Applications of Sulfur Isotopes in Characterizing Reservoir Processes at the Kawerau Geothermal Field, New Zealand

Abstract

Since its early development in 1957, some areas of the Kawerau reservoir have changed in response to production, mainly experiencing cooling brought about by inflow of marginal recharge and injection returns. The chemical signature of these reservoir processes are normally reflected in the fluid chemistry of the producing wells. However, due to the different characteristics of the Kawerau injection fluid (i.e. acid-dosed brine, oxygenated steam condensates or mixed brine and condensate) identifying the source and main reservoir process responsible for the change in fluid chemistry over time is a complex undertaking.
The use of isotopes to enhance the assessment of the reservoir processes affecting the Kawerau field was considered in this paper. Fluid samples from selected production wells and shallow/intermediate aquifer monitoring wells were collected and analysed for their isotopic composition. These include sulfur isotope (?34S in sulfate and sulfide in brine, and ?34S in sulfide in steam) and stable isotopes such as deuterium (?2H in water in brine and steam) and oxygen-18 (?18O in water in brine and steam, ?18O in sulfate in brine).
The isotopic sulfur composition of the production wells at Kawerau displayed similar features to many geothermal reservoirs in the TVZ, such as a homogeneous composition of ?34S in sulfide of around 5 to 7 ‰ and an apparent sulfur isotopic disequilibrium state between the sulfate and sulfide species. The results of a mixing model using reservoir sulfate and the ?34S in sulfate have shown that injection return is the prevailing reservoir process on the eastern side of the reservoir (e.g. in PK06 and PK07) while mixing with marginal recharge predominates on the western side of the field (particularly in KA30 ).
Used in conjunction with the hydrogeological, geological and geochemical data, sulfur isotopes proved to be very useful in the case of the Kawerau field, particularly in identifying the reservoir processes affecting the field. Moreover, the additional information from this study helped in creating a robust reservoir management plan for the field.