Imbibition fronts in porous media: effects of initial wetting fluid saturation and flow rate

We report a series of imbibition experiments in a porous medium, in which the progression of the imbibing fluid (water) displacing oil was followed by means of CT-scan measurements. These experiments, carried out at various fixed injection rates (or capillary numbers), consisted in measuring the water saturation profiles as a function of time. At low rates, the advance of the saturation front proceeds in a hyperdispersive manner, i.e., the water front spreads more rapidly than a tracer obeying normal (Gaussian) dispersion. By contrast, a hypodispersive behavior is observed at high rates, with a front that spreads less rapidly than does a tracer. By analyzing the early time dependence of the saturation profiles, we are able to characterize the divergence, at low water saturation, of the saturation-dependent capillary dispersion coefficient. This coefficient diverges like a negative power of saturation, with an exponent that strongly varies with flow rate. Those results were obtained for secondary imbibition experiments, in which the initial presence of water in the porous medium seems to be responsible for the strong hyperdispersion observed at low rate.