EFFECTS OF GEOMETRICAL COMPLEXITY AND THE MAGNETIC RESPONSE OF MATERIALS ON THE NMR OBSERVABLES IN POROUS MEDIA

Abstract

Observables in nuclear magnetic resonance (NMR) experiments of fluids confined in porous spaces are primarily affected by the geometrical complexity and the magnetic response of the solid matrix and the fluid dynamics inside it. We report a numerical study of a porous system built from 50 random spheres containing brine in cylindrical solid matrix, which can be either quartz (low susceptibility contrast) or sediment (significant susceptibility contrast). By changing a single parameter, the geometrical complexity of the system can be modified. The internal magnetic field distributions in solid and fluid are determined. The results show that in the case of low contrast, a Carr-Purcell-Meiboom-Gill (CPMG) <i>T</i>₂ experiment cannot distinguish the geometrical complexity of the pores, but if contrast is increased, it can. Moreover, we show that, in some cases, the CPMG value obtained for <i>T</i>₂ in a low-contrast experiment corresponds to the average value for the distribution of <i>T</i>₂ calculated from the Brownstein-Tarr model (Brownstein, K.R. and Tarr, C.E., Importance of Classical Diffusion in NMR Studies of Water in Biological Cells, Phys. Rev. A, vol. 19, p. 2446, 1979).