PULSED-FIELD-GRADIENT NMR MEASUREMENTS OF RESTRICTED DIFFUSION AND THE RETURN-TO-THE-ORIGIN PROBABILITY

Previous investigations have shown that pulsed-held-gradient (PFG) NMR measurements can be used to measure a characteristic length scale in monodisperse porous media. In materials with broad distributions of length scales, those methods are not useful. It is shown that, in general, PFG NMR data can be used to calculate the probability that diffusing molecules will return to (or have not left) the vicinity of their initial positions after a given diffusion time. The return-to-the-origin (RTO) probability thus quantifies the dynamic connectivity of a complex pore space. RTO analyses are presented for three natural rock materials which were selected on the basis of their widely differing pore connectivities. We further use the RTO method to show that molecular diffusion in the rock samples is restricted anisotropically on the time scale of NMR experiments. (C) 1995 Academic Press, Inc.