A method for measuring the diffusivity of a liquid into a porous matrix

Abstract

A method for measuring diffusivity of a liquid into a saturated porous matrix is introduced. The experiment involves monitoring, by the “whole sample” NMR, the diffusion of D2O water into a H2O-saturated porous cylinder indirectly by detecting the proton NMR signal as a function of immersion time. An H2O-saturated hardened cement cylinder was immersed into a D2O bath, and the diminishing proton magnetization was monitored over 400 h of immersion. The average radial diffusivity which characterizes this porous medium was found to be 1.9×10−7 cm2/s (±10%), which is two orders of magnitude smaller than the water diffusion constant. The proton spin-spin relaxation time and its stretched exponential parameter α of water were found to be independent of immersion time. This means that, throughout the porous medium, the pores of different size are uniformly distributed. The movement of a radial advection front of D2O towards the center of the porous cylinder describes the phenomenon only for short immersion times. If this process is monitored by magnetic resonance imaging, a distinct dividing line between the D2O advection “front” and H2O is observed only during the beginning of the process. As the diffusion proceeds, the line between H2O and D2O domains becomes progressively more blurred. This problem is avoided if the signal of the whole cylinder is recorded, as in the whole-sample NMR.