Effect of magnetic pore surface coating on the NMR relaxation and diffusion signal in quartz sand

Abstract

Magnetic impurities are ubiquitous in natural porous media such as sand and soil. They generate internal magnetic field gradients because of increased magnetic susceptibility differences between solid and liquid phase in the pore space and because of the presence of magnetic centers. These internal gradients accelerate NMR relaxation rates and thus might limit the possibility of pore space characterization using NMR. In this study, we investigate the effects of coating the surface of natural sands by the antiferromagnetic iron oxyhydroxide goethite on NMR relaxation and diffusion properties. We found a non-quadratic dependence of the relaxation time distributions on the echo time indicating that the relaxation experiments were not performed in the fast diffusion limit, while the weak dependence on the external magnetic field strength is explained by the preponderance of the surface relaxation over the effect of diffusion in internal gradients. The surface to volume ratio of the pore space, determined by NMR diffusimetry ((S/V) ) remains approximately constant, whereas the same quantity, determined from gas adsorption ((S/V) ) increases proportional to the coating density. This is because gas adsorption measures surface roughness on sub-nanometer scale, whereas NMR diffusimetry averages over structures smaller than few microns. This has consequences for the calculation of the surface relaxivities. The usage of the (S/V) leads to constant values, whereas the usage of (S/V) leads to apparently decreasing relaxivities with increasing coating, which is unrealistic. Copyright © 2016 John Wiley & Sons, Ltd.

Do Goethite Surfaces Really Control the Transport and Retention of Multi-Walled Carbon Nanotubes in Chemically Heterogeneous Porous Media?

“…reported that a similar goethite coating technique produced a 75% surface coverage on the sand. Similarly, Duschl et al reported that our goethite coating technique produced a high goethite surface density of 6 to 27 particles per μm 2 . Collectively, the literature and Figure S1 indicate a high density of goethite particles coated the GQS.…”

Effect of Mineral Surface Properties on Water Behaviors in Pores Constructed by Calcite and Silica Particles

“…Peak 2 arises from the fast exchanges among large pores, and Peak 1 is mainly resulted from the relaxation in very small pores which have slow exchanges with other pores. Similar bimodal phenomena have also been identified in previous studies. ,− The T 2 value of Peak 2 for the calcite specimen is 102.341 ms, while for the silica specimen, it is 95.477 ms at t e = 0 min. As the t e increases, which corresponds to the decrease of water content, Peak 1 and Peak 2 gradually move left and finally merge to one peak when t e is 25 min for calcite and 20 min for silica specimens, and later, the merged peak continues to move left.…”

Application of Low-Field Nuclear Magnetic Resonance (LFNMR) in Characterizing the Dissociation of Gas Hydrate in a Porous Media

“…In the conversions, the ratio of pore volume to pore surface area (V p /S p ) and transverse surface relaxivity (ρ 2 ) are assumed to be r/3 and 30 μm/s. 30 It is observed that, for the three samples, the amplitude of T 2 time in the slow relaxation domain (right side) is much larger than that of the fast relaxation domain (left side).…”

“…To show the pore size distributions (PSDs) of the tested samples, the T 2 relaxation time was converted to the pore radius based on the correlation in eq , then added into Figure with an upper-X-coordinate. In the conversions, the ratio of pore volume to pore surface area ( V p / S p ) and transverse surface relaxivity (ρ 2 ) are assumed to be r /3 and 30 μm/s . It is observed that, for the three samples, the amplitude of T 2 time in the slow relaxation domain (right side) is much larger than that of the fast relaxation domain (left side).…”