Deposition of amorphous silica in porous packed beds — predicting the lifetime of reinjection aquifers

Predicting deposition rates of dissolved silica in geothermal reinjection aquifers is dicult due to a lack of reliable scaling rates and the complexity of modelling ¯uid transport simulta- neously with deposition. In order to develop techniques, understand the problems and improve our predictive capabilities, we have undertaken ®eld experiments at Wairakei geothermal ®eld, New Zealand, to determine amorphous silica deposition rates in 25 mm diameter pipes packed with 2 mm diameter zirconia beads. These pipes served as model aquifers. Five experiments using ¯ashed ¯uid containing 530 ppm total silica were completed at temperatures between 71 and 129 C and at ¯owrates between 0.002 and 0.02 kg sÿ1. The residence times in the pipes were shorter than the induction period required for silica polymerisation from solution. The scaling rates in the beds, measured over a month, were about 12 mg cmÿ2 yearÿ1 and inde- pendent of ¯owrate between 80 and 129 C. Scaling at 129 C was unexpected, because the dissolved silica was expected to be undersaturated with respect to amorphous silica. At 71 C the rates were higher (up to 23 mg cmÿ2 yearÿ1) and were proportional to ¯owrate. At Wairakei the 130 C ¯uid used in these experiments is disposed of by injection into a reservoir at 80 C. Using our ®eld deposition rates, we estimate that 2.6 105 kg of amorphous silica would precipitate in 10 years around the injection well, assuming an injection rate of 50 kg sÿ1 into a 100 m thick reservoir of radius 500 m with permeability 100 mdarcy and a porosity of 0.2. # 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.