Tauranga Basin geothermal reservoir model

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Pearson, S.C.P.; Alcaraz, S.A. 2013 Tauranga Basin geothermal reservoir model. Lower Hutt, N.Z.: GNS Science. GNS Science report 2013/13 54 p.

Abstract: This report describes a reservoir model of the Tauranga Basin created by GNS Science for the Bay of Plenty Regional Council. A numerical model of heat and fluid flow was created using the Petrasim interface to TOUGH2 software. The model was calibrated against well temperature data, and simulates the effects of withdrawing warm water at currently consented rates. The model covers 130 km by 70 km to 2 km depth, and was calibrated against temperatures measured in 17 wellbores. Modelling shows that bulk permeabilities of -14 m2 in the sedimentary cover and -16 m2 in the underlying volcanic rocks are required to maintain the (primarily) conductive heat flow regime. The permeability in the sedimentary cover is comparable to values estimated at high-temperature geothermal systems in the Taupo Volcanic Zone, while the permeability in the volcanic rocks is at least an order of magnitude less than that observed in high-temperature, convection-dominated geothermal systems worldwide. The best-fit model (R2=0.9) corresponds to thermal conductivities of 1.25 and 1.8 W/m2 for sedimentary and volcanic rocks respectively, and maximum heat flux of 350 mW/m2. The total surface heat flow is estimated to be 258 MW over 2200 km2. Heat flux is highest under Tauranga City, which may be attributable to enhanced permeability associated with inferred local faults, or to a greater source of heat in this area. The geothermal system provides energy that has been used for over 40 years, and which continues to be used for commercial, irrigation and residential uses. This production and reinjection was added to the natural-state model created. Results showed that once production and reinjection began, simulated pressures dropped rapidly over a few months and then restabilised, while simulated temperatures declined more slowly and consistently. Modelling suggests that after 400 years of use at 10% of allocated rates (the estimated actual usage), pressure will have dropped by up to 25%, and temperature by 5%. With allocated use, modelled extraction of more than 5 kg/s in a 1 km2 area results in cooling to less than 5°C in as little as 38 years. Effects appear to be localised, but severe. Currently allocated use includes eight areas with more than 5 kg/s of extraction in a 1 km2 area, and 63 areas with less than 5 kg/s extraction. (auth)