Bromley, C.J.; Rae, A.J.; Alcaraz, S.A.; Kissling, W.M.; Rivera, J.M. 2024 Inventory of New Zealand supercritical geothermal resources. Lower Hutt, N.Z.: GNS Science. GNS Science report 2023/50. 29 p.; doi: 10.21420/053X-5618
Abstract
The objective of this report for the Geothermal Next Generation (GNG) research programme is to undertake a preliminary inventory assessment and an initial quantification of New Zealand’s supercritical (>374°C, >220 bara) and/or superhot (>374°C) geothermal resources, at potentially drillable depths. [Please see Appendix 1 for definition of ‘supercritical’]. The inventory focusses on the Taupo Volcanic Zone (TVZ), in the Waikato and Bay of Plenty Regions, along with the Northland Region (Ngawha). Specifically, the purpose is to target the ‘deep roots’ of known subcritical (<374°C) geothermal systems, combined with any suspected ‘hidden’ superhot resources. The assessment is based largely on geophysics data. The outcome is a draft compilation, or inventory, of such potential resources for the purposes of long-term energy planning. An approach to superhot geothermal resource assessments, applying the revised and updated UNFC-2009 / UNECE-2022 categorisation approach, is utilised and discussed. An important challenge for constructing this pre-drilling, preliminary inventory of such resources in New Zealand is the unknown parameter of superhot reservoir permeability (both for the natural state condition and for a development-modified or stimulated state), which will govern the potential economic energy deliverability, and extraction sustainability of such resources. It is also challenging to predict the mineral composition and gas content of anticipated discharges from these superhot fluid reservoirs. For the resource inventory, an analogy is proposed based on the observed physical and chemical conditions of superhot fluids that have been encountered in other countries. Large scale mathematical models and simulations of natural state mass and heat flow through the TVZ using supercritical capable codes provide constraints on likely rock and reservoir properties at the depths of interest (3–7 km). These properties influence the likely productivity and sustainability of supercritical or superhot fluid resources. Consequently, although there are insufficient data, at present, to construct robust simulations of individual prospective resources, representative development scenarios of a typical superhot prospect can be simulated through economically viable extraction lifetimes, informing the preliminary inventory assessment. The conclusion of this assessment is that potential superhot energy resources equivalent to about 3.5 GWe of installed capacity, and 30 TWh/year of baseload generation, from the depth range of 3.5 to 6 km, are likely to be available for utilisation in the future, once technical challenges are overcome. The UNFC methodology categorises the inventory of potential projects reported here, at E3.2, F4.2, G4.1 (344), reflecting the current level of uncertainty, prior to deep drilling. At the conclusion of the GNG programme, the geophysical data coverage and modelling provides a partial assessment of New Zealand’s superhot potential. The inventory has been established such that, as data coverage is extended and models refined, or drilling depths increased, the assessed potential can be updated. At an individual project level, the UNFC-2019 classification will be revised as exploratory drilling and pilot studies are undertaken, enlarging the knowledgebase of a specific superhot geothermal project.
