PORE-SIZE DISTRIBUTIONS AND TORTUOSITY IN HETEROGENEOUS POROUS-MEDIA

The diffusion coefficient, measured at long observation times by pulse d-held-gradient NMR, can in principle be used to estimate the tortuosi ty of a porous medium. This method is useful for glass-sphere packs, b ut we find that it does not generally work for porous sedimentary rock . Natural sedimentary rocks are characterized by complex microgeometri es and broad distributions of pore sizes, which cannot be adequately s ampled by diffusing molecules in experimentally accessible observation times. The time-dependent diffusion coefficient D(t) can be distinctl y irregular for rocks with very large pores. In heterogeneous porous m edia, determination of pore-size distribution by relaxation-time measu rements and tortuosity by PFG diffusion measurements are mutually excl usive. (C) 1995 Academic Press, Inc.