Quantitative analysis of diffusion regimes in iron ore with low field NMR

Abstract

SUMMARY The use of nuclear magnetic resonance (NMR) techniques allows in situ characterization of geophysical properties such as moisture content, permeability and wettability. However, the accuracy and applicability of such measurements is limited by internal magnetic field gradients which are a consequence of magnetic susceptibility differences at solid–fluid interfaces. Such effects are particularly prominent in iron ore rock samples which contain ferrimagnetic and ferromagnetic mineralogy leading to high magnetic susceptibility. Multiple echo time Carr–Purcell–Meiboom–Gill (CPMG) NMR pulse sequences are commonly used to capture the influence of internal gradients, with the intention of deconvoluting diffusion in effective internal gradients (geff) from true transver relaxation (T2). The interpretation of such measurements is complicated by the presence of multiple diffusive regimes: the short-time (ST), motionally averaged and localization regimes respectively. We introduce a new model for diffusive NMR signal attenuation, called the multiregime model which is intended to better capture diffusive behaviour across the three regimes. The multiregime model is compared against previous methods for quantifying diffusive decay (the ST only and generalized inversion models). Multi-echo measurements of iron ore samples are fit with each model in order to quantify 2-D T2–geff distributions. The resulting distributions demonstrate how the multiregime model can provide insight into the relative influence of the different diffusive regimes in a given sample. This assists in understanding the influence of diffusive decay on measurement accuracy, for example the increased measurement error with increasing prevalence of the localization regime. The multiregime model provides a key step in accurately segregating surface relaxation and diffusive relaxation, which is crucial for accurately estimating pore size distributions, permeability and wettability in high magnetic susceptibility samples using NMR.