Modelling the transport of iron-based colloids in saturated porous media

The aim of this work is to develop a numerical model able to describe the transport of highly concentrated suspensions of iron particles, in particular during their injection in the subsurface, and in the early stages of migration through the porous matrix. The model is intended to be used as a tool in the development of an efficient injection technology for field-scale applications of nano- and microscale iron suspensions. Due to the complexity of phenomena governing the transport or iron colloidal suspensions, the first part of this study (Chapters 1 to 4) focuses on assessing the influence of hydrochemical parameters on the mobility of simple, well-known colloidal systems, namely latex microspheres. Latex colloids were chosen as they are often used in the literature as model particles when studying the transport of natural nano- and microparticles in porous media. Colloid transport is governed by advection-dispersion mechanisms, filtration, particle-particle and particle-soil interactions. The latter result in dynamic deposition of the particles on (and release from) the grains of the porous medium. Hydrochemical parameters are known to greatly influence colloid deposition/release phenomena, but little or no systematic studies are found in the literature that explicitly include these effects in a numerical model. Consequently, in the first part of this work the influence of transients in ionic strength on particle deposition and re-entrainment is systematically investigated. Semi-empirical relationships are proposed that tie the deposition and release kinetics to the salt concentration, and embedded in a one-dimensional transport model. The work is structured as follows: • Chapter 1. Phenomena controlling colloid transport in saturated porous media are described, and model equations are stated to simulate colloid deposition and release during transients of ionic strength. The effects of changes in the solution ionic strength are explicitly included into the set of model equations. • Chapter 2. The transient model is solved numerically with the implementation of a finite-differences code, called MNM1D (Micro- and Nanoparticle transport Model in porous media). The partial differential equations for the salt and the colloid concentrations are solved simultaneously, and coupled with empirical functions describing the explicit dependence of deposition and release coefficients on the ionic strength. MNM1D was validated against well-established analytical and finite-elements transport codes that solve colloid transport under stationary hydrochemical conditions, proving to be adequately stable and robust. • Chapter 3. A complete characterization of colloidal suspensions and porous medium are presented, and the protocol adopted for column tests of colloidal mobility under different hydrochemical conditions is described: each experiment included a deposition step at constant ionic strength, followed by release steps induced by abrupt changes in porewater chemistry. • Chapter 4. Experimental results are discussed, and fitting of the breakthrough curves with MNM1D are presented. First, the model is applied to the initial part of the experiments (i.e. colloid deposition at constant ionic strength), and the semi-empirical relationships for deposition and release kinetic parameters are derived from the fitting results. Then, MNM1D is applied for the fitting of the whole experiment. In the second part of the work (Chapters 5 to 7), the model is extended to iron micro- and nanoparticles. In this case, particle deposition and rheological properties of the highly concentrated slurry of iron colloids play a major role (concentrations are up to 20 g/l, and particles are dispersed in non-Newtonian viscous fluids for improving stability). Consequently, the hydrodynamic parameters and fluid properties are no longer independent on the concentration of deposed and suspended colloids, and flow and transport become coupled problems. • Chapter 5. Model equations for the description of coupled flow and transport phenomena of non-Newtonian suspensions of micro- and nanosized iron particles are presented. They are developed from the commonly used equations of solute transport in porous media, and modified to account for changes in matrix properties due to particle deposition (i.e. clogging) and for the non-Newtonian nature of the carrier fluid. • Chapter 6. First, the properties of iron micro- and nanoparticles, porous medium and dispersant fluid used in laboratory transport tests are described. Then, the protocol adopted in column experiments is detailed. Column tests were lead under different hydrochemical conditions. A first deposition step with injection of iron particles dispersed in a xanthan slurry was followed by a release step obtained by flushing the column with water. • Chapter 7. The model equations are implemented in a finite differences code, that represents the extension of MNM1D, and are applied for the fitting of the experimental data, proving adequate to simulate particle transport. Before this study, no model was available in the literature for the simulation of iron transport under the conditions described in the experimental section: although solutions exist for the simulation of clogging in deep bed filtration, or changes in pore space geometry due to mineral precipitation, none of these models can simulate the non-Newtonian nature of the carrier fluid, nor the influence of the concentration of suspended colloids on the fluid properties. The numerical model developed in this work, although implemented for the simulation of 1D laboratory column tests, could be extended to more complex geometries, thus becoming a useful tool for the design of the injection and early stages of migration of iron slurries in field applications.