Understanding Calcite Scaling at Wairakei Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

The reservoir fluid at Wairakei geothermal field is generally oversaturated with calcite. Calcite deposition in the production wells was induced by reservoir changes in response to mass extraction and reinjection strategies. Since field production commenced in 1958, reservoir pressure has rapidly declined that facilitated the inflow of shallow, cooler fluids. Mixing with these low dissolved gas (CO2) and low chloride fluids decreased the calcite saturation levels in the reservoir.
In 1998, when fluid reinjection commenced at Otupu, an increase in reservoir liquid pressure occurred which likely suppressed boiling at shallower part of the field; hence, the deep reservoir is less boiled resulting to higher gas content in the production wells. The higher dissolved CO2 in these wells increased the calcite saturation levels.
The increase in energy generation in 2014 upon commissioning of Te Mihi Power Plant has resulted to further field pressure drawdown. This reservoir pressure drop enhanced boiling and consequently, calcite scaling that resulted in decline in mass flows of several production wells.
Applicable geochemical interpretation techniques were adopted in assessing the response of the production wells to calcite scaling. WATCH Automator was used to run several hundreds of geochemical data that modelled calcite saturation levels. A cross plot of Cl concentration and Cl/Ca ratio has clearly distinguished the wells affected primarily by calcite deposition.
Correlation of geochemical data with well test and production data confirmed calcite deposition in these wells. Understanding the mechanism of calcite formation has been proven to be valuable in formulating a scaling mitigation strategy to sustain fluid production at Wairakei.