Sodium-23 NMR in porous media

The enhancement in relaxation of H-1 spins adsorbed at pore walls is well known, described by the famous solutions of Brownstein and Tarr to the Bloch-Torrey diffusion equation with a partially absorbing boundary. Sodium-23 is an alternative nucleus that can be readily accessed at low field, providing a robust discriminator of brine from organic phases in petrophysics and a marker for aqueous ingress in construction materials. We examine NaCl brine in a chalk selected for its monodispersed pore size distribution. The H-1 T-2 distribution reflects the pore size distribution as expected. In contrast, the Na-23 T-2 distribution is broad and multimodal, consistent with relaxation outside the motional narrowing regime. To understand the Na-23 results, we determine the quadrupole coupling constant QCC and rotational correlation time tau(c), and show that well-defined single values of QCC and tau(c) describe the observed relaxation behavior. Our observations call for a new interpretation for quadrupole relaxation in pores. (C) 2017 Elsevier Inc. All rights reserved.