Deep hydrology of the geothermal systems in the Taupo Volcanic Zone, New Zealand

Abstract

This thesis is a study of the large scale flows of water and heat which give rise to the geothermal fields in the Taupo Volcanic Zone (TVZ), New Zealand. To carry out this study, a super-critical equation of state module has been developed for the geothermal simulator TOUGH2, which can describe the flow of water at the conditions expected deep in the TVZ. The code is used to simulate the behaviour of a range of idealised TVZ models in 2D and 3D settings. Hydrothermal plumes which remain stable for periods comparable to the lifetime of the TVZ can occur when there is a contrast between the high permeability of the inner TVZ 'infill' region and the lower permeability exterior region. In this case, downflows of cool surface fluid in the inner TVZ 'sweep' the geothermal heat across the TVZ at depth to the permeability barrier, where the heated fluid ascends to the surface in discrete plumes. This behaviour occurs in 2D models, where separate plumes form at each side of the high permeability infill region, and also in 3D models of caldera-like structures, where perhaps four hot plumes can form around the perimeter of the caldera. This notion is then applied to the complete TVZ hydrological system, where a permeable ‘envelope’ is defined by the location of the Taupo Fault Belt and the currently known volcanic centres in the TVZ. The permeability within this envelope varies spatially according to the geothermal heat flux, and the region outside has relatively low permeability. The spatial variation of the geothermal heat flux is obtained by summing the measured heat flows from the geothermal fields for a number of areas across the TVZ. In this model, the geothermal fields form about the boundary of the envelope, as in the TVZ, and bear a striking resemblance to the actual TVZ geothermal fields. Finally, a new simulation code, NaCl-TOUGH2, is developed to provide a tool for future modelling involving the commonest chemical species in the TVZ - salt. The code incorporates the complete phase diagram for salt-water mixtures and involves liquid, vapour and solid phases over a wide range of temperatures, pressures and salt concentrations. The code is used to solve a number of simple geothermal and mathematical problems.