Sarah D. Milicich

Exploring structure and stress from depth to surface in the Wairakei Geothermal Field, New Zealand

Structures such as fractures and faults have an important role as fluid flow pathways in geothermal fields, as the reservoir rocks hosting geothermal resources can often have little to no intrinsic permeability. As such, understanding and characterizing this structural network is vital to developing reservoir models and field operation and development plans that will maximize the potential of a geothermal resource. Presented here are the preliminary results of three recent studies, micro-earthquake analysis, borehole logging, and active fault mapping, carried out in the Wairakei Geothermal Field to determine the structural character of the system, if and how it contributes to fluid flow, and how the structural observations from these studies inform and relate to each other. Across all three techniques a dominant NE-SW structure strike orientation is observed with lesser population of N-S, E-W and NW-SE, consistent with the broad Taupo Volcanic Zone observed trend. Further analysis of the data is required to resolve important structural questions around the Wairakei Geothermal Field including: whether the data supports the model of the Wairakei Geothermal Field being an expression of enhanced permeability due to its location in an inferred rift accommodation zone, how the links between observed structures at the surface and subsurface can be resolved, and what role to these structures play in geothermal fluid flow from depth to surface?

3D Geological modelling of the Taupo Volcanic Zone as a foundation for a geothermal reservoir model

ABSTRACT We present a 3D hydrogeological model of the central Taupo Volcanic Zone (TVZ), including seven geothermal fields. A combination of borehole data, surface geology and structures were used to generate representative geological groups at depth. The geological units in the model area have been combined into seven hydrogeological groups based primarily on rock types with similar hydraulic characteristics, with permeabilities ranging from 9 × 10−15 to 2 × 10−9 m2. The fault structures used in the model are simplified to nine faults that are laterally extensive or host significant fluid flow. These faults, other than the caldera collapse structures, strike northeast–southwest, reflecting the dominant structural trend in the TVZ. By visualising hydrogeological groups in 3D, realistic geology and hence permeability input data can be more easily integrated into numerical fluid flow models.