Fractures and fluid flow through naturally fractured geothermal reservoirs, Te Ahi Tupua/Taupō Volcanic Zone, New Zealand

Séminaire de Cécile Massiot, chercheuse au GNS en Nouvelle-Zélande

Te Ahi Tupua (Taupō Volcanic Zone), the volcanic area in the central North Island of Aotearoa New Zealand, hosts 25 geothermal systems. Geothermal resources have been used by Indigenous Māori for centuries, are considered taonga (treasure). Some geothermal systems are now used commercially for heating, bathing, and for electricity generation (18% of New Zealand’s electricity). Some systems remain protected or undeveloped. I will briefly touch on the acceptance, risks and social context related to geothermal use in Te Ahi Tupua, and new types of resources that are being explored. 
Characterising the fracture networks and their contributions to permeability is an important component to sustainably use geothermal energy, and to understand rifting processes. Fractures directly observed in 2-3 km deep geothermal borehole images of Te Ahi Tupua are fantastic datasets to unravel links between fractures and permeability. Borehole images were acquired during the 2019-2021 drilling campaign for the new geothermal power station at Tauhara by Contact Energy, which will generate 3% of New Zealand’s electricity from early 2024. Fracture orientations are consistent with the Taupō Rift with local variations. Permeable zones are associated with reservoir-scale faults, fracture clusters or series of fractures. However, similar fracture characteristics are also found outside permeable zones.
Modelling of flow through discrete fractured networks (DFN) that uses interpretations of borehole image from the Wairakei and Rotokawa geothermal fields shows that the fracture networks are connected are reservoir scale, even though over half of the fractures are not connected. Fluid particles can take numerous pathways, with similar behaviour to real tracer tests. The pressure distributions in individual DFN realisations are variable but mean flows and permeability anisotropies derived from multiple realisations can be used in traditional continuum models at a limited computational cost. 
More information on Cécile’s webpage : https://www.gns.cri.nz/about-us/staff-search/cecile-massiot/