Viscous fingering instability in flow through porous media
17th, July 2019, Thrusday, 4:30pm, ChE Seminar hall
In flow displacement through porous media, one often encounters a situation when one fluid is displaced by another fluid, either immiscible or miscible with the fluid being displaced. If the displacing fluid is less viscous than the displaced fluid, the interface becomes unstable leading to viscous fingering. The instability plays an important role in many industrial and natural processes such as, enhanced oil recovery (EOR), 2 sequestration, chromatographic separation and micro-mixing, to name a few. In most of the instances, the instability is undesirable as it spoils the displacement efficiency. To control the instability, viscosity difference between the two fluids can be changed by modifying the viscous properties of the displacing fluid. Polymers are an effective way to increase the viscosity and hence can be added to the displacing fluid to reduce the viscosity contrast (viscosity ratio of two fluids) which is the main cause of viscous fingering. Most of the polymer solutions exhibit non-Newtonian features, for instance, shear-rate dependent viscosity and elasticity. These features can affect the morphology of the fingering due to the non-uniform viscosity distribution (depends on the shear-rate) and relaxation of the stretched polymer chains. To investigate the role of rheological properties on the onset of instability, linear stability analysis is performed for the flow system when one of the fluids is viscoelastic. Experiments are performed to investigate the non-linear behaviour of the fingers at longer times. Viscoelastic fluids (aqueous solution of polyethylene oxide (PEO) and polyacrylamide (PAM)) having different rheology are used to find the role of shear-thinning viscosity and elasticity on the fingering patterns. Evolution of the fingers shows that strong shear-thinning property creates more complex and ramified patterns involving splitting and side-branching as compared to the Newtonian displacements. To know the impact of elasticity, flow displacements of Boger fluids (purely elastic with constant viscosity) by Newtonian fluids are examined and compared with the Newtonian-Newtonian displacements. The fingering patterns are observed to be similar in both the cases. This may be due to the low values of Weissenberg number (varies between 0.08 and 1.05) based on the convective time scale used in the experiments.