Nuclear magnetic resonance pore size determination for unconsolidated sediments with strong internal gradients

Abstract:

Water distribution and flow within porous media are mainly controlled by the pore space structure. Well established methods for the determination of pore sizes like multistep outflow and gas adsorption isotherms (BET) are often time consuming, expensive, or produce toxic waste. As an alternative fast and non-destructive technique, nuclear magnetic resonance (NMR) is used because it probes hydrogen and therefore the dynamics and interactions of water.

Pore space is most easily characterized by NMR relaxometry where the total relaxation rate is controlled by the surface relaxivity ρ of the porous medium in combination with the surface-to-volume ratio (S/V) [1]. Furthermore, there are contributions of molecular diffusion through local magnetic field gradients which are created by susceptibility differences between solid and liquid phases [2] as well as by paramagnetic impurities [3]. Hence, surface to volume ratios and surface relaxivities of porous media cannot be measured individually with NMR relaxometry. Therefore, NMR diffusion measurements are applied to probe the S/V of pores without other contributions.

In this study, we demonstrate that NMR diffusion measurements are feasible to determine the S/V ratio of the pore space of quartz sand coated with goethite (α-FeOOH) as paramagnetic impurity. Our findings were compared to BET measurements and we found no dependence of the S/V on the coating density with NMR diffusion and a clear dependence between coating density and S/V with krypton BET measurements. Possible explanations are the different characteristic length scales on which the pore space is probed, and the intrinsic fractal nature of porous media [4] together with the roughness of the pore surface on a nm-scale due to the coating process. After isolating the additional contribution of the paramagnetic impurities to the NMR relaxation and the calibration of the NMR relaxation signal for each coating density it was possible to use fast relaxometry experiments to obtain information about the pore size distribution.

[1] Grunewald E. and Knight R., Geophysics, Volume 76(4), G73-G83, (2011)

[2] Stingaciu L. R. et al., Water Resources Research, 45, W08412, (2009)

[3] Anand V., and Hirasaki G. J., J Magn Reson, 190(1), 68-85, (2008)

[4] Stallmach F. et al., Phys Rev Lett, 88(10), 105505, (2002)