Nuclear Magnetic Resonance Measurements on Unconsolidated Sands of variable Grain Size and Mineralogy
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NMR uses a permanent magnet to polarize hydrogen nuclei in water or other fluids. The dephasing time, also termed the relaxation time, or T2, is related to pore body size (T2S), bulk fluid properties (T2B), and fluid diffusion (T2D). In brine saturated, porous sandstones measured in the lab, bulk fluid an diffusional properties (associated with heavy oil and gas, respectively) can be neglected, and the NMR signal is controlled by T2S, the pore size term. Objective: Measure relaxation time in laboratory created sands of variable size and mineralogy, and invert the T2 measurement for pore size distribution. Conclusions. Gelatin has shorter relaxation time than pure water (~3.5s). Relaxation time does not depend on viscosity in this case. Presence of paramagnetic phases result in more rapid decay of the NMR signal that leads to shorter observed T2 times. Presence of paramagnetic phases interfere with the alignment of hydrogen nuclei resulting in the underestimation of porosity. Inversion of NMR T2 for total porosity and pore size distribution must account for changes in mineralogy.