Interpreting Nuclear Magnetic Resonance Relaxation Measurements in Unsaturated Porous Media

Abstract

Near‐surface geophysical applications of nuclear magnetic resonance (NMR) have primarily focused on interpreting NMR measurements under the assumption that the measured volume is fully saturated; however, as NMR is used for a wider set of near surface applications, an improved understanding of NMR measurements of unsaturated porous media will be needed. Our research focuses on using laboratory measurements to understand the relationship between the NMR relaxation time (T2) and the water content of unsaturated porous geologic material. A limited number of recent studies have determined how the shape and magnitude of the T2‐water content relationship varies with the surface‐area‐to‐volume ratio (S/V) of a porous media. Although it is well known that in fully‐saturated porous material T2 is affected by the surface relaxivity (a parameter that quantifies the ability of a pore surface to enhance relaxation) as well as S/V, there have been no studies that explore the effect of surface relaxivity on the T2‐water content relationship. We will present results from a laboratory study designed to understand the effect of both surface relaxivity and S/V on the T2‐water content relationship. NMR measurements were made on nine samples with varying S/V and surface relaxivity values. S/V was varied by using material with different particle‐size distributions. Surface relaxivity was varied by coating the sands with different concentrations of Fe(III). NMR measurements were collected continuously as water was pumped into dry samples until they were saturated. Our results confirm previous studies and show that T2 is a function of water content. Additionally, we found that both S/V and surface relaxivity affect the shape and magnitude of the T2‐water content curve. Our results indicate that an improved understanding of the effect of surface relaxivity on T2 in unsaturated porous media is critical if NMR is to be used to accurately characterize the vadose zone.