MODELLING OF IN SITU SILICA PARTICLE GROWTH AND ITS DEPOSITION UNDER GEOTHERMAL CONDITIONS

Abstract

When cooled geothermal brine is reinjected, colloidal silica may form, deposit, and eventually block the fluid pathways in the aquifer, reducing injectivity. Models of silica deposition, the formation of colloidal silica, especially the growth of silica nanoparticles over time, are of value in the prediction of geothermal well lifetime and the effects of brine treatments and workovers. This paper deals with the transport of silica colloids, once formed.
A 2D Eulerian model to predict the in situ deposition rate of silica is proposed. The driving mechanisms of silica transport considered in the model are diffusion and convection (Levich 1962), and five external forces acting on individual suspended silica nanoparticles: gravity, buoyancy, London-van der Waals force, electrostatic force. The Saffman lift force is shown to be negligible. The modelling results are compared to the observations of Mroczek et al. 2017, in experiments with natural brines in which colloidal silica deposition is suppressed by ageing the brine.