Quantitative evaluation of the density-driven flow during the through-diffusion experiment: experimental and theoretical approach

The through-diffusion experiment is a commonly used method to measure the diffusivity of a solute in a rock. In this experiment, however, an effect of advection, induced by density difference between dense aqueous solution and pure water, has not been considered. To evaluate the effect of density-driven flow, a through- diffusion experiment using Fontainebleau sandstone was conducted for KCl and KI aqueous solutions with various densities. The results showed that the effective diffusion coefficient of a high density solution (1 M KI) was one order of magnitude larger than that of a low density solution (0.1 M KCl). The hydraulic head difference was also found to be positively correlated with the density difference between two solutions, indicating the occurrence of advection. These results were successfully explained by a diffusion-advection equation including Darcy's law. Based on the theory, a diagram to evaluate the condition at which the measured effective diffusion coefficient does not include the effect of advection is presented.