Investigation of thin surface-wetting films in two-phase saturated porous media

Strongly water-wet porous solids were saturated with two immiscible phases, an aqueous and an hydrocarbon phase, in varying proportions. The porous solids studied were a sample of Fontainebleau sandstone and packs of monodisperse glass beads. The diffusive displacements of the aqueous phase (i.e. the wetting phase) were investigated using pulsed field gradient stimulated echo (PGSTE) NMR to measure the one-dimensional displacement probability distribution P-Delta(X). Measurements of P-Delta(X) are presented for the whole accessible water saturation range. At lower water contents the displacement distributions P-Delta(X) show a distinctive shape, which is attributed to the distribution of the aqueous phase in thin surface wetting films connecting pendular rings where the beads are in contact. The data are reproduced well by a random walk computer model based on such a geometry, the simulated propagators being very sensitive to the surface film thickness. The longitudinal relaxation times of the water phase were also measured and found to deviate from the fast diffusion model at low water saturations. Copyright (C) 2002 John Wiley Sons, Ltd.