Direct Measurement of Pore Size and Surface Relaxivity with Magnetic Resonance at Variable Temperature

A variety of magnetic resonance relaxometry methods have been developed to determine pore size in porous rocks. Uncertainties in pore-size estimates may occur due to uncertainty in the relaxation-diffusion regime. We have developed a direct and rapid one-dimensional method based on Brownstein-Tarr theory to help remedy this problem. The correlation between magnetic resonance relaxation behavior and the temperature-dependent self-diffusion coefficient of pore fluids was employed to estimate the pore size and surface relaxivity of a series of reservoir rocks. Relaxation is anticipated to depend on diffusion in the intermediate regime of Brownstein-Tarr theory. Water-saturated glass-bead packs were employed in initial Carr-Purcell-Meiboom-Gill experiments at variable temperature. The calculated pore size matches the estimated geometric pore size. The proposed method was applied to determine the pore size of Berea, Buff Berea, and Nugget sandstones. The pore sizes determined with the three pore geometries are in good agreement with scanning electron microscopy and micro-computed-tomography measurements. The experimentally observed changes in relaxation times and their corresponding intensities indicate an intermediate Brownstein-Tarr regime for all systems examined in this work.