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Definitions of Geothermal Terms

Why have a glossary?

The beginning of wisdom is the definition of terms (Socrates, c.400 BC (Not independently verified)).
There are inconsistent and unclear definitions and uses of geothermal terms across the published literature and technical reports. This causes confusion and reduces the ability to directly compare geothermal systems or locations. It can also impede communication about geothermal-related projects across organisations.

Glossaries and dictionaries standardise the meanings of words so that people with different levels of experience can be aligned and use geothermal words accurately and consistently. Glossaries exist for physics terms, geology terms, engineering terms, etc. The geothermal community would also benefit from standardised definitions for geothermal terms, to align with colleagues and to communicate effectively to people who want to learn more about geothermal.

Glossary theme

This glossary provides a foundation set of terms about geothermal systems and some of the necessary components for a successful geothermal project. Its intention is to provide a quick reference document for a range of geothermal stakeholders.

The geothermal terms in this glossary are grouped into sections. In each section, the terms are listed in the sequence that they impact/cause each other, which is more useful than in alphabetical order. Words that are italicised have definitions elsewhere in the glossary. This is intended to be a living document which will be revisited as needed. Reach out to us with your suggestions to [email protected].


  1. Terms relating to Earth’s internal heat and its use
  2. Geothermal rock and fluid parameters, conditions and locations
  3. Types of geothermal system, geothermal play and thermal energy transfer
  4. Geothermal locations
  5. Geothermal and hydrothermal operations
  6. Stages and scales of geothermal project development
  7. Finance and risk considerations for geothermal projects
  8. References

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 modelled to have a higher energy density than shallower geothermal systems and 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, online). It is not the same as heat.

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

Heat: the flow of thermal energy between two objects or places due to a difference in temperature between them (a temperature gradient).

Heat can be transformed into work (e.g. University of Calgary, online). 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 that originates from a few metres to several kilometres beneath the Earth’s surface (modified after University of Bern, online).

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 (72 to 87 °F/mi) per kilometer 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 or °F per kilometer/mile, Kelvin per kilometer, or milli Kelvin per metre deep.

Hydrothermal: an adjective relating to hot water, the action of hot water, or the products of this action (Mindat.Org, online). 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.

It 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: is 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 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 that concerns the generation, use, conversion, and exchange of thermal energy between physical systems.

Heat Flux: is 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 fluids or magma (Allen and Allen 2013; Boden, 2016).

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) flowing in a circular path, also out of rock that is in contact with moving fluid.

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

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. Britannica, online.

Enthalpy [reservoir]: the measurement of energy or total heat in a thermodynamic system (Oxford English Dictionary).

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

Reservoir enthalpy is described below. It is different from working fluid enthalpy.

  • Low-enthalpy is used to describe low temperature (e.g. reservoir temperatures are less than 90°C) and low pressure conditions. The liquid is generally used to provide direct heat for (or to cool) buildings.
  • Medium-enthalpy is used to describe moderate temperature (reservoir temperatures around 90 to 150°C) 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 are 150 to 374°C) 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) and high pressure conditions. Superhot rock geothermal plays are modelled 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 can be 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 (U.S. Department of Energy, online). 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.

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), and (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.

Pore (rock): a void space between grains or crystals in a rock.

Porosity (rock): the ratio between the volume of pores in a rock and the total volume of the rock.

This rock property is used in geological, engineering and geothermal resource calculations because rock pores are filled with heated fluids. It is stated as a percentage. 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 (The Century Dictionary, 1899).

Permeability: the property or state of being permeable.

This rock property 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 darcies. The antonym is impermeability.

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.

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.

Sedimentary (rock): a variety of rock types formed from fragments of other rocks (e.g. sandstone) or made by once-living organisms (e.g. coral limestone).

They usually occur in low lying areas of Earth’s surface, for example 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).

They 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, outcropping basement, naturally fractured basement reservoir, etc.

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.

It is an essential ingredient of a hydrothermal system because it transfers geothermal energy to Earth’s surface. Geofluid 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: is 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 are [re]injected too.

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 100℃ (705℉) at Earth’s surface.

It has a high thermal capacity, which in turn offers a high thermal conductivity. Numerous geothermal wells have drilled into superheated fluid. 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.

Supercritical (state): a substance at a temperature and pressure where distinct liquid and gas phases do not exist. This would be above 374℃ (705℉) and above 22 MPa (3205 PSI) for pure 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. Modelling 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 2022). 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.

3. Types of geothermal system, geothermal play and thermal energy transfer

There is a variety of geothermal systems and geothermal plays that differ because of their fluid temperature and pressure, reservoir depth, reservoir rock type, type of engineering enhancement, heat transfer mechanism, etc. Some are listed below.

Geothermal system: a connected set of components that operate together to transfer Earth’s thermal energy to Earth’s surface.

Geological components include thermal energy, heated rock, fractures, fluid, and porosity. Human components include wellbores, surface heat exchangers, turbines, etc. Some of these systems are exploited by humans for, “conventional geothermal energy generation”. In other geothermal systems, the geological components need enhancing by engineers, i.e. enhanced geothermal systems and next generation geothermal projects.

A geothermal system is comprised of one or more geothermal plays, that in turn contain one or more geothermal fields, each of which can contain one or more types of geothermal resource.

Geothermal projects are classified in different ways by different organizations (e.g. Rybach, 1981), e.g.:

  • Geological setting and how geological systems formed
  • Depth to the heat source (‘shallow’ and ‘deep’ geothermal system have a heat source above and below 500 meters depth respectively (Pérez, 2019; British Geological Survey, online)).
  • Convective or conductive heat transfer mechanism (e.g. Rybach, 1981).
  • Temperature or enthalpy of the thermal energy.
  • Well test results or responses to long-term fluid production.
  • Type of engineering enhancement needed.
  • Fluid circuit type, e.g. open or closed loop.

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

The commercially operating ones are named 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. All hydrothermal systems are geothermal systems. Not all geothermal systems are hydrothermal systems. 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 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): the term used when engineers artificially create or improve a hydrothermal system.

It is 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 a synonym for Enhanced Geothermal System. Superhot rock is a geothermal play type that will likely require engineering. Modified from Massachusetts Institute of Technology (2007) and Breede et al. (2013).

Enhanced Geothermal System (EGS): a synonym of Engineered Geothermal System (e.g. U.S. Department of Energy) 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 and play 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, 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 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. It is in development for deep and next generation geothermal projects. The advantages of a deep, closed-loop geothermal circuit for superhot rock geothermal plays would 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 (d) adapting methods and logic that already exist for shallow, closed-loop geothermal circuits.

Next generation geothermal [projects]: a variety of hydrothermal and engineered geothermal projects that are in the research or testing phases of development, e.g. superhot rock geothermal.

One or more geothermal components 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.

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; Ho et al. 2018; University of Bern, online).

To extract Earth’s heat, the geothermal reservoir needs to be engineered into a hydrothermal reservoir (see Engineered Geothermal System). Hot Dry Rock geothermal and Superhot Rock geothermal play types are categories of Petrothermal systems.

Hot Dry Rock (HDR) geothermal [play]: a high-enthalpy geothermal play within a petrothermal system.

Superhot rock (SHR) geothermal [play]: a super high-enthalpy geothermal play within a petrothermal system.

SHR systems would require fluid to be circulated into high-temperature, high-pressure rocks then ascend as superhot fluid, superheated fluid or supercritical fluid. The aim of an SHR geothermal project is to bring large amounts of very high heat density to Earth’s surface–e.g. five to eight times the heat of a typical HDR system well (e.g. Johannsson 2016; Reinsch et al., 2017)–to generate very high power density.

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 et al. 2023; Future of Texas Geothermal Energy, 2023)

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

Geothermal play: the portion of a geothermal system that contains one heat reservoir, its subsurface fluid, and a cluster of geothermal fields, geothermal discoveries, geothermal prospects and geothermal leads, all surrounded by impermeable 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.

Aquifer-hosted geothermal [play]: the play type (Ball et al. in prep for Geothermal Rising presentation) that has geofluid circulating through rocks with natural matrix porosity and permeability (modified after University of Bern, online). These plays are usually in permeable sedimentary rocks (Ball et al. in prep for Geothermal Rising presentation).

Sedimentary rock geothermal [play]: the play type (Ball et al. in prep for Geothermal Rising presentation) that has the hydrothermal reservoir within sedimentary rocks. (Ball et al. in prep for Geothermal Rising presentation). These plays are generally low-enthalpy and mid-enthalpy geothermal systems (Ball et al. in prep for Geothermal Rising presentation). They are commonly aquifer-hosted geothermal systems, but introduced geofluids can also be used.

Fracture-hosted geothermal [reservoir]: the reservoir type (Ball et al. in prep for Geothermal Rising presentation) in hydrothermal and engineered geothermal systems where heat transfer occurs in fracture networks within sedimentary rock or crystalline rock (Ball et al. in prep for Geothermal Rising presentation).

Crystalline rock geothermal [reservoir]: the reservoir type (Ball et al. in prep for Geothermal Rising presentation) 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 (Ball et al. in prep for Geothermal Rising presentation). Superhot rock geothermal resources are likely to be in crystalline rock geothermal reservoirs.

Geothermal lease (noun): 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 from

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, geothermal plays and/or geothermal fields.

Geothermal lead (noun): 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 from Jahn, F.; Cook M.; Graham M. (2008). Hydrocarbon exploration and production. Developments in petroleum science. Vol. 55. Elsevier. p. 444. ISBN 9780444532367.&

Geothermal prospect (noun): 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. Modified from…. and,a%20lead%20to%20a%20prospect.

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

There is 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 be several kilometers in height from the deep geothermal resource to Earth’s surface (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 a separate entity to a geothermal field, geothermal discovery, geothermal play and geothermal system. Electrical power can be produced via a variety of technologies, depending on the temperature and nature of the fluid brought to surface.

4. Geothermal locations

Igneous province: a region close to Earth’s surface 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 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, e.g. 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.

5. Geothermal and hydrothermal operations

Base load [power] (also stated as baseload): a term used for: (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]: are 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 have low operation costs due to their use of lower-cost fuels (energy).

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).

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

Open loop operation: a geothermal plant that flash geothermal fluids can emit naturally [dissolved] gas [e.g. N2, H2, CO2, CH4) to come out of solution and to be released into the atmosphere.

Mitigation steps can be introduced but they are not always required (Fridriksson et al 2017; Ball 2021).

Closed loop operation: a geothermal plant prevents naturally occurring gas to exit subsurface working fluid and be released into the atmosphere (CARB. California’s 2000–2014 Greenhouse Gas Emission Inventory, California Environmental Protection Agency, Air Resources Board, Air Quality Planning and Science Division, pp.174, ghg_inventory_00-14_technical_support_document.pdf (2016).Also known as Binary system that takes advantage of ORC turbine technology.

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 footprint, greenhouse gas emissions footprint, subsurface footprint, etc. 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, 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. 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.

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, (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 United States Geological Survey, online).

Fracking (sometimes spelled frac’ing): a colloquial term for hydraulic fracturing. Fracking is used for engineered geothermal systems. It is more benign than fracking to extract oil and gas (CATF, 2022).

Seismic: an adjective relating to a vibration, tremor or earthquake in Earth. Modified after Miriam Webster, online.

Seismicity: the distribution of vibrations, tremors and earthquakes in space and time. Modified after Fowler (1997).

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.

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

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 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, online.

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

6. Stages and scales of geothermal project development

A new geothermal project and plant is developed and tested through a sequence of phases and milestones. They include proof of concept (POC), prototype, and pilot phases. That development sequence needs to include: (a) a redesign of a geothermal system into a hydrothermal system (b) a redesign of existing processes for analysing and integrating information, and (c) persuading and training potential stakeholders into the new way of thinking. There are also a few categories of project readiness: technological, commercial and market readiness.

There are a variety of published geothermal project workflows that seem to be for conventional geothermal systems (e.g. Aragón-Aguilar et al., 2018; Hanson, 2019a, b). While they provide a general sequence of tasks, they seem to assume that no new technology or innovation is needed. However, superhot rock and next-generation geothermal projects will need innovation, demonstration, and learning-by-doing.

Proof of concept (POC) phase: a small exercise to test a discrete design idea or assumption.

The primary objective 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 studies of the geology and non-geology, geoscience surveys at the earth’s surface, and 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.

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 (Utah FORGE, online). Another example is the Derek Riddell Eavor-Lite™ Demonstration Facility by Eavor, a geothermal research and development company (Eavor, online).

Pilot phase: when the system’s product is available for a subset of the market/stakeholders to test.

The purpose is to get 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 organisations outside the stakeholder group. For example, ST1’s Otaniemi Engineered Geothermal System is exploring the technical implementation for the heating plant and how to intensify injected water flows in the deep bedrock fractures (ST1 2021, online).

Demonstration scale: a limited-scale project that is run 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, etc. Modified from Geothermal Canada, online.

Commercial scale: the scale of geothermal project that is generating sufficient heat/power for its customer and sufficient monetary revenue for its investors.

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.

Next of a kind: the new geothermal power plants, innovations or technologies that stemmed from the first of a kind. They seek to generate better innovations, new plant designs, increases in scale and lower costs.

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.

7. Finance and risk considerations for geothermal projects

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) and 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.

LCOE can be a misleading comparative measure of renewable power when electric power/energy product is required across an entire time period, e.g., a week, because the same degree of reliability and other attributes needed by a consumer/electricity grid are not provided by intermittent energy sources as base load sources.

Therefore, measures that are more comprehensive than LCOE should be used to compare the equivalent benefits of geothermal power with fossil fuel energy sources, that include lifecycle costs and risks.


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