Miscible displacement fronts of shear thinning fluids inside rough fractures

ALEJANDRO BOSCHAN, HAROLD AURADOU, IRENE IPPOLITO, RICARDO CHERTCOFF, JEAN-PIERRE HULIN

WATER RESOURCES RESEARCH

AMER GEOPHYSICAL UNION

Año: 2007 vol. 43 p. 1 - 10

0043-1397

The miscible displacement of a shear-thinning fluid by another fluid with the samerheological properties is studied experimentally in a transparent fracture by an opticaltechnique that images relative concentration distributions. The fracture walls havecomplementary self-affine geometries and are shifted laterally in the direction perpendicularto the mean flow velocity U: The flow field is strongly channelized and macrodispersioncontrols the front structure for Pe´clet numbers above a few units. The global front widthincreases therefore linearly with time and reflects the velocity distribution between thedifferent channels. In contrast, at the local scale, front spreading is similar to Taylordispersion between plane-parallel surfaces. Both dispersion mechanisms depend strongly onthe fluid rheology, which shifts from Newtonian to shear thinning when the flow rateincreases. In the latter domain, increasing the concentration enhances the global front widthbut reduces both Taylor dispersion (due to the flattening of the velocity profile in the gap ofthe fracture) and the size of medium scale front structures.