Large-Scale Natural Convection in the Wairakei-Tauhara Geothermal Field

Abstract

The Wairakei-Tauhara geothermal system, located within the Taupo Volcanic Zone, has been the subject of extensive geological and geophysical research, and many numerical modelling studies. In the present study the structure of large-scale convection in the Wairakei-Tauhara system is of interest. The variables investigated are the permeability structure, the topography and the heat sources. The data used for calibration are the surface mass flow and surface heat flow, based on pre-exploitation field data. A three-dimensional model was created that is larger, deeper and simpler than current Wairakei-Tauhara models. It is large enough that the Rotokawa geothermal field is also included within the model. Many different numerical simulations were carried out using TOUGH2, exploring the effects of different permeability and heat input structures. Initial models had a very simple structure and were investigated mainly to confirm the effects, already reported in literature, of different permeability structures on the number and spacing of convective plumes. As further simulations were performed it was found that the most appropriate form of the large scale permeability structure to generate the required plume spacing was non-uniform and anisotropic. Anisotropy, when combined with a cap rock layer near the surface, and a non-uniform heat flux as a bottom boundary condition, produced three plumes with surface outflows in approximately the correct locations to represent the Wairakei, Tauhara and Rotokawa fields. The mass and heat outflows for each of the fields shows a good correlation to the field data.