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 le ngth scales, those methods are not useful. It is shown that, in genera l, PFG NMR data can be used to calculate the probability that diffusin g molecules will return to (or have not left) the vicinity of their in itial positions after a given diffusion time. The return-to-the-origin (RTO) probability thus quantifies the dynamic connectivity of a compl ex pore space. RTO analyses are presented for three natural rock mater ials which were selected on the basis of their widely differing pore c onnectivities. We further use the RTO method to show that molecular di ffusion in the rock samples is restricted anisotropically on the time scale of NMR experiments. (C) 1995 Academic Press, Inc.