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 difficult due to a lack of reliable scaling rates and the complexity of modelling fluid transport simultaneously with deposition. In order to develop techniques, understand the problems and improve our predictive capabilities, we have undertaken field experiments at Wairakei geothermal field, 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 flashed fluid containing 530 ppm total silica were completed at temperatures between 71 and 129 degreesC and at flowrates between 0.002 and 0.02 kg s(-1). 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 cm(-2) year(-1) and independent of flowrate between 80 and 129 degreesC. Scaling at 129 degreesC was unexpected, because the dissolved silica was expected to be undersaturated with respect to amorphous silica. At 71 degreesC the rates were higher (up to 23 mg cm(-2) year(-1)) and were proportional to flowrate. At Wairakei the 130 degreesC fluid used in these experiments is disposed of by injection into a reservoir at 80 degreesC. Using our field deposition rates, we estimate that 2.6x10(5) kg of amorphous silica would precipitate in 10 years around the injection well, assuming an injection rate of 50 kg s(-1) into a 100 m thick reservoir of radius 500 m with permeability 100 mdarcy and a porosity of 0.2. (C) 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.