Core analysis by low-field NMR

NMR core measurements have been made customarily in high magnetic fields and have emphasized T₁ for pore-size evaluation or have used chemical shift spectroscopy for quantitative differentiation of oil and water. However, new NMR logging tools use T₂ measurements made at low field strengths, and these measurements should be supported by core analysis. This paper illustrates that low-field NMR T₂ measurements made on water-saturated rock cores can be used to measure total porosity and extract T₂ distributions. The T₂ distributions are shown to be closely related to pore-size distributions. Our experiments demonstrate that low-field measurements are quantitative, even for cases where high-field measurements fail. T₂ distributions from water-saturated plugs are used to estimate producible fluids for sandstones and carbonates and to estimate clay bound water and matrix permeability. Although chemical shift spectroscopy is not feasible at low fields, a method is demonstrated on native state diatomites that permits the measurement of oil, water, and gas saturation. In the case presented, it is also possible to determine in-situ oil viscosity.