Geochemical Controls on Nuclear Magnetic Resonance Measurements

Abstract

Proton nuclear magnetic resonance (NMR) is used in the Earth Sciences as a means of obtaining information about the molecular-scale environment of fluids in porous geological materials. Laboratory experiments were conducted to advance our fundamental understanding of the link between the NMR response and the geochemical properties of geological materials. In the first part of this research project, we studied the impact of both the surface-area-to-volume ratio (S/V) of the pore space and the surface relaxivity on the NMR response of fluids in sandclay mixtures. This study highlighted the way in which these two parameters control our ability to use NMR measurements to detect and quantify fluid saturation in multiphase saturated systems. The second part of the project was designed to explore the way in which the mineralogic form of iron, as opposed to simply the concentration of iron, affects the surface relaxation rate and, more generally, the NMR response of porous materials. We found that the magnitude of the surface relaxation rate was different for the various iron-oxide minerals because of changes in both the surface-area-to-volume ratio of the pore space, and the surface relaxivity. Of particular significance from this study was the finding of an anomalously large surface relaxivity of magnetite compared to that of the other iron minerals. Differences in the NMR response of iron minerals were seen in column experiments during the reaction of ferrihydrite-coated quartz sand with aqueous Fe(II) solutions to form goethite, lepidocrocite and magnetite; indicating the potential use of NMR as a means of monitoring geochemical reactions. The final part of the research project investigated the impact of heterogeneity, at the pore-scale, on the NMR response. This work highlighted the way in which the geochemistry, by controlling the surface relaxivity, has a significant impact on the link between NMR data and the microgeometry of the pore space. Introduction and Research ObjectivesNuclear magnetic resonance (NMR) is a measurement technique, developed in the 1940's, that can provide an incredible amount of information about molecular-scale processes and properties in a wide range of materials. Of specific interest in our research is proton NMR, which investigates the molecular-scale environment of hydrogen nuclei. In the Earth sciences proton NMR is most commonly used to measure the response of water in porous geological materials, as a way of obtaining information about the water content and microgeometry of the water-filled regions of the pore space. Despite many years of the use and study of NMR for borehole logging in the petroleum industry there still remain key questions about the basic mechanisms governing NMR relaxation measurements. As a result, the potential of NMR as a means of determining and studying the physical, chemical and biological properties of Earth materials is far from being fully realized. 2The goal of our research was to advance the fundamental understanding of the link between the NM...