Abstract

A laboratory study was conducted to measure the effect of the mineralogic form and concentration of iron(II) [Fe(II)] minerals on nuclear magnetic resonance (NMR) relaxation rates of water-saturated sand mixtures. We measured mixtures of quartz sand and three common Fe(II)-bearing minerals in granular form: siderite (FeCO3), pyrite (FeS2), and pyrrhotite (Fe1xS; 0<x<0.2) at two concentrations of iron by weight. The NMR response of these samples was used to calculate four transverse relaxation rates for each Fe(II) mineral mixture: total mean log, bulk fluid, diffusion, and surface relaxation rates. The surface area of the samples was used to calculate the surface relaxivity of the sample and the magnetically active surface. For each iron mineral, the mean log and surface relaxation rates were greater for samples with higher Fe(II) concentration. For the siderite, pyrrhotite, and high-concentration pyrite mixtures, surface relaxation was the dominant relaxation mechanism. Bulk fluid relaxation contributed significantly to the total relaxation for the siderite and pyrite mixtures; for the low-concentration pyrite mixtures, bulk fluid relaxation was the dominant relaxation mechanism. For the pyrrhotite mixtures, the diffusion relaxation rate was nonzero and slower than the surface relaxation rate; for the siderite and pyrite mixtures, the diffusion relaxation rate was zero. Surface relaxivity calculations revealed that, for the pyrite mixtures, relaxation occurred in the fast diffusion regime; for the siderite and pyrrhotite mixtures, relaxation did not occur in the fast diffusion regime. The range of surface relaxivity values calculated depends on mineralogic form. We conclude that Fe(II) concentration and mineralogic form are important factors in determining relaxation rate.

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