NCG-NACL Reservoir Modelling In Waiotapu-Waikite-Reporoa Geothermal Areas

Abstract

Natural state simulation of the Waiotapu, Waikite and Reporoa geothermal fields was previously carried out, with a saturated atmosphere model, by Kaya et al. (2014). In the present study, the model is modified so that the effects of saline fluids (NaCl) and non-condensable gases (NCG), on the Great Waiotapu system, can be considered. The previous saturated zone model is extended to include a gas/water interface. This enables the unsaturated zone, close to the ground surface, to be included in the model, and also increases the ability of the model to represent the infiltration zones. Additional surface manifestations are considered (i.e. surface heat discharges from Maungakakaramea and Maungaongaonga) for a more accurate representation of the Waiotapu region. The model is recalibrated in order to achieve agreement with available natural state downhole temperature data, magnitudes and locations of heat flow to the surface, as well as, measured chloride data. For the best fit model, the quantity of total mass upflow was assigned to be approximately 720 kg/s, which includes NaCl content ranging from 0.04 to 0.32% and CO2 content of 0.024%. The result of this model gives a total heat loss of ~685 MW and estimates Cl flux across the system to be 244 g/s.