Release of clay particles from an unconsolidated clay-sand core: experiments and modelling

This work identifies the main phenomena that control the peptisation and transport of clay particles in a sand core. Clay can be dispersed into small particles in an aqueous solution of low ionic strength. This property is used to generate clay particles with NaCl concentration varying from 0.5 M to 0.015 M. For this purpose, a chromatographic column is initially packed with a 5 % clay-sand mixture. The monitored decrease of the NaCl concentration of the feed solution allows the control of transport of the particles without plugging the porous medium. In this conditions, it is shown that, in a column of a given length, the amount of clay particles, released into solution and available to transport, depends only on NaCl concentration. Some clay particles are available to migration when the NaCl concentration of the feed concentration is between 0.16 M and 0.05 M (first domain) or between 0.035 M and 0.019 M (second domain). An empirical function, Pd([NaCl]), accounts for this particle generation. Transport is mainly dependent on the hydrodynamic characteristics of the porous medium that vary during the elution, probably due to the particle motion inside the column. A phenomenological modelling is derived from these results, coupling the particle generation term, Pd([NaCl]), with an adapted nonequilibrium transport solute model. Similarly to the solute, particles were attributed a characteristic time of mass transfer between mobile and immobile water zones. This is sufficient to take into account the kinetic limitations of particles transport. The values of the parameters are determined by independent experiments. Finally, breakthrough curves of clay particles are predicted when a column of a given length is flushed by a salinity gradient of NaCl in various conditions. Introduction Particles are suspected to be involved in the transport of pollutants in soils (Avogadro and De Marsily, 1984). The possible phenomena involved in the control of particle transport are linked to the physical properties of the particles in solution (flocculation or dispersion according to the solution), the interaction between the particles and the porous medium (mechanical retention (filtration), adsorption (DLVO theory or sorption reaction)) and of course, the hydrodynamic properties of the porous medium ( porosity, permeability, dispersion...). It is well known that, exchanging the salt water present in a sand core with a fresh water, reduces the permeability of the core due to clay particles migration (Van Olphen, 1977). This problem is of interest for petroleum engineers and agricultural researchers. They usually focus their studies on the evolution of the permeability of the sand core until plugging occurs. Few quantitative studies exist on the clay migration (Khilar and Fogler, 1987). These works show that the behaviour of a sand