MODELLING THE TAURANGA LOW TEMPERATURE GEOTHERMAL SYSTEM

Abstract

A low temperature geothermal system underlies the Tauranga Harbour to Maketu area of the Bay of Plenty Region. The temperature of the system ranges from cool (18 °C) to warm groundwater (less than 70 °C). The resource is not mineralised, but freshwater. There are no active hot geothermal surface features and the resource is confined but not free-flowing artesian. The resource is tapped by direct use bores, and the variance of temperature across the field means a range of usage including: municipal supply, irrigation, commercial pools and domestic space and water heating /pool water.
The 1991 Resource Management Act stipulates that water 30°C and above is geothermal. For allocation purposes this has caused difficulties in the Tauranga area as the aquifer is both groundwater (<30°C) and geothermal (>30°C). They are one-in-the-same resource, but allocation from one can impact on the other.
To understand and simulate heat and mass flows through this low-temperature geothermal field a TOUGH2 numerical model has been created. This model covers a significantly larger area than typical Taupo Volcanic Zone field models. As regular monitoring of production rates is not widespread in the Tauranga System, finding suitable data to calibrate the model proved to be a challenge. In the end the calibration dataset included downhole temperature profiles, and water level changes due to seasonal production changes. Seasonal changes in production data required for this calibration were estimated from the metered data of a selected set of production wells.
In this paper, we present the details of the model and the data used for calibration. Simulation of the allocated and estimated actual use patterns of the resource were undertaken to understand the likely impact of continued abstraction of geothermal groundwater on pressure and temperature in the Tauranga Geothermal System.