Coupled electric and transport phenomena in porous media

Abstract

The coupled electrical and transport properties of clay-containing porous media are the topics of interest in this study. Both experimental and numerical (pore network modeling) techniques are employed to gain insight into the macro-scale interaction between electrical and solute transport phenomena in clayey porous media. The electrical properties are determined by the electrical double layer formed on the mineral-solution interface. Due to the unique surface properties, the external electric field is applied across the natural and the plug-like flow is generated which is referred as electro-osmotic flow. Different from the traditional hydraulic flow field used for contaminants removal in the subsurface, the electro-osmotic flow have advantages in less spreading of solute movement. The numerical simulations for solute dispersion under electro-osmotic flow and hydraulic pressure flow are carried out, and the reduced dispersion coefficient for electro-osmotic flow is quantified. With the knowledge of surface properties of clay minerals, several geophysical methods are employed to investigate fluid flow and solute transport properties in natural soils. The self-potential method has been intensely applied in the hydrogeological communications. An experimental set-up for self-potential measurement in a column filled with different types of porous media (i.e. clean sand vs clayey sand) is built-up. By conducting the salt tracer tests in the columns, the self-potential signals induced by solute concentration gradient are observed and the behavior of time-lapse self-potential in different medium is different. The time-lapse self-potential at each location are well reproduced by the continuum scale governing equation. Compared the experimental self-potential signals with the numerical model for a given solute gradient, the effective self-potential coefficient of each type of porous media is derived. The results show that the self-potential coefficient for clayey sand needs to consider the effects of the electrical double layer. The overlapping of electrical double layer is significantly affect the movement of ions inside the pore space and eventually determined the effective electrical and transport coefficient. To detail the pore scale electrokinetic process in clay-free or clay-rich materials, the pore network model is employed to investigate the effective coefficients based on the microscopic material properties (pore radius, surface charge density) and solution property. The quasi-static response of the electrical potential to the salt plume movement in clay-rich materials is well captured by the pore network model. The evaluated solute diffusivity and self-potential coefficient for the clean sand and clay are in good agreement with those obtained in the laboratory experiment, and exhibit the similar behavior of the tendency with respect to the extent of overlapping for the electrical double layer.