Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite

Abstract

Kunigel-V1 bentonite was analyzed by proton nuclear magnetic resonance (NMR) spectroscopy over a wide range of water content and temperature at 0.47 T. Kunigel-V1 is a bentonite clay consisting of ~50 wt% Na-rich montmorillonite from Yamagata, Japan, and represents a candidate for engineered barriers of underground nuclear waste disposal sites in Japan. The NMR-related properties of bentonite are also important with respect to application of the material as a mud in boreholes for NMR well logging in the geophysical exploration of disposal sites. The proton relaxation times, surface relaxivity of montmorillonite, and H2O self-diffusion coefficient, were determined in this study for water-rich gel samples of bentonite at 11.0 to 70.0°C and for bentonite weight fractions of 0 to 37.7 wt%. The proton relaxation times (T1 and T2) were measured by the inversion recovery method and Carr-Purcell-Meiboom-Gill method, respectively, and the self-diffusion coefficient of H2O molecules (D) was measured by the pulsed-gradient spin-echo method. The results showed that T1, T2, and D increased with increasing temperature, and decreased with increasing bentonite weight fraction (w). The T1 and T2 surface-relaxivities were on the order of 10–7 m/s and also decreased with temperature. The activation energies of the T1 and T2 relaxation for the bentonite gels were significantly lower than those for bulk water (i.e., about 50 to 70%), whereas the activation energy of the diffusion process for the gels was nearly equal to that for bulk water. As a result, the normalized H2O self-diffusivity, D/D 0, obeys a temperature-independent master curve described by In(D/D 0)=1.54[exp(—0.0377w)— 1], where D 0 is D in bulk water and w is in wt%.