Session 4B: Super Hot Geothermal

Current geothermal power plants are characterized by geofluid production temperatures ranging from about 100-250 °C, with research underway to enable much higher temperatures. The range of 300 °C and above is considered herein, encompassing the region of “superhot rock”, or SHR, defined as exceeding 375-400 °C. To quantify the potential of these systems, the exergetic power of a production well is assessed at its maximum flow capacity, as determined empirically by the onset of erosion. This indicates that at a given temperature, the power output potential is maximized where the geofluid production is most dense. Performance plateaus around the critical point, beyond which higher exergy is approximately offset by reduced mass flow rate. Three types of power cycles were considered: dry steam, flash, and binary, with each cycle corresponding to specific production conditions. Dry steam cycles were found to confer superior exergetic efficiency, but relatively low gross power output because of the characteristically low production density. Flash and binary cycles were found to be a significant improvement on an equal-temperature basis, with the lower exergetic efficiency compensated by a much higher mass flow rate. Single-, double-, and triple-stage cycles were considered for both flash and binary plants, and optimized for net work using gradient ascent algorithms. This yielded heuristics for the optimal design of both cycle types. For binary cycles specifically, pure water was selected as the working fluid of choice on the basis of its superior performance at high temperatures, as compared to the hydrocarbons characteristic of organic Rankine cycles (ORCs). The performance of flash and binary plants was found to be highly similar in the sense of thermal and exergetic efficiency. Diminishing returns were observed in both cycles, with a significant improvement from adding a second stage, and only marginal improvement from a third. The choice between flash and binary may depend more on practical considerations such as geochemistry, reinjection requirements, and capital costs. Altogether, the plants considered herein have the potential to produce about 30 MWe gross per production well with an 8.5 inch minimum inner diameter, up to an order of magnitude more than is typical of current geothermal systems.

Well casings in geothermal wells that are used to produce from high-temperature fields are prone to fail due to a combination of factors, e.g. the quality of the cemented annulus and the degree of temperature change, that can result in structural overload. The main failure modes have been identified as collapse and tensile rupture of the casing. Casing collapse is an effect of annular pressure buildup in the cemented annuls primarily due to excess water and/or closed off water in the cement sheath that expands as the well warms up for the first time after drilling. Whereas tensile failure is an effect of permanent deformation in compressive (hot) state where the yield strength is surpassed generating plastic strain in the material that in tensile (cold) state results in tensile failure. Such casing failures can cause significant production problems, both in terms of wellbore flow restrictions and operational safety. Additionally, current lessons learned from drilling and producing from superheated/supercritical deep geothermal wells brings forward the conclusion that the metal casings are a bottleneck for reliable structure and/or a successful project. These challenges need to be solved to continue the pursue of deep drilling into superhot conditions. Two technological advancements have been in development within ÍSOR, (i) Flexible Couplings are intended to solve the axial thermal expansion problem by allowing displacement of every casing joint into the connection and (ii) pressure relief system to relief temporary overpressure within a cemented annulus that is intended to prevent casing collapse. These two solutions work together in increasing the structural integrity of the casing throughout its lifetime. The patented Flexible Couplings have been developed within ÍSOR from the year 2015 within several research projects (EU Horizon 2020 supported GeoWell and DEEPEGS, and GEOTHERMICA project GeConnect) and the annular pressure relief system is being developed in the EU Horizon Europe supported project COMPASS.

Commercial utilization of superhot rock geothermal energy could make a significant contribution to global decarbonization by making high-enthalpy geothermal power available across diverse geographical settings. This paper aims to conduct an assessment of the optimal energy production pathways of superhot rock energy downstream of the geofluid production at the surface and identify any remaining equipment gaps for dealing with geothermal fluid above 400˚C in a power plant system. The focus lies on the needs to foster innovation and identifying an efficient system that is not currently available off-the-shelf. Setting a benchmark of 500MW for energy production represents an approach aligned with industry trends in high-temperature, high-pressure, hot-dry rock geothermal systems. This benchmark sheds light on critical technological gaps that must be addressed to unlock the full potential of superhot rock energy and facilitate a seamless transition towards sustainable energy sources.

By undertaking this evaluation, the paper provides valuable insights into the current state of superhot rock energy technology but also to pave the way for future research and demonstration initiatives. Ultimately, the findings of this study are poised to inform strategic decision-making processes, guiding investment priorities and shaping the trajectory of superhot rock energy production towards a more sustainable and resilient energy landscape.

Superhot geothermal appears an attractive opportunity for New Zealand (NZ), from 2037 through to at least 2050. It provides a potentially zero-emissions, reliable, and cost-competitive energy source for electricity generation or high-enthalpy direct-use industrial applications. Superhot geothermal, and even conventional geothermal, has been under-represented in the deliberations and consideration of renewable energy sources by energy planners that NZ might utilise in the future. Policy discussed in several planning documents has focused on solar and wind deployment and has under-appreciated the full potential of geothermal in plans for official endorsement.

The NZ research program, “Geothermal - the Next Generation”, has been investigating the potential of supercritical geothermal, and has completed two important assessments for NZ. These challenge the current under-representation of geothermal potential by building electricity scenario models to include utilisation of superhot geothermal resources.

The first assessment is a preliminary resource inventory assessed over a depth range of 3.5 to 6 km, and at temperatures between about 375 oC and 500 oC. These inferred resources are located in the Taupō Volcanic Zone (Waikato and Bay of Plenty Regions) and at Ngawha (Northland). The cumulative resource potential is determined to be 4.6 GWe (38 TWh/yr at 95% operating capacity factor). If the resource volumes within current regulatory protected geothermal systems are excluded, the assessed total becomes 3.5 GWe (29 TWh/yr).

The second assessment uses electricity market modelling to provide an estimate of the likely commercialisation process for superhot geothermal power plants. The analysis, adopting the grid system operator’s assumptions, forecasts that superhot geothermal power plants could contribute an additional 1.4 or 2.1 GWe capacity (depending on whether or not some gas-fired electricity generation is permitted for peak loads) to the NZ grid between 2037 and 2050. Using economically optimised scenarios, the modelling shows that the addition of superhot geothermal power plants effectively doubles the expected contribution from conventional geothermal power plants, and would still be economical to construct, even if superhot projects prove to be twice the cost of conventional geothermal projects.

The vision is to build a new clean energy industry to catalyze energy transition globally by creating Enhanced Geothermal Systems (EGS) to harvest heat from Super-Hot Rock (SHR) resources (rocks above 375 °C). The goal is to generate terawatt-scale, carbon-free power from such unconventional resources that will be globally accessible, secure, and cost less than $45 per MWh. Two critical technology areas of focus for us to achieve sustained power generation from SHR-EGS developments are:

• High integrity well construction: design of reliable, long-life injection and production wells for hostile SHR environments.

• Creation of a thermal lattice: a novel stimulation process to create high-rate pathways that maximize contacted rock volume in SHR. Flow from injection well to production well is primarily through this Engineered Rock Volume (ERV).

Our internal discussions on how to guide our development of the wells and the thermal lattice led to adopting the use of models. Commercial geothermal models were judged to be not fully applicable for the conditions of the planned implementation. Through an evolutionary process we internally developed an Integrated Asset Management (IAM) model for the full system cycle of injection wells, heat harvesting from the ERV, production wells, and electrical generation. This model, coupled with economic evaluation, has advanced the project well design, stimulation treatment, and operational strategy. It has given greater understanding to unconventional heat harvesting, and it is an enabler towards the goal of cost effective power generation.

The model is based on fundamental physics of heat and mass transfer that includes conduction, convection, and advection for heat harvesting from SHR geothermal wells. It is a transient pressure and enthalpy balance solved as a finite element model with dynamic cells. Water transports through the cells a) down injection wells, b) through the thermal lattice, c) up production wells, d) through steam turbines for electrical generation, and e) finally, recycled back to the injection wells. The integrated framework solves the entire system to best meet a specified, electrical power target.

The SHR EGS model must address the operational challenge above 375 °C when water becomes supercritical and its thermophysical properties change dramatically. These supercritical changes affect the production well pressure temperature characteristics differently compared to conventional geothermal wells. These changes also affect the flow in the wells and in the thermal lattice and guide the needs of the wells and the stimulation treatments to create the reservoir. Finally, an important benefit is that supercritical water has a much higher enthalpy capacity than at conventional temperatures. Therefore, SHR-EGS considerably reduces water requirements while yielding well power capacities substantially beyond conventional EGS.

The goal for any model should be reliably assessing and resolving the challenges of SHR-EGS, and it is about addressing the issues need that to be tackled, and that haven’t been addressed, in unconventional heat harvesting. A model must be living and learning that will evolve based on data and findings from the field.

For half a century Landsvirkjun, the national power company of Iceland, has been exploring and utilising some of world's hottest geothermal resources. Wells drilled into superheated resources believed to be around or above 360°C have proven to be very powerful but hard to operate due to various challenges including corrosion, particle erosion, silica scaling potential, thermal expansion etc. The most significant effort to date was well IDDP-1 drilled into 950°C hot magmatic body in the Krafla Geothermal Field in 2009. The well was flowtested in 2010-2012 where various tests were carried out providing valuable learning for future projects. The well was flowtested with various wellhead pressure but enthalpy was constant around 3150 kJ/kg and superheat of 160°C (above boiling temperature). Landsvirkjun is currently considering next steps towards utilisation of superhot geothermal in near-magma environment.

This paper presents a brief overview of the experience, challenges and learning from drilling and operating superhot geothermal wells in Iceland and oulines the outlook for further efforts in the near future, including the Krafla Magma Testbed project, KMT, plannied in 2025-2029.