Validation of NMR relaxation exchange time measurements in porous media.

Two-dimensional T(2)-T(2) NMR relaxation exchange spectroscopy has been applied to model porous media composed of mixtures of nonporous borosilicate and soda lime glass spheres in water. The spheres had a mean diameter of 100 microm, thus providing an approximately constant characteristic pore dimension throughout the structures, while the use of two glass types ensured that water in different pore-space regions had significantly different T(2) relaxation rates. The packed beds were constructed in various ways with controlled glass type domain sizes to rigorously validate a model for region-to-region exchange of water. From the determined exchange times, the corresponding length scales were calculated based on the molecular self-diffusion of water; these agreed to better than +/-25% with the expected domain sizes. Furthermore, exchange distances on the order of the pore size were observed in thoroughly mixed systems. Depending on the relaxation rates present in the sample, this technique can provide estimates of length scales ranging from microns to millimeters.