Modelling field-scale injection of shear thinning slurries of microscale iron particles: coupled flow and transport in radial and 3D geometries

The use of shear thinning solutions of green biopolymers (eg. guar gum, xanthan gum, etc.) have been considered in recent years as an effective approach for the colloidal stabilization and improved delivery of concentrated suspensions of microscale and nanoscale zerovalent iron particles (MZVI and NZVI, respectively) for groundwater remediation. The particles are injected into the subsurface close to the source of contamination, thus creating a reactive zone. Therefore, the design of a field-scale pilot test or remediation intervention requires a reliable estimate of iron distribution after the injection. The transport in porous media of concentrated MZVI and NZVI slurries is affected by the rheological properties of the dispersing fluid (shear thinning) and by particle deposition and filtration in the porous matrix, which result in porous medium clogging (i.e. reduction of porosity and permeability). Moreover, the kinetics of particle retention is strongly influenced by flow rate and fluid viscosity, as evidenced by experimental results (Gastone et al. 2013). Therefore, the strong coupling of non-Newtonian flow (due to the shear thinning properties of the guar gum solutions) and particles transport requires the development of specific modelling tools. E-MNM1D (Tosco and Sethi, 2010; www.polito.it/groundwater/software) was previously developed by the authors for the simulation of one-dimensional transport of MZVI and NZVI non-Newtonian dispersions at the laboratory scale, while MNM1D was developed to simulate the influence of transients in water salinity on colloid transport in the absence of clogging (Tosco et al., 2009). In this work, the two modelling approaches are extended to more complex geometries. The model equations for the simulation of colloid transport under transients in ionic strength, solved in 1D geometry by MNM1D, are here adopted for more general scenarios of particle transport, and implemented in other commercial and freeware software, namely RT3D (Clement et al., 1998). The flow and transport of MZVI slurries is solved in a radial domain for the simulation of field-scale injection, incorporating the abovementioned relevant mechanisms. The governing equations and model implementation are presented and discussed, along with examples of injection simulations. The work was partly co-funded by the EU projects AQUAREHAB (FP7 - G. A. Nr. 226565) and NANOREM (FP7 - G. A. Nr. 309517).