Formation and Neutralisation of Corrosive Fluids in the Shallow Injection Aquifer, Rotokawa Geothermal Field, New Zealand

Abstract

Severe external casing corrosion has occurred in several wells in the Rotokawa Geothermal Field both prior to and following development of the resource for electrical power production. This paper presents the results of alteration mineralogy, microgravity, fluid chemistry and corrosion product studies that have led to the development of a model for the formation and neutralisation of corrosive fluids in the shallow injection aquifer at Rotokawa. The occurrence of kaolinite, alunite, dickite and goethite in drill cuttings reveals the presence of both acid sulphate and CO2-rich/ bicarbonate fluids in the shallow aquifer (<500mRSL). The occurrence of CO2-rich/bicarbonate fluids is also supported by analysis of casing and cement samples and downhole fluid chemistry. The formation of these fluids is attributed to the ascent of hot (>250°C) reservoir fluids, leaking through an otherwise impermeable reservoir cap rock, boiling and condensation of CO2 and H2S into a cooler shallow groundwater aquifer. Microgravity measurements support the existence of this two-phase zone in the shallow aquifer prior to the start of production, and subsequent resaturation and “neutralisation” of this zone by shallow injection during the first eight years of production, apparently slowing the rate of casing corrosion. This model for the formation and neutralisation of corrosive fluids has become a vital tool in the management of the Rotokawa Geothermal Field.