Sodium-23 NMR in porous media

Abstract

The enhancement in relaxation of 1H 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 1H T2 distribution reflects the pore size distribution as expected. In contrast, the 23Na T2 distribution is broad and multimodal, consistent with relaxation outside the motional narrowing regime. To understand the 23Na results, we determine the quadrupole coupling constant QCC and rotational correlation time τc, and show that well-defined single values of QCC and τc describe the observed relaxation behavior. Our observations call for a new interpretation for quadrupole relaxation in pores.