Silica scaling in cooled silica saturated geothermal water: Comparison between Wairakei and Ohaaki geothermal fields, New Zealand

Abstract

Cooling and aging of silica saturated geothermal water as a means of minimizing and controlling silica scaling prior to disposal by injection has been trialled and utilised at many geothermal fields. These show that although scaling may be reduced, it is typically insufficient to be an effective means of control with the risk of scaling in reinjection wells and prematurely reducing the permeability of the surrounding aquifer.We describe small scale field experiments undertaken at the Wairakei and Ohaaki Geothermal Fields (New Zealand) to measure the silica scaling potential of cooled separated geothermal water and to assess whether it could be safely disposed of by injection.The Wairakei experiments showed that rapid cooling followed by aging for several days successfully suppressed silica scaling. Based on this result Contact Energy Ltd undertook a large scale outfield injection trial of cold (30°C) geothermal water where 270t/hr of separated geothermal water containing total silica of∼730mg/kg was injected for 10 months. The injectivity of the wells did not show any indications of reduced permeability due to deposition of silica either in the well bore or in the formation. This was the first major large scale injection of cold geothermal water undertaken in New Zealand. Subsequently, after the commissioning of the Te Mihi Power Station in 2013, the injection of geothermal water diluted (average 42%) by steam condensate has continued to date.In contrast, at Ohaaki, scaling observed in rapidly cooled separated water aged for 30min at 80°C (total silica 840mg/kg) suggested that the injection of this fluid would result in impairment at or close to the well bore. Therefore, no subsequent field injection trial was undertaken at Ohaaki.