Geometrical and Taylor dispersion in a fracture with random obstacles : an experimental study with fluids of different rheologies

A. BOSCHAN, I. IPPOLITO, R. CHERTCOFF, H. AURADOU, L. TALON AND J.P. HULIN

WATER RESOURCES RESEARCH

AMER GEOPHYSICAL UNION

Año: 2008 vol. 44 p. 1 - 11

0043-1397

The miscible displacement of Newtonian or shear-thinning fluids by anotherone of same rheological properties is studied experimentally in a model transparent frac-ture with one flat wall and a random distribution of cylindrical obstacles on the other.Using a dyed fluid allows for a high resolution mapping of the local relative concentra-tion: its variations on individual pixels verify a Gaussian convection-dispersion relation,allowing for the determination of a mean transit time ¯t(x, y) and a local dispersivity ld(x, y).A map of the local front velocities v(x, y) is determined from the spatial variations of¯t(x, y): the correlation length is shown to be independent of the P´eclet number and offluid rheology while the width of the velocity distribution increases both with the rhe-ology and the shear-thinning character of the fluid. The variation of the local dispersionwith the P´eclet number indicates that it results from a combination of geometrical andTaylor dispersion: Taylor dispersion is reduced for shear-thinning fluids and controls dis-persion at high P´eclet numbers. Geometrical dispersion is dominant at lower P´eclet num-bers and is increased for shear-thinning fluids (except at the lowest P´eclet numbers forwhich the shear thinning solutions display Newtonian characteristics).