The Geochemical Response of the Rotokawa Reservoir to the First Five Years of Nga Awa Purua Production

Abstract

Best practice geothermal field management requires identification of the key processes acting within the reservoir and understanding of the relationships between reservoir and process chemistry changes with the potential to impact resource and infrastructure sustainability. Following the initial development of the 34MWe Rotokawa Power Station, the Rotokawa reservoir exhibited relatively stable production geochemistry characteristics. In 2010 the 138MWe Nga Awa Purua Power Station (a joint-venture between Mighty River Power and Tauhara North No.2 Trust) was commissioned which increased steam production and brine injection in the Rotokawa reservoir. As anticipated, this generated a number of transient hydrogeologic changes which were monitored closely through regular Tracer Flow Testing (TFT), produced fluid geochemistry evaluation, in addition to a variety of other reservoir testing activities. When integrated and evaluated within a geoscientific context, these data provide important insights into both transient and long-term reservoir changes which can occur in response to operational activities. This paper will discuss the geochemical response of the Rotokawa reservoir to Nga Awa Purua start-up and operation over the last 5 years. Production well geochemistry has identified distinct geochemical responses to three main processes occurring within the reservoir: 1) injection chemical breakthrough, 2) boiling, and 3) dilution from marginal recharge.