An Updated Numerical Model of Rotorua Geothermal Field

Abstract

The Rotorua geothermal field is situated at the southern margin of the Rotorua Caldera in the Taupo Volcanic Zone, New Zealand. The Rotorua system lies beneath a small city and has an abundance of natural features of great cultural, economic and scientific value. However from the 1950’s onwards, intensive extraction of fluid and heat from over 900 shallow wells for commercial and industrial purposes resulted in a decline of the surface features. In 1986, a bore closure program was introduced and geyser activity and hot springs have rejuvenated progressively with some springs overflowing recently for the first time in over 30 years. Two three-dimensional numerical models of the Rotorua system have been developed to study the response of surface features to production and reinjection, called here UOA Model 3 and 4. UOA Model 3 and 4 differ from previous models by having a much finer layer structure in the shallow zone − minimum block size of 250m2 x 10m and 125m2 x 5m respectively − and by including an unsaturated zone. This enables a better representation of near-surface mass and heat flows. They also include the complex structural and lithological structures associated with the asymmetrical caldera collapse setting of the Rotorua geothermal system. Both models have been calibrated against a large number of shallow temperatures, water table levels, the locations and magnitudes of surface activity and pressure transients.