainly in larger aperture regions.

Shear thinning fluids reduce wall friction and enable transport in narrower zones.

Abstract

The possible transport of fibers by fluid flow in fractures is investigated experimentally in transparent models using flexible polyester thread (mean diameter $280 \mu\mathrm{m}$) and Newtonian and shear thinning fluids. In the case of smooth parallel walls, fibers of finite length $\ell = 20-150 \mathrm{mm}$ move at a constant velocity of the order of the maximum fluid velocity in the aperture. In contrast, for fibers lying initially at the inlet side of the model and dragged by the flow inside it, the velocity increases with the depth of penetration (this results from the lower velocity - and drag - in the inlet part). In both cases, the friction of the fiber with the smooth walls is weak. For rough self-affine walls and a continuous gradient of the local mean aperture transverse to the flow, transport of the fibers by a water flow is only possible in the region of larger aperture ($\bar{a} \gtrsim 1.1 \mathrm{mm}$) and is of "stop and go" type at low velocities. Time dependent distorsions of the fiber are also often observed. When water is replaced by a shear thinning polymer solution, the fibers move faster and continuously in high aperture regions and their friction with the walls is reduced. Fiber transport becomes also possible in narrower regions where irreversible pinning occurred for water. In a third rough model with no global aperture gradient but with rough walls and a channelization parallel to the mean flow, fiber transport was only possible in shear-thinning flows and pinning and entanglement effects were studied.