Gravity driven instabilities in miscible non-Newtonian fluid displacementsin porous media

Gravity driven instabilities in model porous packings of 1 min diameter sph eres are studied by comparing the broadening of the displacement front betw een fluids of slightly different densities in stable and unstable configura tions. Water, water-glycerol and water-polymer solutions are used to vary i ndependently viscosity and molecular diffusion and study the influence of s hear-thinning properties. Both injected and displaced solutions are identic al but for a different concentration of NaNO3 salt used as an ionic tracer and to introduce the density contrast. Dispersivity in stable configuration increases with polymer concentration - as already reported for double poro sity packings of porous grains. Gravity-induced instabilities are shown to develop below a same threshold Peclet number Pe for water and water-glycero l solutions of different viscosities and result in considerable increases o f the dispersivity. Measured threshold Pe values decrease markedly on the c ontrary with polymer concentration. The quantitative analysis demonstrates that the development of the instabilities is controlled by viscosity throug h a characteristic gravity number G (ratio between hydrostatic and viscous pressure gradients). A single threshold value of G accounts for results obt ained on Newtonian and non-Newtonian solutions. (C) 2001 Elsevier Science B .V. All rights reserved.