fast diffusion

1. n. [Formation Evaluation]
The rapid rate of diffusion of molecules in pore fluids during a nuclear magnetic resonance (NMR) measurement. In fast diffusion, the hydrogen within a certain volume diffuses fast enough that only one T2 peak is observed for the whole volume. This is the case in a single pore, because the surface relaxation is not strong enough for observation of separate T2 peaks, for example, for water near the surface of a grain and water in the middle of the pore. Fast diffusion is also considered to occur between most clay- and capillary-bound water, between normal pores and micropores within some carbonates, and in some other systems.