In situ determination of surface relaxivities for unconsolidated sediments

Abstract

NMR relaxometry has developed into a method for rapid pore-size determination of natural porous media. Nevertheless, it is prone to uncertainties because of unknown surface relaxivities which depend mainly on the chemical composition of the pore walls as well as on the interfacial dynamics of the pore fluid. The classical approach for the determination of surface relaxivities is the scaling of NMR relaxation times by surface to volume ratios measured by gas adsorption or mercury intrusion. However, it is preferable that a method for the determination of average pore sizes uses the same substance, water, as probe molecule for both relaxometry and surface to volume measurements. One should also ensure that in both experiments the dynamics of the probe molecule takes place on similar length scales, which are in the order of some microns. Therefore, we employed NMR diffusion measurements with different observation times using bipolar pulsed field gradients and applied them to unconsolidated sediments (two purified sands, two natural sands, and one soil). The evaluation by Mitra's short-time model for diffusion in restricted environments yielded information about the surface to volume ratios which is independent of relaxation mechanisms. We point out that methods based on NMR diffusometry yield pore dimensions and surface relaxivities consistent with a pore space as sampled by native pore fluids via the diffusion process. This opens a way to calibrate NMR relaxation measurements with other NMR techniques, providing information about the pore-size distribution of natural porous media directly from relaxometry.

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

“…The values for all samples obtained by the BET method are generally more than threefold higher than those obtained by NMR. This was also observed for different quartz sand and soil materials in a previous study . In addition, we find a dependence of the surface to volume ratios determined by BET on the coating process, which is not detected by NMR diffusometry.…”

“…This was also observed for different quartz sand and soil materials in a previous study. [38] In addition, we find a dependence of the surface to volume ratios determined by BET on the coating process, which is not detected by NMR diffusometry. The reason is that NMR diffusion and relaxation measurements yield values averaged over the distance, which a water molecule covers by diffusion during the observation time.…”

“…In a preceding study, we have shown how the determination of surface relaxivities depends on the experimental approach to measure the surface to volume ratios of unconsolidated sediments. [38] Specifically, we found that the rescaling of relaxation rates into surface relaxivities using surface to volume ratios (S/V)NMR from NMR diffusometry gave more consistent data than using (S/V)BET determined from gas adsorption as the length scales of diffusometry and relaxometry are comparable, but the length scale of gas adsorption measurements is smaller. Furthermore, we could only determine a lower limit of the surface relaxivities because of violations of the fast diffusion assumption.…”

“…NMR relaxation experiments, performed with increasing echo times and at different magnetic field strengths, will be used to identify the effects of magnetic impurities on longitudinal and transverse relaxation time distributions for a series of quartz sand samples with variable coverage of goethite. Secondly, we compare the surface to volume of these samples determined by either NMR diffusometry or gas adsorption (BET), according to the method of our preceding study . Surface relaxivities are finally calculated using both estimates of ( S / V ).…”

“…Secondly, we compare the surface to volume of these samples determined by either NMR diffusometry or gas adsorption (BET), according to the method of our preceding study. [38] Surface relaxivities are finally calculated using both estimates of (S/V). The first question to be answered is which length scale of the (S/V) determination yields suitable surface relaxivities while taking into account the chemical environment at the pore walls.…”