Chloride Model of Ngatamariki Geothermal System

Abstract

The Ngatamariki geothermal field in the Taupo Volcano Zone of New Zealand provides power to a full binary 82 MWe net plant commissioned in 2013. Over its 7 years of production, the plant and production wells have observed stable enthalpy. The field has shown stable pressure after an initial small drawdown following plant start-up in its reservoir monitoring wells. To search for any indication of future changes in the field, the chemistry of the produced fluid is inspected. The chloride concentration is slightly diluting. The Ngatamariki numerical reservoir model has been improved to include chloride data. A new method is used and described here. TOUGH3 is used for the reservoir simulator and is coupled with a wellbore model. TOUGH3 allows for faster modelling than TOUGH2 due to its ability to run in parallel. EOS1 with two water components is used as a tracer for chloride concentration. Fresh water in shallow aquifers is water #1. Geothermal upflow rich in chloride is water #2. Pressure, temperature, and chloride concentration are modelled throughout the reservoir and wellbore. The model is calibrated to latest production data and new information from drilling. The process of calibrating the model to the latest data has led to greater understanding of the geothermal system. The model is calibrated to match shut PTS, flowing PTS, quarterly chloride concentrations, continuous downhole pressure, and plant enthalpy. Learnings are described regarding the temperature distribution and flow paths within the reservoir. The dilution trend is captured in the model to improve forecasts of future production, and therefore, the updated model allows for better resource and risk management.