Mineral Precipitation in the Rotokawa Power Station, NZ

Abstract

Scaling in production pipelines and heat exchangers in the Rotokawa geothermal power station occurs through changes in physico-chemical properties of the fluids. Reservoir fluids at ~330?C are flashed to 265?C with an accompanying pressure reduction from 70 to 50 bar abs. This major pressure reduction causes the precipitation of altaite, hessite, sphalerite and pyrite aggregates in a chalcopyrite matrix. Separated liquids at 220?C enter the brine heat exchanger and undergo conductive cooling. This causes the deposition of altaite, hessite, chalcopyrite, pyrite, wurtzite, quartz and sodic feldspar. Pyrrhotite and Fe oxide are deposited as corrosion products and well scale from the RK5 two phase production pipe line has also been remobilised and redeposited here. Scaling in the brine + condensate heat exchanger occurs as alternating bands of stibnite and kaolinite, with rare tellurides: coloradoite, telluroantimony, altaite and stutzite. Scaling throughout the power station has been thermodynamically modeled using CHILLER and SOLVEQ. Results suggest that deposition of these minerals can be directly related to changes in temperature, pressure, pH and concentration of H2S.