Non-Fickian dispersivity investigation from numerical simulations of tracer transport in a model double-porosity medium at different saturations

Abstract

It is generally admitted that dispersivity is an indicator of the heterogeneity scale of porous media. This parameter is assumed to be an intrinsic property which characterizes the dispersive behavior during the transport of a tracer in a porous medium. When the medium is saturated by two fluid phases (water and air), dispersivity depends strongly on saturation. “Double-porosity” medium concept can be attributed to a class of heterogeneous soils and rocks in which a strong contrast in local pore size characteristics is observed. In this work, we characterized non-Fickian dispersivities of a double-porosity medium at different saturations, by performing numerical simulations for a series of one-dimensional experiments of tracer dispersion under different initial and boundary conditions. The physical double-porosity model was composed of solidified clayey spheres, distributed periodically in a more permeable sandy matrix. Using a two-equation macroscopic model, numerical simulations reproduced very well the experimental data, thus allowing to determine the dispersivity for different transport scenarios. For the first time, the existence of a unique dispersivity of a double-porosity medium at a given saturation was demonstrated for different transport scenarios of initial and boundary conditions. The saturation dependence of the dispersivity in the double-porosity medium was established and compared with the trends obtained for the single-porosity soils in previous studies.