Influence of rheology on buoyancy driven instabilities of miscible displacements in 2D micromodels

Abstract

The stability of miscible displacements of Newtonian and shear-thinning fluids of slightly different densities (Δρ/ρ ≈ 3× 10−4) with a mean flow velocity U is investigated in a 2D transparent network of channels (average width = 0.33 mm). Concentration maps providing information at both the global and local scale are obtained through optical absorption measurements and compared in gravitationally stable and unstable vertical flow configurations; the influence of buoyant flows of typical velocity Ug is characterized by the gravity number Ng = Ug/|U|. For Ng < 0.2, the spreading of the mean relative concentration profile is diffusive for both types of rheologies and characterized by a single dispersivity value ld = D/U. For the Newtonian water-glycerol solution, ld is only the same in the stable and unstable configurations for |Ng| < 0.01. For 0.01 < Ng < 0.2, ld is increased by buoyancy in the unstable configuration and increasingly large front structures are observed on the concentration maps; for Ng > 0.2, front spreading is not diffusive any more. In the stable configuration, in contrast, the front is flattened by buoyancy for Ng < -0.01 and ld reaches values of the order of the length of individual channels. For the shear thinning water-polymer solution, both the concentration maps and the value of ld are the same in the stable and unstable configurations over the full range of U values investigated: this stabilization is explained by their high effective viscosity at low shear rates keeping Ng below the instability threshold even at the lowest velocities.