PERMEABILITY ESTIMATION FROM NMR DIFFUSION MEASUREMENTS IN RESERVOIR ROCKS

It is well known that in restricted geometries, such as in porous medi a, the apparent diffusion coefficient (D) of the fluid depends on the observation time. From the time dependence of D, interesting informati on can be derived to characterise geometrical features of the porous m edia that are relevant in oil industry applications. In particular, th e permeability can be related to the surface-to-volume ratio (SN), est imated from the short time behaviour of D(t), and to the connectivity of the pore space, which is probed by the long time behaviour of D(t), The stimulated spin-echo pulse sequence, with pulsed magnetic field g radients, has been used to measure the diffusion coefficients on vario us homogeneous and heterogeneous sandstone samples. It is shown that t he petrophysical parameters obtained by our measurements are in good a greement with those yielded by conventional laboratory techniques (gas permeability and electrical conductivity). Although the diffusing tim e is limited by T-1, eventually preventing an observation of the real asymptotic behaviour, and the surface-to-volume ratio measured by nucl ear magnetic resonance is different from the value obtained by BET bec ause of the different length scales probed, the measurement remains re liable and low-time consuming. (C) 1998 Elsevier Science Inc.