Flow and Transport Simulation of Chemical Grouts in Porous Media: Shear Effects on Gelation

Chemical grouting is emerging as a technology for containment of contaminant transport in porous media. When chemical grouts, such as sodium silicate, colloidal silica (CS), polyacrylamide and acrylates, are reacted with cross-linkers, the grout solution becomes viscous and eventually forms a gel or semi-solid depending up on CS concentration and the reaction time (age). However when the solution is flowing through porous media, gelation is retarded due to shear effects. CS, a stable dispersion or sols of discrete particles of amorphous silica, is adopted as grout in this study. The objective of the present study is to develop a code that can simulate the flow and transport of chemical grouts, subjected to shear, in porous media and analyse the effect of shear on gelation. Chemical grouting of colloidal silica solution into porous media is simulated by non-iteratively coupling MODFLOW (3-D groundwater flow simulation model) and RT3D (3-D multi-species reactive transport model) and adding modules for gelling and grout age. The flow field is calculated by MODFLOW. The grout age and concentration field determined by RT3D and shear rate (in terms of velocity) obtained in the current time step are used to calculate the viscosity of grout in the porous media as part of MODFLOW during the next time step. Gel viscosity is indirectly incorporated in MODFLOW by changing the hydraulic conductivity in each cell based on gel viscosity. The modified hydraulic conductivity is used to determine the flow field followed by concentration field for the next time step. The model may also be used for other chemical grouts by incorporating respective viscosity parameters. A three dimensional model is set up to simulate the grout curtain formation. After the completion of grouting simulation, the performance of the barrier is assessed in terms of its water tightness, by testing a MODFLOW problem of the same domain with different boundary conditions to obtain effective hydraulic conductivity of the curtain. The comparison of results, obtained when the shear is considered in gelation and when it is not considered, indicated that shear is an important property that needs to be incorporated in the gelation of chemical grouts. The grout injection process is found to be affected by shear.