Monitoring of tracer dispersion in porous media by NMR imaging

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Abstract
The dispersion of a paramagnetic tracer has been measured by NMR imaging in a non-consolidated porous medium made of 2 and 6 mm diameter glass beads. The technique allows one to obtain 3D images of the tracer repartition as well as instantaneous 1D concentration profiles along the flow axis. Flow reversibility was checked by examining concentration profile variations in both forward and backward flows. Dispersion coefficients were obtained by fitting 1D profiles with the solutions of the convection diffusion equation. They are completely consistent with usual results on bead packings for experiments performed with tracer injection stable with respect to gravity. Moreover, a total irreversibility of the dispersion is established after displacement lengths of a few bead diameters. The tracer distributions as observed from 3D images differ markedly in stable and unstable experiments. For gravitationally unstable injections, on both profiles and images, an additional, partly reversible, convective spreading was observed-it is probably due to permeability inhomogeneities of the bead packing.