Geothermal energy resources can be subdivided into three classes based on the temperature ranges of their sources and produced fluids, and the resulting applications. Low-Temperature (<150°C/300°F) resources are a standard and widely used approach for local as well as utility-scale thermal energy production, primarily for heating and storage. Ultra-High Temperature (>250°C/480°F) resources are being investigated in research projects but do not yet have commercial potential due to the engineering challenges of drilling and measuring under these conditions. Between these two extremes, High-Temperature (150-250°C/300-480°F) resources offer a favourable combination of factors including temperatures that enable efficient electricity generation and acceptable drilling and measuring costs. But even after more than a century of development efforts, only a very small share of global electricity supplies is derived from geothermal sources due mainly to geological, engineering and cost constraints.
However, over the past few years, High-Temperature geothermal resources have successfully been accessed for electricity generation by the introduction of ‘next generation’ production methods from unconventional oil and gas production including horizontal drilling and hydraulic stimulation. These methods include the Enhanced Geothermal Systems (EGS) approach that has evolved rapidly to enable significantly increased geothermal electricity generation. The key benefit is its ability to overcome one of the main limitations of previous methods by drastically increasing the surface areas that enable thermal energy to be exchanged from rocks to producing fluids.
The reference research project for EGS is Utah FORGE which is funded by the U.S. Department of Energy (DOE), and the reference commercial project is the ongoing development by Fervo Energy of the Cape Station geothermal power plant in Utah, USA. Cape Station construction started in late 2023, after the concept had been tested in pilot projects, and accelerating technical learning curves enabled 200,000 feet (60,000 m) of drilling to be performed during a single-rig drilling programme in 2024, with 50% of the drilling in hard rocks. Electricity delivery from a 500 MW plant is now expected to start in 2026 after about 3 years of drilling/construction and will make Cape Station the 5th largest geothermal plant in the world. Permits are already in place for the project to produce up to 2 GW so that Cape Station could become the largest geothermal power plant in the world, providing clear proof of the ability of the EGS approach to rapidly deliver new utility-scale baseload electricity supplies.
These new developments have important implications for worldwide geothermal exploration and the particular combination of thermal characteristics and reservoir conditions enable Utah FORGE and Cape Station to be used as an ‘EGS’ geothermal exploration play concept with some similarities but also major differences compared to conventional geothermal exploration and production.
Exploration for geothermal targets might initially appear to be simple: search for the highest possible temperatures at the shallowest possible depth, for example 200°C at 2000 m depth. Mapping surface expressions of high temperatures, such as hot springs, is a good starting point, however studies in basins with favourable geothermal conditions such as the Great Basin in the USA have indicated that up to 75% of the potential geothermal systems may be ‘blind’, i.e. have no surface expressions such as fumaroles or hot springs. Regional geothermal mapping is therefore a significantly more complex process and can only be performed by detailed analyses of specific geological characteristics and processes.
Thermal gradients are frequently used to indicate the increase in subsurface temperatures with depth. Their apparent simplicity is however a major pitfall as the underlying thermal data is often not QC’ed, heat transfer processes are not taken into account and the gradients are not well defined and often misused, for example for inter- and extrapolations, resulting in oversimplifications of actual subsurface temperature distributions. However, thermal modeling is one of the foundations of Basin and Petroleum Systems analysis and thermal modeling tools have been used by petroleum explorationists for more than half a century. Basin and petroleum systems modeling experts should therefore be closely involved in thermal data QC and evaluation for geothermal screening and portfolio management.
As EGS production methods are very closely related to unconventional petroleum production, geothermal explorationists also need to have access to the related technical expertise to assess the required reservoir properties in prospective geothermal areas of interest. Reservoir simulators used for hydraulically fractured reservoirs have been successfully adapted for EGS applications including long-term circulation and concomitant thermal analyses. An understanding of next-generation geothermal production engineering developments and the required reservoir properties, especially for Enhanced Geothermal Systems (EGS), is therefore an essential prerequisite for geothermal play to prospect risking.