Modelling the Effects of Direct Use at the Tauranga Low-Temperature Geothermal System

Abstract

Tauranga is underlain by an extensive low-temperature geothermal system covering approximately 40 km by 60 km. Warm springs in the area are between 22ºC and 39°C, and well temperatures have reached 67°C at 750 m depth. The resource has been used for the last 40 years, for heating, cooling, glasshouses, bathing and tropical fisheries. In order to better delineate the system and estimate the effects of current and future withdrawal of warm water, a numerical model of the field has been developed. A heat and fluid flow model of the area was created using TOUGH2 software. It comprises two layers representing volcanic units and overlying sediments. It was calibrated against temperature depth profiles in 17 wells, and checked against a further 437 mainly single well-temperature measurements. A reasonable fit could be obtained with this simple model, with an R² of 0.9. The model suggests that the highest heat flux is centred under Tauranga City. Modelling current estimated usage in the study area shows an initial rapid drop in pressure by up to 25% which then stabilises, while temperature gradually and consistently declines by up to 5% over 400 years. Other scenarios show the effects of higher extraction rates of warm water, providing a tool to help with management of the low-temperature geothermal resource.