NUMERICAL MODEL OF THE DEEP HYDROTHERMAL CONVECTION IN THE TAUPO VOLCANIC ZONE, NEW ZEALAND

Abstract

The Taupo Volcanic Zone (TVZ) is a NNE-trending rifting arc, characterized by extensive volcanism. Associated with this volcanism high-temperature (>250°C) geothermal fields occur with convective circulation to depths of 7–8 km. The purpose of this study is to create a representative model of the deep hydrothermal convection of the TVZ. Study investigates the variables that are affecting the major features controlling the geothermal fields distributed throughout the area. Model encompasses 21 geothermal fields, namely Kawerau, Rotoma, Tikitere/Rotoiti, Taheke, Rotorua, Horohoro, Waimangu, Waiotapu-Waikite, Reporoa, Te Kopia, Ohaaki, Orakeikorako, Ngatamariki, Atiamuri, Rotokawa, Ongarato, Mangakino, Mokai, Wairakei, Tauhara, and Lake Taupo. The model was divided into two main regions, the interior region and exterior region. Interior region is bounded by the Kaingaroa Fault to the East, with the Taupo Fault Belt (TFB) presented in the centre of the TVZ. The heat flow at the base of the exterior region is set to the normal terrestrial flow of 0.09 W/m2, while the heat distribution at the base of the interior region is varied to achieve the measured heat output at the surface. Dominating geological structures such as caldera boundaries, distribution of greywacke basement and faults that provide the permeability needed to facilitate the movement of fluids in the system were inferred from recent geological and geophysical studies. The distribution of heat at the base of the model (approximately 6.7 km) and the permeabilities of geological structures were the parameters to be estimated. The model was calibrated to predict the location of discrete plumes, magnitude of heat discharge from each field and reservoir temperature.This work investigates the possible impacts of infield reinjection of CO2 in two-phase liquid-dominated geothermal reservoirs using an earlier computer model of the Wairakei-Tauhara system (O’Sullivan and Yeh 2007) as a representative case study. Various reinjection scenarios were applied to test alternative reinjection strategies. Different injection rates of CO2 were used along with the separated geothermal water and its effects on: reservoir pressure, temperature, production enthalpy, steam and CO2 production were investigated. The breakthrough of CO2 was also monitored since it can result in lower power recovery and higher gas (CO2) production, hence higher practice load. The modelling results showed that the injection of CO2 helps maintaining the reservoir pressure, but at the same time it suppresses boiling which results in reduction of the enthalpy of the produced fluid.