A laboratory study of the effect of magnetite on NMR relaxation rates

We conducted a laboratory study to measure the effect of magnetite concentration and grain size on proton nuclear magnetic resonance (NMR) relaxation rates of sand mixtures and to determine the dominant mechanism by which relaxation occurs. We measured mixtures of quartz and three different forms of magnetite: a powdered synthetic magnetite; a small-grained, natural magnetite; and a large-grained, natural magnetite. The powdered synthetic magnetite was mixed with quartz in five concentrations ranging from 0.14 to 1.4% magnetite by weight; both sizes of natural magnetite were mixed with quartz in concentrations of 1 and 2% magnetite by weight. The NMR response of the water-saturated samples was measured and used to calculate four averaged relaxation rates for each magnetite concentration: the total mean log, bulk fluid, surface, and diffusion relaxation rates. The results of this study show that: 1) surface relaxation was the dominant relaxation mechanism for all samples except the powdered synthetic magnetite sample containing 1.4% magnetite; 2) the surface relaxivity is a function of the fraction of the surface area in the sample composed of magnetite; 3) there is no clear dependence of the diffusion relaxation rate on the concentration of magnetite.