Investigating internal magnetic field gradients in aquifer sediments

Abstract

Internal magnetic field gradients in porous materials, if sufficiently large, can be a source of error in nuclear magnetic resonance (NMR) measurements of the transverse relaxation time [Formula: see text] and the diffusion coefficient [Formula: see text]. Given that these measurements can provide information about the pore fluid and the pore geometry, it is important to determine the magnitude of internal gradients and assess their potential impact. We estimated the effective internal gradients in aquifer sediment samples using three methods. We used a 2D NMR method to map the distribution of internal gradients versus [Formula: see text] and found gradients up to [Formula: see text] with peak gradient values in the range of [Formula: see text] for most of the samples. The average effective gradient values, calculated from the slope of the mean log relaxation rate versus the squared echo time, typically fell above the peak gradient values in the 2D distributions, with a range from 12 to 230 G/cm. The maximum effective gradients, calculated from the magnetic susceptibility of the samples, were found to be the upper bounds for most of the gradient distributions. The mean gradient was found to increase with increasing magnetic susceptibility of the sample; however, pore size was also found to impact gradient magnitudes. Given that the distribution of internal gradient magnitudes is determined by the properties of the sediment and by the magnitude of the background field, our results have implications for the acquisition of logging and surface NMR data. We expect the internal gradients in many aquifer sediments to impact NMR logging measurements; this should be considered when selecting logging parameters and interpreting NMR logging data. In contrast, we expect internal gradients to have a negligible impact on surface NMR measurements because of the much smaller magnitude of the background magnetic field.