| Abstract |
This paper describes the development of the main conceptual model elements that have been used as a basis for numerical modelling and well targeting at the Rotokawa Geothermal Field. These elements are largely based on characterising the natural state of the reservoir, prior to production, but significant new knowledge has been gained from monitoring the response of the reservoir to the start of the 138 MWe Nga Awa Purua (NAP) plant in 2010. Characterising the natural state pressure, temperature and geochemistry of the system has been crucial to the conceptual model development, particularly the location of deep upflows and outflows. The lateral extent of the convecting permeable reservoir has been interpreted using a combination of natural state temperature and MT resistivity surveys. A significant confined aquifer called the “intermediate aquifer” exists above the reservoir and contains variably mixed geothermal fluids and groundwater in the Rotokawa area. Flows within this aquifer and between the deep reservoir and intermediate aquifer have been characterised based on natural state temperatures, alteration, MT surveys and geology and topography. Low permeability caps occur above both the deep reservoir and the intermediate aquifer, formed by smectite-altered formations that are identified in wells via conductive temperature profiles and methylene blue measurements, imaged by MT surveys as low resistivity zones. Major geologic structures have been identified within the field based on detailed analysis of stratigraphic offset between the wells and microseismic data. These structures appear to be controlling the compartmentalised response of the reservoir under production, which has been identified through pressure and geochemistry monitoring since the start of production. The depth of microseismicity has provided some indication of the effective base of the reservoir for numerical modelling, however this is unconfirmed by drilling. Hydrology of the shallow unconfined aquifer is based on the geochemistry of surface features, shallow monitoring well pressures, temperatures, chemistry and by surface geology and topography. |