Sodium diffusion in heterogeneous porous media: Connecting laboratory experiments and simulations

Sodium has been suggested as a tracer for brine in reservoir formations where a significant amount of sodium ion is found mainly in the aqueous phase. Fortunately, the most abundant sodium isotope, 23Na, is Nuclear Magnetic Resonance (NMR) active, making it possible to study the structure and dynam-ical properties of both bulk and pore confined fluid systems. We investigated the diffusion of key dis-solved sodium species in bulk solution and porous media as a function of composition, ionic strength, and permeability of the carbonate samples using NMR relaxometry and pulsed-field gradient (PFG) NMR techniques. We use Indiana limestone as an example of natural porous media and water as a freely diffusible tracer and carrier. We demonstrate apparent diffusion measurement of sodium based on changes in spin-spin relaxation time (T2) signal in pore confinement. The diffusion rate of sodium decreases with increasing counter-ion size. This effect is greater at higher ionic strengths and lower chemical potentials in porous media. The reactive transport code, CrunchFlow, was used to complement the NMR experiments to simulate diffusion behavior in porous media. Combining these two methods provides a powerful approach to estimating effective diffusion coefficients in heterogeneous matrices. The modeling considers the influence of physical properties (porosity and tortuosity) and chemical prop-erties (geochemical composition and chemical gradients). The results presented in this work highlight the advantage of measuring apparent diffusivity using NMR T2 relaxometry in conjunction with numerical simulation to derive effective diffusivity and the corresponding matrix properties (i.e., tortuosity) of the system.(c) 2022 Published by Elsevier Ltd.

Automated summary

Sodium can be used as a tracer for brine in reservoir formations, and this study investigates the diffusion behavior of sodium in both bulk solution and porous media using nuclear magnetic resonance (NMR) techniques. The results show that the diffusion rate of sodium decreases with increasing counter-ion size, with a greater effect observed at higher ionic strengths and in porous media. By combining numerical simulation and experimental methods, effective diffusion coefficients and matrix properties can be estimated.