Silica scaling in geothermal heat exchangers and its impact on pressure drop and performance: Wairakei binary plant, New Zealand

• Scaling in geothermal heat exchangers is mainly related to the deposition of silicate.
• Hard silica scale deposits inside the heat exchanger tubes at Wairakei binary units.
• This silica scaling (called shark skin) is dense, monomeric and 1–2 mm thick.
• To quantify the impact of scaling on production, a lumped parameter model was used.
• Silica saturation index <2 should be maintained if no chemical inhibitor to be used.

Scaling (fouling) has a dramatic impact on the long term operational performance of geothermal heat exchangers. Scaling affects both the flow hydrodynamics and the heat transfer resistance. A review of reported scaling problems in geothermal heat exchangers shows that silicate (pure silica and metal silicate) is the main deposited mineral. Scanning electron microscope images show that: silica deposition at the Wairakei binary plant is very dense as result of molecular (monomeric) deposition. This work attempts to characterize this impact on the binary plant, using long term production data. The geothermal brine flow rate and pressure drop within the plant are useful to assess the plant performance. These are interdependent quantities hence taken alone do not allow a proper understanding of the impact of silica scaling on performance. A new simple model was proposed to quantify the performance impact (resistance) of scaling. The available chemistry data showed that the recent (late 2009) increase in scaling rate and flow resistance is mainly related to the introduction of new wells with high silica content to the production system. Recommendations were given to maintain silica saturation index to less than two and use scaling inhibitors.