A three dimensional numerical model of the Waiotapu, Waikite and Reporoa geothermal areas, New Zealand

• A numerical model of the Waiotapu, Waikite and Reporoa systems has been developed.
• Underground fluid flow and the gross permeability structures investigated
• Model results are in good agreement with the measured field data.
• Reporoa could not be successfully modelled as only an outflow from Waiotapu.
• Our model is able to represent Waikite as an outflow from Waiotapu.

A large-scale three-dimensional numerical model of the Waiotapu, Waikite and Reporoa geothermal fields, New Zealand, is presented. The model covers an area of 450 km2 and extends to a depth of about 3.6 km. It is based on geological, geophysical and geochemical data and related interpretations available in the literature. The model has been calibrated by varying its permeability structure and the fluid and heat input at its base so that the temperature distribution in the model associated with the convective and advective fluid flow matches the natural state conditions in the three fields. Good agreement has been obtained between model results and measured downhole temperature data as well as reproducing the location and magnitude of the surface heat discharge for each field.In the best-fit model, 665 kg/s of deep thermal fluid is transferred to the surface and to concealed shallow aquifers. The subsurface extent of the geothermal reservoirs can be assessed from model blocks at temperatures greater than 220 °C. The Waiotapu reservoir covers about 30 km2 at a depth of 1 km below sea level, whereas the hot zone in the Reporoa Caldera associated with the Reporoa reservoir extends over about 13 km2. A separate deep thermal upflow is required in the model in order to get a satisfactory match to temperatures in the Reporoa field. The modelling study shows that the Waikite field is an outflow from the Waiotapu field and the high temperature region beneath the Waikite field only appears at shallow depths (− 500 mRL and above). The model therefore well defines the Waiotapu–Reporoa geothermal system which hosts the three geothermal fields at Waiotapu, Waikite and Reporoa.