Superhot Rock Energy Glossary

The beginning of wisdom is the definition of terms (attributed to Socrates, c.400 BCE).

This glossary provides a foundational set of terms about geothermal systems and some of the necessary components for a successful geothermal project. It focuses on superhot rock energy, a high-temperature form of geothermal energy. The intention of this glossary is to provide a quick reference document for a range of geothermal stakeholders, and to streamline the often inconsistent and unclear definitions of geothermal terms used across published literature and technical reports. We hope that this glossary will help the geothermal industry communicate more clearly both internally and externally.

The geothermal terms in this glossary are grouped into sections. In each section, the terms are listed in sequence based on their relationships with each other. Italicized words have definitions elsewhere in the glossary.

This glossary is a project of the Superhot Rock Energy team at Clean Air Task Force. Clean Air Task Force (CATF) is a global nonprofit organization working to safeguard against the worst impacts of climate change by catalyzing the rapid development and deployment of low-carbon energy and other climate-protecting technologies. With 25 years of internationally recognized expertise on climate policy and a fierce commitment to exploring all potential solutions, CATF is a pragmatic, non-ideological advocacy group with the bold ideas needed to address climate change. CATF has offices in Boston, Washington D.C., and Brussels, with staff working virtually around the world.

CATF’s Superhot Rock Energy team is a group of scientists, industry practitioners, and policy visionaries dedicated to decarbonizing the energy sector through superhot rock energy. Our goal is to achieve demonstration and commercialization of SHR anywhere in the world, providing affordable access to the largest untapped energy source on the planet. Learn more at catf.us/superhot-rock.

Credits

Key contributors: Dr. Graham Banks, Principal Geoscientist, Route to Reserves Consulting Inc.; Dr. Philip Ball, Chief of Geothermal Innovation, Clean Air Task Force; Project Manager: Terra Rogers, Superhot Rock Energy Program Director, Clean Air Task Force

Other contributors and reviewers: Dr. Bruce Hill, Ann Garth, Jenna Hill, Clean Air Task Force; Prof Dornadula Chandrasekharam, Izmir Institute of Technology, Turkiye; Bastien Poux, Aetna Geothermal Limited; Dr. Amy Whitchurch, The Geological Society of London

©Clean Air Task Force, 114 State Street, Boston, MA 02108 USA – catf.us

This is intended to be a living document which will be revisited as needed. Reach out to us with your suggestions at [email protected]. CATF is not responsible for the accuracy of any versions of this glossary in locations outside this website.

All rights in the SHR Glossary belong to Clean Air Task Force (CATF). The SHR Glossary is intended to be a living document to be updated and revised as needed in CATF’s discretion. CATF invites you to send any suggested updates and revisions to [email protected]. CATF reserves the right to accept or reject any suggested update or revision. All contributors to the SHR Glossary agree that their contributions, if accepted, shall be incorporated into, and become part of, the SHR Glossary. Contributors provide their contributions to CATF for inclusion in the SHR Glossary without royalty or fees, and such contributors understand and agree that they shall acquire no right, title, or interest in the SHR Glossary as a result of having their contributions accepted. Contributors may request permission from CATF to post the SHR Glossary in its entirety on their websites or in other media. CATF hereby grants to all users of the SHR Glossary a limited, royalty-free license to cite, quote, copy, and use portions of the SHR Glossary provided that any use is accompanied by the following attribution: “This text is taken from the SHR Glossary, a work created and owned by Clean Air Task Force.”

CATF makes no warranties, express or implied, regarding the accuracy, adequacy, completeness, legality, reliability, usefulness for any particular purpose, freedom from contamination by computer viruses, and/or non-infringement of proprietary rights in the SHR Glossary, and CATF disclaims all such warranties. CATF does not assume liability for any action or decision based on or made in reliance on the SHR Glossary or for any damages caused by use of the SHR Glossary.

1. Terms relating to Earth’s internal heat and its use

Energy: the ability (capacity) to do work. Energy is measured in Joules (J), calories or British Thermal Units (BTU).

Energy density: the total amount of energy within a system per unit volume, e.g., the calories in a packet of peanuts.

Geothermal energy is a favorable type of renewable energy because it has a higher energy density for an area on Earth’s surface than the footprint areas of other renewable energy sources. Superhot rock geothermal plays are modeled to have a higher energy density than shallower geothermal plays.

Thermal: an adjective relating to heat or temperature.

Thermal energy: a category of energy that is stored/contained within a system and responsible for its temperature (Khan Academy, 2023). It is not the same as heat.

Thermal conductivity: a measurement of the ability of a material (fluids, rocks, and soils) to conduct or transfer heat. This is extremely important for the design of geothermal wells and underground energy storage systems (e.g., Oliveira, 2021). The reciprocal of Thermal conductivity is thermal resistivity.

Heat: the flow of thermal energy between two objects or two places (e.g., from rock to groundwater) due to a difference in temperature between them (a temperature gradient).

Heat can be transformed into work (UoC, 2023). Geothermal systems harness natural heat from within Earth to work devices that warm (or cool) buildings, to generate electric power, or for other industrial uses.

Geothermal: an adjective relating to heat within Earth.

Geothermal energy: thermal energy inside the Earth. Geothermal energy is currently extracted from a few metres to a few kilometers beneath the Earth’s surface, to use as a practical source of energy (modified after UoB, 2023).

Geothermal energy is a clean, renewable source of energy throughout the daily cycle of power demand.

Geothermal gradient: the rate of temperature change with increasing depth into Earth.

Generally, the temperature rises by 25 to 30°C per kilometer of depth (0.014 to 0.016°F per 100 feet of depth) in most of the Earth’s upper crust (Fredleifsson et al., 2008). Some geological settings like igneous provinces and rift systems have a steep geothermal gradient, enabling geothermal reservoirs just a few kilometers deep to be very hot and within reach of a wellbore. The geothermal gradient is measured as °C per kilometer or °F per 100 feet.

Hydrothermal: an adjective relating to hot water, the action of hot water, or the products of this action (Mindat, 2023). Refer to hydrothermal system and conventional geothermal energy generation for insight about hydrothermal parameters in geothermal projects.

Heat density: the total amount of heat per unit volume, e.g., joules in one cubic meter of rock.

Heat endowment: the quantity and quality of geothermal heat in a particular geographic region or depth.

Heat endowment can be used to compare and rank the natural heat ‘wealth’ of different regions. Assessment inputs include heat density, depth to reservoir, geothermal play type, and available technology.

Heat transfer: the movement of heat from one substance or material to another, e.g., through rocks, from rock to fluid, or from fluid to turbine.

Commercial hydrothermal systems are often located where Earth’s internal heat transfer or geothermal gradient is higher than the average of Earth’s upper crust. The main modes of heat transfer in geothermal systems are advection, conduction, and convection. Heat transfer is also a branch of engineering that concerns the generation, use, conversion, and exchange of thermal energy between physical systems.

Heat flux: heat transfer per area per timeframe, e.g., joules of energy per second per square meter. Synonyms include thermal flux, heat flux density, heat flow density, and heat flow rate intensity, although “heat flow” is unnecessary because flow is incorporated within the word heat.

Conduction [geothermal]: heat transfer through rocks or fluids that are in contact, but not moving.

Conduction is the most common type of heat transfer in Earth, from a higher temperature region to a lower temperature region (e.g., Earth’s surface). It is a less efficient heat transfer mechanism than convection for geothermal projects. Petrothermal systems have conductive heat transfer until they are engineered to increase their convective heat transfer. Closed-loop geothermal circuits rely on heat conduction from the rock into the pipe that contains the subsurface working fluid.

Advection [geothermal]: rapid heat transfer of heat due to magmatic or hydrous fluids (Allen and Allen, 2013).

Advective heat transfer is controlled by the speed of fluid flow, for example through permeable rock from a higher temperature region inside Earth to a lower temperature region (Boden 2016).

Convection [geothermal]: heat transfer by the physical movement of molecules (i.e., fluid). The word is used in two contexts: (a) for fluids flowing in a circular path, and (b) heat transfer out of rock and into a moving fluid that comes into contact with the rock.

The circular flow path is caused by fluid buoyancy (hot less-dense fluid rises and cool denser fluid sinks) or geothermal gradients (Fowler, 1990). This term is widely used to encapsulate the combination of conduction and advection. Superhot rock geothermal plays will likely need to be engineered to increase heat and fluid convection in deep, superhot rocks. An example is an open-loop geothermal circuit.

Thermodynamics: a branch of science about the relationship between heat, work, temperature and energy.

In broad terms, thermodynamics deals with the transfer of energy from one place or form to another (Drake, 2023).

Enthalpy [reservoir]: the measurement of energy or total heat in a thermodynamic system.

Enthalpy is used to relate the energy of a system, heat transfer, and work done (Libretexts, 2023). Geothermal systems can be low-, medium-, high-, or super-high-enthalpy.

Reservoir enthalpy is different from working fluid enthalpy.

• Low-enthalpy is used to describe low temperature (e.g., reservoir temperatures are less than 90°C, <194°F) and low pressure conditions. The liquid is generally used to provide direct heat or to cool buildings.
• Medium-enthalpy is used to describe moderate temperature (e.g., reservoir temperatures from 90°C to 150°C, 194°F to 302°F) and moderate pressure conditions. The fluid is used to provide direct heat and/or generate electrical power.
• High-enthalpy is used to describe high temperature (e.g., reservoir temperatures from 150°C to 374°C, 302°F to 705°F) and moderate pressure conditions. They produce steam, which is used to generate electrical power. They are generally located in regions of Earth’s crust that have high temperatures at shallow depth, for example in active igneous provinces and young rift systems.
• Super-high-enthalpy is used to describe very high temperature (e.g., reservoir temperatures greater than 374°C, >705°F) and high pressure conditions. Superhot rock geothermal plays are modeled to be super-high-enthalpy. They would deliver a large amount of heat up to Earth’s surface in supercritical and superheated fluids with a very high energy density, to generate a lot of electrical power.

Power [geothermal]: the amount of energy transferred from one component of a geothermal system to another per second.

Typically, geothermal power plants use heat-carrying fluids extracted from hydrothermal reservoirs to produce electrical power or heat. Power is measured in Watts (W).

Power density: the power that produced per unit of mass, area or volume; for example, the electrical power produced per square meter on Earth’s surface.

Power density is useful for comparing the amount of power generated by individual power sources.

Capacity: a group of terms relating to measurements that include (a) storing or providing a substance or energy or (b) generating, transmitting, and purchasing power.

Geothermal power plants have a high-capacity factor compared to other renewable energy sources, which means they can operate at maximum capacity nearly all the time. The efficiency of a geothermal system is how much of its heat capacity (megawatts of heat brought to Earth’s surface) can be converted into electric power capacity (megawatts of electricity). Geothermal-generated electricity is measured by capacity and energy or heat.

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2. Geothermal rock and fluid parameters, conditions and locations

Matrix [rock]: the solid part of a rock mass which contains the pores, fractures and fluids.

Reservoir: a term used for either (a) a rock volume or magma volume that contains thermal energy (strictly a geothermal reservoir) or (b) a rock volume containing fluid-filled pores or fractures where Earth’s heat is transferred into the fluid from the rock (strictly a hydrothermal reservoir).

Heat and fluids are retained in the reservoir by surrounding impermeable rocks. Geothermal wells are drilled to extract heat from reservoirs through hydrothermal systems, closed-loop circuits, engineered geothermal systems, or other methods for geothermal heat extraction.

Pore [rock]: a void space between grains or crystals in a rock that can contain fluids

Porosity [rock]: the ratio between the volume of pores in a rock and the total volume of the rock, i.e., the capacity of rock to contain a fluid. It is expressed as a percent of rock volume.

Porosity is used in geological, engineering and geothermal resource calculations because rock pores are filled with heated fluids. Pores need to be connected to other pores for hydrothermal fluids to convect heat. There are several porosity types pertinent to geothermal resources including total porosity, primary (matrix) porosity, fracture (secondary) porosity, isolated porosity, and connected (effective) porosity (e.g., Hook, 2003; Tiab and Donaldson, 1996).

Permeable [rock]: the ability of a rock to permit the passage of fluids without rupture or displacement of pores (TCD, 1899). The antonym is impermeable. An impermeable rock is sometimes informally called “dry rock.”

Permeability: the capacity of a porous rock to transmit fluids.

Permeability is used in geological, engineering, and geothermal resource calculations because heat transfer in convective geothermal systems requires permeability. There are several permeability types pertinent to geothermal systems including primary (matrix) permeability, secondary (fracture) permeability, and effective permeability (e.g., Tiab and Donaldson, 1996). It is measured in the unit called darcy.

Impermeable [rock]: A rock that does not permit the passage of fluids through it (Oxford Learner’s Dictionaries, 2023a).

Fracture [verb]: the action of cracking, dividing or breaking a rock.

Fracture [noun]: a general term for several types of breaks in a rock– for example faults and joints–that may contain pore spaces and fluids.

Connected networks of open fractures enhance the permeability and storage capacity of a hydrothermal reservoir. This enables fluids and heat to migrate to a geothermal production well.

Permeability creation: the action of various engineering techniques to create fluid pathways within rock, such as fracture enhancement or creation by hydraulic pressure or imposed temperature differential.

Examples include widening natural fractures (stimulation), creating new fractures, or installing a closed-loop geothermal circuit. The purpose is to increase the speed and amount of heat extraction out of the thermal reservoir. Permeability creation is how a thermal reservoir is modified into a hydrothermal reservoir.

Sedimentary [rock]: a variety of rock types formed from fragments of other rocks (e.g., sandstone), made of compressed organic matter (e.g., coal), or made by once-living organisms (e.g., coral limestone).

Sedimentary rocks usually form in low lying areas of Earth’s surface, such as sedimentary basins in rift systems. Some sedimentary rocks and basins host conductive hydrothermal reservoirs. They generally provide higher natural porosity and permeability, and easier drilling conditions, than crystalline rock reservoirs.

Crystalline [rock]: a variety of rock types formed by crystallization from either: (a) magma or lava (i.e., igneous rocks) or (b) rocks modified by elevated pressures and/or temperatures (i.e., metamorphic rocks).

Crystalline rocks host conductive geothermal systems and convective hydrothermal systems. They may have lower hydrothermal reservoir capacity than sedimentary rock reservoirs because their tightly interlocked crystals have low matrix porosity and permeability. However, they may have taller fractures and vertical permeability than sedimentary rock reservoirs, which could bring heat from deep in Earth to a shallow depth. This makes crystalline rocks attractive reservoirs for superhot rock geothermal plays. Crystalline rock can be engineered to create convective hydrothermal systems.

Crystalline rock is sometimes called “basement”, “basement rock,” or “basement reservoir.” However, “basement” is not a recommended term for the geothermal industry unless it is used with qualifiers, e.g., economic basement, seismic basement, naturally fractured basement reservoir, etc. (Banks and Peacock, 2020).

Crystalline rock is sometimes called “bedrock.” However, “bedrock” is not a recommended term for the geothermal industry because bedrock can be made of crystalline or not crystalline rock types.

Subsurface working fluid: the geofluid or the introduced working fluid that is heated in natural or engineered hydrothermal reservoirs, and in open-loop and closed-loop geothermal circuits.

Geofluid: naturally occurring brine, water, steam or supercritical fluid within Earth or at Earth’s surface.

Geofluid is an essential ingredient of a hydrothermal system because it transfers geothermal energy to Earth’s surface. It has little monetary value. The value is in the geothermal energy that it transfers. An exception is geothermal brine that may contain minerals such as lithium in high enough concentrations to be extracted and sold. Geofluid is sometimes referred to as groundwater or formation water (e.g., Moeck, 2014), but these terms are discouraged because they do not communicate that geofluids may be supercritical vapor, steam, or brine.

Introduced working fluid: fluid sourced by engineers and introduced into a geothermal reservoir that has insufficient geofluid.

This introduced fluid can be a mixture of water, steam, brine, or carbon dioxide. The terms “artificial fluid” and “injected fluid” (e.g., Moeck, 2014) are not recommended, because an introduced working fluid can still be natural and geofluids can also be (re)injected.

Superhot [fluid]: a colloquial term to represent very hot fluids that are produced from deep in Earth’s crust.

The temperature, pressure and physical state are not defined.

Superheated [fluid]: a substance that stays in its liquid state above its boiling temperature (Roy, 2001) or a pure water steam that exceeds 374°C (705°F) at Earth’s surface.

Superheated fluid has a high thermal capacity, which in turn offers a high thermal conductivity. Superheated fluid could transfer heat with very high energy density out of a superhot rock geothermal reservoir. This could be used to generate a higher power density than hydrothermal systems. Superheated fluid differs from supercritical fluid because it has not reached the fluid’s critical pressure (see Figure 9). Numerous geothermal wells have drilled into superheated fluid.

Supercritical [state]: a substance at a temperature and pressure where distinct liquid and gas phases do not exist. This would be above 374°C (705°F) and above 22 MPa (3205 PSI) for water.

Supercritical water is a vapor that has the heat-transferring capacity of liquid yet flows with the ease of steam. Supercritical fluid could transfer heat with very high energy density out of the thermal reservoir of a superhot rock geothermal reservoir. This could be used to generate a higher power density than conventional hydrothermal systems. Modeling suggests supercritical fluid from a superhot rock geothermal play could transfer five to eight times the thermal energy to Earth’s surface than fluid from conventional geothermal energy generation (Johannsson, 2016; CATF, 2022b). Few wells have encountered supercritical fluid (Reinsch et al., 2017). Research is ongoing to use supercritical fluids to harvest geothermal resources.

Circuit: a set of components that form a circular path or route for fluid flow, e.g., to make open-loop geothermal circuits and closed-loop geothermal circuits.

Superhot rock: A subsurface geologic rock resource existing in-situ at or above the supercritical temperature of water, 374°C in de-ionized water (or higher in brine).  

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3. Types of geothermal systems and thermal energy transfers

System: A group of things, components, pieces of equipment, etc. that are connected or work together (Oxford Learner’s Dictionary, 2023b).

Hydrothermal system: the category of geothermal system with convective heat transfer in a naturally occurring geofluid.

All hydrothermal systems are geothermal systems. Not all geothermal systems are hydrothermal systems. Current commercially-operating hydrothermal systems are conventional geothermal energy systems. Their heat-carrying geofluid, reservoir storage capacity, and rock permeability are naturally of sufficient quality to circulate heat towards Earth’s surface. Hydrothermal systems can be created from low permeability geothermal systems by engineering interventions (see engineered geothermal system).

Conventional geothermal energy [generation]: delivery of thermal energy from shallow or deep hydrothermal reservoirs to Earth’s surface without an introduced working fluid.

The technique is well-established with technologically mature and commercially available methods (Deb, 2021). Heated geofluid is pumped up production wells and used in a geothermal plant. That cooled geofluid is then pumped down reinjection wells to recirculate through the hot, permeable reservoir and reheat. The amount of geofluid and natural reservoir permeability is sufficient to support large-scale heating or power generation. Some generate electricity at competitive prices (Malek et al., 2022). The antonym is unconventional geothermal (e.g., Deb, 2021) energy generation.

Unconventional geothermal energy generation: delivery of thermal energy from shallow or deep low-permeability geothermal reservoirs to Earth’s surface due to introduced working fluid (in open-loop or closed-loop circuits) and/or engineering the reservoir’s permeability. These techniques are recently proven or still in development. Superhot rock geothermal resources will be used to generate unconventional geothermal energy. The antonym is conventional geothermal energy generation.

Engineered geothermal system (EGS): a geothermal system that engineers have artificially created or improved.

Engineered geothermal systems are used in a variety of geothermal plays that have hot rocks but insufficient reservoir quality. Human intervention enhances the heat exchange from the hot rock to the subsurface working fluid, via: (a) permeability creation and reservoir stimulation (modified after Moeck, 2014), (b) increasing the initial amount of subsurface working fluid by injecting fluid into the rock, and/or (c) reinjecting and recirculating the subsurface working fluid. EGS is often used as a synonym for enhanced geothermal systems. Superhot rock is a geothermal play type that will likely require engineering. (Tester et al., 2006, Breede et al., 2013.)

Enhanced geothermal system (EGS): a synonym for engineered geothermal system (e.g., DOE, 2016) and next generation geothermal projects that is entrenched within geothermal literature.

Open-loop [geothermal circuit]: a circuit containing subsurface working fluid that is heated in the hydrothermal reservoir during direct contact with rock pores and fractures.

Open-loop circuits currently operate in shallow, deep, hydrothermal, and engineered geothermal system types. The fluid ascends a production well and is used to work a heat or power device at Earth’s surface. The cooled fluid descends a reinjection well back into the hydrothermal reservoir rock, absorbs more heat, and then recirculates to a production well. Open-loop circuits could operate in superhot rock geothermal plays. They may require large volumes of introduced fluid because some fluid may be lost into the hydrothermal reservoir during each fluid circulation.

Closed-loop [geothermal circuit]: a circuit containing subsurface working fluid that is heated in the reservoir without direct contact with rock pores and fractures. Instead, the subsurface working fluid stays inside a closed loop of deeply buried pipes that conduct Earth’s heat.

Shallow, closed-loop geothermal systems have been operating for decades, and deep and next generation closed-loop geothermal projects are in development. The advantages of a deep, closed-loop geothermal circuit include: (a) no need for a geofluid, (b) no need for the hot rock to be permeable, (c) all the introduced fluid could be recirculated, and (d) the ability to adapt methods and logic that already exist for shallow, closed-loop geothermal circuits.

Next generation geothermal [projects]: an umbrella term for a variety of hydrothermal and engineered geothermal projects that are in the research or testing phases of development, including superhot rock geothermal.

One or more components of a next generation geothermal project has insufficient quality for commerciality and needs to be enhanced with innovative geoscience and engineering (Deb, 2021). The aim is to enhance underground heat exchange from the hot rock into the subsurface working fluid.

Superhot rock energy: Geothermal energy extracted from superhot rock to generate heat and/or power.  

Petrothermal system: a type of geothermal system that is assumed to lack producible geofluids because the subsurface heat exchanger is in impermeable rock below a hydrothermal system/groundwater aquifer (modified after Moeck, 2014; Min et al., 2018; UoB, 2023).

To extract Earth’s heat, the geothermal reservoir needs to be engineered into a hydrothermal reservoir (see engineered geothermal system). Hot dry rock geothermal is one category of petrothermal systems.

Hybrid geothermal systems or multi-system hybrids: systems that couple (a) two geothermal system types, such as engineered geothermal systems and advanced geothermal systems, or conventional hydrothermal systems, or (b) two different energy systems such as solar and geothermal, direct air capture and geothermal, hydrogen and geothermal, energy storage and geothermal, etc.

These systems can be deployed in a variety of rock types (Beard and Jones, 2023).

Advanced geothermal system (AGS): a colloquial term applied to some deep, closed-loop geothermal circuits and some types of next generation geothermal concepts.

Fracture-hosted geothermal [reservoir]: the reservoir type in hydrothermal and engineered geothermal systems where heat transfer occurs in fracture networks within sedimentary rock or crystalline rock.

Crystalline rock geothermal [reservoir]: the reservoir type in hydrothermal and engineered geothermal systems with the subsurface heat exchanger within crystalline rocks. It can be in a low-, medium-, high-, or super-high-enthalpy geothermal system, and a conductive or convective geothermal system. Superhot rock geothermal resources are likely to be in crystalline rock geothermal reservoirs.

Geothermal lease: a deed with which a landowner authorizes exploration for–and production of–geothermal fluids or heat on their license area, usually in consideration of a royalty (modified after Law Insider, 2023a).

Lease categories include technical study leases, exploration leases, appraisal leases, and production leases, depending on the exploration and production maturity of the project. A lease may contain geothermal leads, geothermal prospects, and/or geothermal fields.

Geothermal play: the portion of a geothermal system that contains a heat reservoir, its subsurface working fluid (either native or introduced), and/or a cluster of geothermal fields, geothermal discoveries, geothermal prospects, and/or geothermal lead – that resemble each other geologically and share common risks (modified after Doust, 2009) – all surrounded by low permeability rocks.

The play is a useful construct to describe the spatial locations and monetary value of a portfolio of similar geothermal fields. The play concept enables geothermal exploration strategy planning, risk management, land evaluation, prediction of future geothermal exploration success, and ultimately the optimal strategic decisions about groups of geothermal fields and their geothermal resources. Many geothermal plays have insufficient quality to be a commercial asset in their natural state. One or more geothermal play components, usually the subsurface heat exchanger, need enhancing or creating.

Geothermal lead: a vaguely-defined underground geological feature that has the potential to contain a geothermal resource.

A geothermal explorer will carry out an initial screening of the earth below a geothermal lease to identify possible leads. Further work is then focused on the locations of those leads, with the intention of ‘upgrading’ some of them into geothermal prospects. (modified after Jahn et al., 2008).

Geothermal prospect: an underground, geological structure or region with indications that it could contain a hydrothermal reservoir or geothermal reservoir, which could be economically exploited.

A significant amount of geological investigation must be conducted to ‘upgrade’ a geothermal lead into a geothermal prospect. The next objective is to define a drilling location to test whether the geothermal prospect contains a hydrothermal reservoir or geothermal reservoir that could return a profit on the investment. A group of geothermal prospects of a similar nature constitutes a play.

Geothermal discovery: an underground geological structure or region in which a hydrothermal reservoir or geothermal reservoir has been proven, usually with an exploration well.

A geothermal discovery provides insufficient information to state the size and type of geothermal resource and whether it has economic potential.

Geothermal resource: the portion of a reservoir’s total geothermal energy that is technically recoverable from the earth.

The energy is either recoverable today (i.e., reserves), recoverable in the future after a technological, political, fiscal or other contingency is removed (i.e., contingent resources), or possibly recoverable in the future after more geological and engineering analysis to define its size (i.e., prospective resources) (modified after Tester et al., 2006).

Geothermal field: a localized volume of Earth’s crust where a geothermal resource can be extracted from a reservoir by subsurface working fluids and then delivered to a place of use (modified after Moeck, 2014).

A geothermal field can span several kilometers of depth from the deep geothermal resource to Earth’s surface (modified after Grant and Bixley, 2011). It incorporates the sub-surface geology, fluids and wells and well completion equipment.

A geothermal field can be owned by one or more lease-holding entities. One example of a geothermal field owned by multiple owners across different geothermal leases is Geysers geothermal field in California. To optimize geothermal lease management, each owner’s portion is decided through a unitization process.

Geothermal power plant: an industrial facility built on Earth’s surface to generate electrical power.

Synonyms include geothermal power station, generating station or generating plant. A geothermal power plant is separate from a geothermal field, geothermal discovery, geothermal play, and/or geothermal system. Electrical power can be produced in a geothermal power plant using a variety of technologies, depending on the temperature and nature of the fluid brought to surface.

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4. Geothermal locations

Igneous province: a region of the Earth’s surface and shallow subsurface that contains features related to volcanoes, hot magma, or rocks that contain magmatic and/or naturally radioactive heat. These geological settings are obvious targets for geothermal exploration. A sub-category of igneous province is a magmatic hot spot of the Earth’s crust, for example the Iceland and Hawaii hot spots. A few documents use “magmatic tectonic province,” but this term is geologically ambiguous and not recommended.

Rift system: a low land region of Earth’s surface that can be as much as several kilometers deep, tens of kilometers wide and hundreds of kilometers long, such as the East African Rift System. A rift system comprises a group of geological features that form due to tension (stretching) and subsidence (sinking) within or between tectonic plates. Many geothermal power plants are located in rift systems because of their large geothermal gradients, high heat density, and networks of large fractures (e.g., Buiter et al., 2022; Goutorbe et al., 2011). Superhot rock geothermal plays could occur in rift systems.

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5. Geothermal and hydrothermal operations

Reservoir management: the operations conducted to obtain the maximum possible economic recovery from a reservoir on the basis of facts, information, and knowledge. Sound reservoir-management practice relies on: use of financial, technological, and human resources; safety; and an attempt to minimize capital investments and operating expenses to maximize economic recovery of fluid from a reservoir (modified after Thakur, 1996).

Base load [power] (also stated as baseload): (a) the minimum power required during the daily cycle of power demand, i.e., when most of the population is asleep, and (b) the amount of power made available by an electric energy producer (e.g., a power plant) to meet that minimum level of power demand.

Base load [power sources]: the power sources that operate continuously to generate reliable and dependable power to consistently meet base load power demand.

Base load power plants supply the basic demand in a power network. They can operate for long periods of time at or near full load, and they can have low operation costs due to their use of lower-cost fuels.

Dispatchable [energy or generation of power]: a source of energy/power that can be controlled in a short amount of time, i.e., turned on, turned off or adjusted to market needs.

The term dispatchable is also used to imply a reliable source of energy/power. In contrast, non-dispatchable energy/power sources are less able to adjust their output to match a change in power demand, for example if the energy source is weather-dependent (adapted after Baroni, 2022), like in the case of wind or solar power.

Firm [power]: uninterruptible or guaranteed power. Firm power is (a) intended to be available at all times and/or (b) contracted to be supplied by the seller (e.g., Law Insider, 2023b).

Open-loop plant [power]: a geothermal plant in which flash geothermal fluids can emit naturally dissolved gas (e.g., nitrogen, hydrogen, carbon dioxide, and/or methane), which comes out of a solution and is released into the atmosphere. Mitigation steps can be introduced to limit emissions from open-loop operations, but they are not always required (Fridriksson et al., 2017; Ball, 2021a, b).

An open-loop plant is different from an open-loop circuit, which refers to the flow of fluid and heat in the subsurface.

Closed-loop plant [heat or power]: a geothermal plant that prevents naturally occurring gas to exit subsurface working fluid and be released into the atmosphere (CARB, 2016). This is also known as binary system, taking advantage of organic Rankine cycle (ORC) turbine technology.

A closed-loop plant is different from a closed-loop circuit, which refers to the flow of fluid and heat in the subsurface.

Direct-contact: a type of surface heat exchanger and a type of condenser inside some geothermal power plants, where heat and mass are transferred out of steam by mixing it with cold water (e.g., DiPippo, 2016b).

The term is also used to describe the heat transfer directly out of hot rock and into subsurface working fluid within open-loop, conventional or enhanced hydrothermal reservoirs.

Footprint: a term for the impact created by a geothermal plant or geothermal investigations during a geothermal project’s lifespan.

There are several categories of geothermal footprint, for example: land footprint, environmental footprint, carbon emissions footprint, greenhouse gas emissions footprint, subsurface footprint, and more. For example, a geothermal plant’s land footprint is the total area of land disturbed during direct and indirect activities during geothermal exploration, construction, operation, and remediation phases. Footprint can be stated in land area units, such as acres or square kilometres; energy or power density units, such as 200 MW per acre; and geothermal capacity units, such as square metres per kilowatt-hour.

Well [geothermal]: a conduit for fluids and information into/out of the thermal reservoir. Categories of geothermal well include production well and reinjection well.

Wellbore: a machine-drilled hole in the ground that hosts the well.

Production well [geothermal]: a type of geothermal well that transmits heat-containing fluids up from a hydrothermal reservoir to Earth’s surface (see Figure 21). It is also used to collect information about the reservoir and fluid.

Reinjection well [geothermal] (also stated as injection well): a type of geothermal well used to reinject heat-depleted fluid and dissolved minerals back into the hydrothermal reservoir after thermal energy has been extracted in the power plant (see Figure 21).

Fluid reinjection fulfils three requirements to maintain a power/heat plant’s production and associated commerciality: (a) extract more thermal energy from the reservoir, (b) replace the fluid previously removed from the reservoir, and (c) keep fractures open and the subsurface fluid pressurized so the fluid keeps flowing.

Stimulation [reservoir]: enhancement of a geothermal reservoir or hydrothermal reservoir to increase the amount or speed of subsurface working fluid flow (“reservoir quality”). This can be done by thermal, mechanical, or chemical methods. The objective of stimulation is to either: (a) restore permeability that was reduced when well drilling fluids plugged the rock’s pores; (b) enhance the natural, near-well permeability; or (c) connect the well to permeable fractures that were not intersected by the well (modified after Axelsson, 2012).

Hydraulic fracturing: one method of reservoir stimulation. High pressure fluid is injected into rock to create new fractures and/or increase the size, extent, and connectivity of existing fractures (modified after USGS, 2019).

Fracking (sometimes spelled fracing): a colloquial term for hydraulic fracturing. Fracking is used to create engineered geothermal systems.

Seismic: an adjective relating to a vibration, tremor or earthquake in Earth (modified after Merriam Webster, 2023).

Seismicity: the distribution of vibrations, tremors, and earthquakes in space and time (modified after Fowler, 1990).

Natural seismicity: vibrations, tremors and earthquakes that occur during natural adjustments of stress and strain within Earth’s crust.

Induced seismicity: vibrations, tremors and earthquakes that result from human-induced adjustments of stress and strain within Earth.

Causes could include: (a) movement along rock fractures triggered during reinjection of high-pressure water into a hydrothermal reservoir, (b) reinjected fluid cooling the hot rock or altering the reservoir’s natural stress field, or (c) project operating conditions causing changes in rock pressure (modified after Stober and Bucher, 2021). Induced seismicity risk should be estimated in engineered and next generation geothermal projects, and precautions against induced seismicity can be taken at geothermal plants (modified after Dincer and Ezzat, 2018; DOE, 2012).

Microseismic [events]: natural seismicity or induced seismicity events within Earth that are unnoticed at Earth’s surface because they release very little energy, are of very short duration, and are of very low magnitude.

Seismic risk: the probability of a natural or human-induced vibration, tremor, or earthquake occurring and causing damage within a given time interval and region (modified after Natural Resources Canada, 2021).

A seismic risk assessment should be conducted at each development phase of a geothermal project (modified after Stober and Bucher, 2021).

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6. Finance and geothermal project development

New technologies are emerging to overcome barriers to geothermal development and facilitate innovation in the geothermal sector. Innovative geothermal projects and plants are developed and tested through a sequence of phases and milestones.

Technology readiness level (TRL): the TRL scale is a widely used tool for a maturity assessment, with specific levels corresponding to different levels of development of a new technology.

The TRL scale allows a consistent comparison of maturity between different types of technologies and helps readers understand technology evolution, regardless of their technical background (De Rose et al., 2017).

Proof of concept (POC) phase: a small exercise to test a discrete design idea or assumption (see Figures 22-24).

The primary objective in the POC phase is to prove if a solution could be viable, e.g., determining if a piece of paper can fly by folding it into an airplane shape and throwing it. In a geothermal project, this phase would include desk-based feasibility studies and surveys related to the geological, geophysical, environmental and non-technical of the sub-surface and surface of the proposed test site, as well as the first wellbores to test if a geothermal resource and reservoir exists (e.g., Hanson, 2019a).

Prototype phase: a simulation of the full system, or a relevant part of it, to determine how parts of the system would behave at larger scales of operation (see Figures 22-24).

In a geothermal project, this phase could be field development and a limited-scale power plant (e.g. Hanson, 2019b). An example is Utah FORGE: an underground geothermal field laboratory with two geothermal wells (and four seismic monitoring wells) to develop, test, and accelerate engineered geothermal system technologies (FORGE, 2023). Another example is the Derek Riddell Eavor-Lite™ Demonstration Facility by Eavor, a geothermal research and development company (Eavor, 2023).

Pilot phase: when the system’s product is available for a subset of the market/stakeholders to test (see Figures 22-24).

The purpose of a pilot phase is to gain a better understanding of how the system will be used in the market and how to further improve the system. A synonym is “minimum viable product” (MVP). The pilot phase can feature (a) an internal pilot phase when the system is tested within the project’s stakeholders and (b) an external pilot phase when the system is tested on organizations outside the stakeholder group. For example, ST1’s Otaniemi engineered geothermal system is exploring the technical implementation for a heating plant and how to intensify injected water flows in deep bedrock fractures (ST1, 2021).

Field testing: testing a technology in the environment in which it is used.

Field R&D: the research and development of a technology in the environment in which it is used.

Demonstration scale: a limited-scale project that is conducted to demonstrate whether (a) the technology can run successfully at full-scale, (b) data can be generated to verify the company’s modeling of technical performance in a variety of real-world applications, (c) the project can extract required thermal energy, (d) any unproductive hydrothermal wells can be retrofitted to become productive, (e) pilot-phase models of the commercial-scale system have valid technical and economic assumptions, and so on (see Figures 22-24) (modified after GreenFire, 2020).

Commercial scale: a geothermal project that is generating sufficient heat/power for its customer and sufficient monetary revenue for its investors (see Figures 22-24).

An example is the enhanced geothermal system at Soultz-sous-Forêts (France). It has been generating commercial electricity since 2016, when reservoir stimulations and fluid injection were performed down to 5000 meters to increase the permeability and the fracture connectivity between the hot, dry crystalline rock, geothermal reservoir, and geothermal wells (e.g., Baujard et al., 2021).

First-of-a-kind: the first successful demonstration of a commercial-scale geothermal power plant, innovation or technology (see Figure 25).

Next-of-a-kind: new geothermal power plants, innovations, or technologies that stem from the first-of-a-kind. They seek to generate better innovations, new plant designs, increases in scale, and lower costs (see Figure 25).

Nth-of-a-kind (NOAK): power plants, innovations, or technologies that have been fully designed and optimized. Continued cost declines are unlikely at this phase (see Figure 25).

Levelized cost of electricity or energy (LCOE): a reporting standard to assess and compare the cost of electricity between energy sources (modified after Lazard, 2019; EIA, 2020).

The LCOE of an energy-generating power plant is the average cost of building and operating the power plant per unit of total electricity generated over its assumed lifetime. It is a comparative metric to (a) determine whether or not to move forward with a project, i.e., if it will break even or be profitable, or (b) assess and compare alternative methods of energy production.

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