The Use of Geothermal Reservoir Modelling and Resource Assessment in Geothermal Regulation and Sustainable Resource Management

Abstract

This thesis was written as a multi-disciplinary effort to analyse the use and management of geothermal resources from a regulatory, economic, and technical perspective with the final aim being to contribute to their sustainable use. The main research question to be answered is: ‘How sustainable is the use and management of geothermal energy resources in New Zealand, and what improvements to resource assessment, management, and monitoring may be desirable to achieve better sustainable management of the resource?’ Chapter 1 outlines the background for this thesis and research question. The consideration of this question is split into two parts. The first part (Chapter 2) focuses on the first, regulatory part of the above research question, and explores in detail how (geothermal) sustainability is defined, around the world and in New Zealand. In this thesis, the chosen definition of sustainability is considered a process for managing natural, renewable resources, reaching some level of equilibrium between resources needed for development in the present (rate of use) and the rate of renewal/conservation of resources available for future generations, while avoiding irreversibility in damages, permanent depletion, and continual diminution/collapse to renewal rates. It is also argued that sustainability is not an absolute concept but should be measured on a spectrum or scale and along multiple dimensions, with possible trade-offs in practice. This thesis focuses on the availability (longevity) and management of (geothermal) resources per se for future generations. A scale is defined from very weak - weak - strong - very strong sustainability. Chapter 2 continues to describe the implementation of sustainable geothermal management in New Zealand under the Resource Management Act (RMA, 1991) through regional policies, plans, and reporting practices. Although the original intent of the RMA (1991) might have been for strong sustainability and the definition of hard ‘bottom lines’, in its final formulation and implementation in reality over the last three decades, a weaker form of sustainability has generally prevailed. However, a comparison with some of the other main geothermal countries around the world shows that New Zealand still has one of the stricter definitions and implementations of sustainable geothermal resource management, comparable only to Iceland. One of the central policy choices in New Zealand’s regions has been to use a hierarchical, portfolio approach whereby a significant share of the geothermal resources is protected, while a limited number of systems are allowed for development under controlled depletion over 50-100 years, followed by allowing such systems to lie fallow for a similar period and subsequent recuperation. This way, ‘weak’ sustainability on an individual development system basis is complemented by protection, rotation, and recuperation on a portfolio scale (weak+ sustainability). However, the monitoring and management of this overall policy is hampered by the lack of a harmonised geothermal resource assessment framework and infrequent monitoring and reporting, especially on the quantification of potentially additional resources available for future development and for conservation. Improvement of such resource assessment and management frameworks is one of the main recommendations. The second part of this thesis (Chapters 3 and 4) focuses on the main engineering part of the above research question and compares different geothermal vs petroleum and mining resource assessment and reporting frameworks. This thesis chooses the new UNFC resource assessment framework and geothermal specifications (UNECE-IGA, 2016), and adapts it to the New Zealand regulatory requirements. This framework is then applied to a detailed case study – Ohaaki – over the more than 50 years of its exploration and development. Technical lessons on which units to use, categories to report, assumptions to clarify, etc. are discussed, as well as lessons to implement this adapted UNFC framework to all NZ geothermal resources. The first results of this analysis are presented in the final Chapter 5, as well as a discussion on how this could assist in improving the sustainability of New Zealand geothermal resource management.