| Abstract |
Tauranga, on the north coast of the North Island of New Zealand, is the site of a fairly extensive low-temperature (60°C at 800 m) geothermal resource that is currently used for hot pools, swimming baths, domestic use, greenhouses and tropical fish growing. As the population of the area grows and interest in direct use of geothermal resources increases, the system comes under increasing demand. In this study, a TOUGH2 heat and fluid flow model of the Tauranga geothermal field is used to determine the extent of the system, and the possible effects of withdrawing hot fluid from the area. The TOUGH2 model covers a 70 by 130 km area and extends to 2 km depth. Modeled temperatures matched measured well temperatures using surface heat flow rates to constrain the heat input at depth. The high temperature gradient observed in the top 500 m was replicated using a low thermal conductivity of 1.05 W/m°C in the shallow Tauranga Formation sediments. A good match could be obtained over the majority of the field using a homogeneous 2-layer model and two zones of basal heat influx. The model shows that heat flow is conductive to the northwest, but convective to the southeast. The geothermal system appears to be stable over long periods of time in its natural state. When warm water is extracted, the pressure of the system re-equilibrates within a few months. However, there is a permanent decrease in temperature. After extraction has ended, the system takes hundreds of years for the temperature to return to its natural-state levels. Therefore it is important that these systems are carefully managed, and that modeling is carried out to ensure that they are not over-produced. |