New experimental observations of the behavior of sodium ions in saturated rock samples

¹H NMR relaxometry of saturated rock samples has become a useful tool for the characterization of porosity and transport phenomena of enclosed fluids. The pore size can be measured using the difference between inverse relaxation values of protons absorbed by the saturated rock and that present in the bulk fluids. These experiments are usually performed at low magnetic fields to reduce the influence of the diffusion on the relaxation values in the presence of Internal Gradient Fields. Recently, sodium ions have become objects of investigation. The main advantage of sodium ions over protons as measured spins in the Petrophysic NMR experiments is their presence in water and not other phases like oil and gas. However unlike protons, sodium ions can have slow motion properties like appearance of the bi-exponential relaxation and residual quadrupolar distribution, which can lead to complex behavior of spins inside the pores. Here, we describe eight ²³Na NMR experiments at 9.39 T external magnetic field, in which we have investigated the behavior of sodium ions in 4 saturated rock samples: Berea 500, Fontainebleau 1, Bentheimer sandstones, and Austin chalk. We show that the reduction of spin-spin relaxation is caused by anisotropic motion and not diffusion in the presence of Internal Gradient Fields. There can be a link between free diffusional and motional averaging regimes regardless of the size of environment in which the measured spin ions are. Using different NMR sequences, we reveal and quantitatively describe bi-exponentially relaxing spins and spins with residual quadrupolar coupling. This work demonstrates unique model for the behavior of ions inside porous media, which is different than known models (Brownstein-Tarr model and "agarose gel model").