Restrictive diffusion from uniform gradient NMR well logging

Nuclear magnetic resonance (NMR) has long been recognized for its potential as a well logging method for measuring pore size and permeability. NMR spin-lattice and spin-spin relaxation times, T₁ and T₂ respectively, are typically used in these applications because they are related to the surface-to-volume ratio of the pore system via the surface relaxivity. Another NMR measurement, restrictive diffusion at short interecho spacings, is also proportional to surface-to-volume ratio but has no dependence on surface relaxivity. A new pulsed NMR logging tool, the MRIL, is capable of making continuous restrictive diffusion measurements because its permanent magnet generates a uniform magnetic field gradient of 25 gauss/cm. Restrictive diffusion logs run in Shell's Johnson City test well clearly show the effect of nearly unrestricted diffusion in the permeable zones. The MRIL logs are confirmed by laboratory NMR measurements of restrictive diffusion on core plugs from the test well. Several NMR relations are then compared for permeability estimation using bound water, T₁, T₂, and restricted diffusion data.