| Title | The Natural-State Geochemistry of the Rotokawa Reservoir |
|---|---|
| Authors | Jeffrey Winick, Thomas Powell, Ed Mroczek |
| Year | 2011 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Rotokawa, geochemistry, conceptual model, numerical model |
| Abstract | The geochemistry of fluid discharges from historical and post-production wells and thermal features have been evaluated as part of a conceptual model update for the Rotokawa reservoir. Early geochemistry investigations by the DSIR laid the foundation for much of the current understanding of the geothermal system. Information available at the time was limited to seven crown wells. A conceptual model update in 2006 considered wells up to RK18 and focused on natural-state temperatures, pressures, alteration patterns, permeability distribution, and geophysical data. At this time, a comprehensive review of the field-wide geochemistry was not performed. As of 2011, 14 additional deep production and injection wells have been drilled and a significant quantity of additional geochemistry data now exists to help characterize reservoir processes within the Rotokawa system. Significant gradients in terms of Cl, Cl/B, Na-K-Ca geothermometry, and non-condensable gasses (NCGs) are observed across the field. The deep reservoir fluids also appear to be in disequilibrium with respect to most geochemical systems. Relationships of enthalpy-Cl, major and trace liquid constituents, and NCGs do not easily resolve a unique parent fluid to the system, yet the possibility of one cannot be discounted. An updated conceptual model has been developed on the basis of new understanding which accommodates both possibilities. The conceptual model incorporates elements of a hot, deep, two-phase upflow which rises and boils, outflowing from the system toward the north. Progressive dilution along the outflow by marginal fluids is largely responsible for the observed geochemical gradients. A large structural feature in the centre of the field provides a zone of enhanced permeability, allowing deep reservoir fluids to rise and boil within an intermediate aquifer. The intermediate aquifer is a complex mixture of gas-rich steam condensates, groundwater, boiled reservoir fluids, and acid-SO4-Cl fluids from a shallow, steam-heated aquifer in the vicinity of Lake Rotokawa. |