(262b) Instability in Miscible Flow Displacement Involving Flexible Polymer Solutions: Experiments and Stability Analysis

A displacement process involving two fluids is often unstable when displacing fluid is less viscous than the displaced one. The unfavourable viscosity difference generates finger-like patterns around the diffused interface. This instability, termed as viscous fingering, may be desirable (for enhanced micro-mixing) or undesirable (for enhanced oil recovery, filtration etc.) depending on the application. The dynamics of fingering patterns is well understood for the Newtonian displacements. However, strikingly different patterns have been observed when the instability is for the viscoelastic fluids. In order to understand the mechanism responsible for these complex patterns, we attempt to disentangle the influence that different non-Newtonian flow properties have on the instability. To do so, we examine the phenomenon of fingering in different polymer solutions exhibiting both shear thinning and normal stress effects.

The present study performs rheological characterization, stability analysis as well as experimental studies to examine the miscible viscous fingering in the displacement of viscoelastic polymer solutions. First, the role of shear-thinning and elastic features on the onset of viscous fingering instability is analysed using linear stability theory. In comparison to the Newtonian fluid, shear-thinning fluids are found to significantly enhance not only the growth rate but also the critical wavelength of fingering instability. On the other hand, growth rate is found to be greatly reduced due to the elastic feature of the polymer solutions. The subsequent growth of the flow instability at later stage is examined with the aid of experiments in rectilinear Hele-Shaw cell which is a prototype for the homogeneous porous media flow. Shear-thinning feature of the polymer solutions results into more complex, thinner and ramified patterns than for the Newtonian fluids while elasticity makes the fingers smaller and wider. The experimental measurements are found to be good in agreement with the predictions of the stability analysis. Thus, using polymer concentration which influences both viscous and elastic properties as tuning parameter, the extent of viscous fingering instability can be controlled to improve the performance in practical porous media flows.