Echo tracer dispersion in model fractures with a rectangular geometry

Abstract

We report an experimental study of tracer dispersion in model rectangular fractures with rough or smooth walls and with different mean apertures. We use an echo dispersion technique in which tracer is first injected into the fracture and then pumped back through a detector. In a parallel flow regime, echo dispersion combines a geometrical mechanism due to the fracture roughness and a Taylor mechanism related to the parabolic velocity profile between walls. The latter effect is dominant at high velocities and the dispersivity variations with velocity allow one to determine the effective aperture of the fracture. The low-velocity dispersivity limit that should be related to the geometrical characteristics of the roughness was found to be independent of the mean fracture thickness for the two models that were studied. We show experimentally and numerically that velocity variations in the direction perpendicular to the flow lines result in additional dispersion resulting from molecular diffusion of the tracer particles across the flow lines.