RELATING NMR RELAXATION TIME DISTRIBUTIONS TO VOID SIZE DISTRIBUTIONS FOR UNCONSOLIDATED SAND PACKS.

Abstract

Nuclear magnetic resonance (NMR) is used in near surface geophysics to understand the pore scale properties of geologic material. The interpretation of NMR data in geologic material assumes that the NMR relaxation time distribution (T2-distribution) is a linear transformation of the void size distribution (VSD). This interpretation assumes fast diffusion and can be violated for materials with high surface relaxivity and/or large pores. In this laboratory study we compare the T2-distribution to the VSD using the particle size distribution (PSD) as a proxy for the VSD. Measurements were collected on water-saturated sand packs with a range of grain sizes and surface relaxivities, such that some samples were expected to violate the fast diffusion assumption. Samples were prepared from silica sand with three different average particle sizes and were coated with the iron oxide mineral hematite to vary the surface relaxivity. We showed analytically that outside the fast diffusion regime, the T2-distributions are broader than in the fast diffusion regime, which could lead to misinterpretation of NMR data. The experimental results showed that, the T2-distributions were not linear transformations of the PSDs. The PSDs were a single peak independent of the hematite coating. The T2-distributions were broader than the PSDs and the center of the distribution depended on the coating. Using an equation that does not assume fast diffusion to transform the T2-distributions to an NMR-estimated VSDs resulted in distributions that were centered on a single radius. However, our attempts to recover the true VSDs were unsuccessful; the NMR estimated VSDs were broader and yielded average pore radii that were much smaller than expected. We conclude that the approach outlined here is useful for determining relative VSDs from T2-distributions; however, additional research is needed to improve our understanding of the link between the T2-distribution and the VSD for unconsolidated sands.