Investigating the effect of internal gradients on static gradient nuclear magnetic resonance diffusion measurements

Abstract

Nuclear magnetic resonance (NMR) methods can be used to measure the diffusion coefficient D of fluids. In porous materials, diffusion of the pore fluid is restricted by pore boundaries, such that D may be smaller than the diffusion coefficient of the bulk fluid. This reduction in D provides information about the geometry of the pore space. Significant overestimates of D can, however, occur due to internal gradients caused by magnetic susceptibility contrasts between the pore fluid and the solid phase. We have investigated the way in which internal gradients can impact the measured diffusion coefficient and obscure the link to pore geometry in unconsolidated sediments. We focus on measurements of D obtained with a static gradient diffusion-editing sequence, which can be used with NMR logging tools to measure D in subsurface sediments. Laboratory measurements of D measured with a diffusion-editing D-T 2 sequence indicate significant impacts from internal gradients, including D values several orders of magnitude larger than D of bulk water. The log-mean D values were found to be highly correlated with estimated internal gradient magnitudes and indicate no clear relationship to pore size. Samples with heterogeneous magnetic susceptibility of the solid phase indicate D distributions with multiple peaks, reflecting the nonuniform distribution of internal gradients in the sediment. We found evidence of high internal gradients impacting a majority of our samples, resulting in increased D values that do not reflect pore size even in samples with low magnetic susceptibility.