NartIntel

The Glossary · Updated August 2026

The language of
the reservoir.

Every term you need to evaluate a geothermal opportunity, explained in plain language. Figures cited (LCOE, PPA prices, tax-credit status) reflect the market as of mid-2026. Treat them as anchors rather than constants.

Section 01

Resource Types & Subsurface

Hydrothermal system

A conventional resource where heat, permeability, and fluid all occur naturally in one reservoir. That is the geothermal analog of a conventional oil & gas trap. Requires the "right place" rather than engineering.

Blind geothermal system

A hydrothermal resource with no surface expression: no hot springs, fumaroles, or altered ground to give it away. Historically invisible to conventional exploration, which relied on surface manifestations. Machine learning over gravity, magnetic, geochemical and legacy well data is now surfacing them: Zanskar's Big Blind in Nevada (December 2025) was the first blind system confirmed commercial in the U.S. in over 30 years. Blind systems are the reason "AI-enhanced hydrothermal discovery" is treated as its own developer category rather than a research curiosity.

Next-Generation Geothermal (NGG)

The umbrella term for technologies that break geothermal's historic dependence on rare, naturally occurring hydrothermal reservoirs, engineering a reservoir, or a sealed loop, where nature did not provide one. Covers EGS, geopressured (GGS), advanced closed-loop (ACL), and superhot rock (SHR). Two boundaries worth holding onto. NGG means power generation, so direct-use heat and ground-source heat pumps sit outside it. And AI-enhanced hydrothermal discovery (Zanskar, Ignis) is not strictly NGG either, since the resource it finds is conventional. It is tracked alongside because it competes for the same capital and answers the same question.

Enhanced Geothermal System (EGS)

Hot rock with too little natural permeability, made productive by drilling and hydraulically fracturing an artificial reservoir, then circulating fluid between injector and producer wells. Borrows the shale playbook directly; Fervo's Cape Station is the flagship.

Advanced Closed-Loop (ACL)

Circulates working fluid through a sealed wellbore network, with no fluid exchange with the formation. Eliminates induced-seismicity risk and water loss at the cost of lower power density, because conduction through the pipe wall is slower than convective flow through a permeable reservoir. Eavor's Geretsried, Germany plant delivered the first commercial closed-loop power. Also written AGS (Advanced Geothermal Systems) in older sources; ACL is the more precise term and the one used throughout NartIntel.

Geopressured Geothermal System (GGS)

Taps naturally over-pressured, fluid-filled sedimentary formations, using the earth's own lithostatic pressure to do work that EGS does with pumps. A "huff-and-puff" cycle injects fluid into fractures, heats it in place, then recovers it. The distinguishing feature is that the same wellbore can run as long-duration energy storage, charging on cheap power and discharging at peak, which no other geothermal architecture offers. Viable sites need high pressure, high temperature and high permeability at once, which is rare. Sage Geosystems brands its version "Pressure Geothermal."

Superhot Rock (SHR)

Ultra-deep resources targeting supercritical conditions (>374°C, >221 bar), where energy density per well runs 5–10x conventional. Still R&D-stage in 2026. The gap is drilling wells that survive the temperature.

Hot Dry Rock (HDR)

The 1970s–90s precursor concept to EGS. Superseded as terminology, but it still appears in older literature and DOE program names.

Conductive vs. convective systems

Convective systems (volcanic/hydrothermal) move heat via circulating fluid and cluster near the surface in specific regions. Conductive systems (sedimentary basins, EGS targets) require deeper drilling but exist almost everywhere, which is the basis of the "geothermal anywhere" thesis.

Geothermal gradient

The rate temperature rises with depth, typically 25–30°C/km in stable crust, highly variable elsewhere. The key exploration parameter.

Sedimentary vs. magmatic geothermal

Basin-hosted heat overlaps with mature O&G basins and their data, so lower temperature and lower geologic risk. Magmatic systems (Great Basin, Iceland, Indonesia) run hotter but are geographically constrained.

Co-produced geothermal

Power or heat from hot water already produced by existing oil & gas wells, reusing the wellbore and subsurface data in place. A growing niche given the large inventory of idle wells.

Supercritical fluid

Fluid above its critical point (374°C / 221 bar for water), carrying dramatically more energy per unit mass. That is the prize SHR technology is chasing.

Non-condensable gases (NCG)

CO2, H2S, and other gases dissolved in geothermal fluid that must be extracted and managed. They materially affect plant design, parasitic load, and permitting.

Silica scaling

Silica precipitating out of cooling brine into pipes, heat exchangers, and injection wells. A major operating-cost and reliability driver, and a key due-diligence item on any operating asset.

Induced seismicity

Earthquakes triggered by fluid injection, most associated with EGS stimulation. A central permitting and community-acceptance risk, mitigated by traffic-light protocols and real-time monitoring. It is also the reason closed-loop developers market its elimination.

Section 02

Downhole Operations & Drilling

Directional / deviated drilling

Standard O&G technique now central to EGS and closed-loop alike. It places multiple laterals from one pad and building the horizontal sections closed-loop designs require.

High-temperature MWD/LWD

Measurement/logging-while-drilling tools rated for geothermal heat. Legacy oilfield tools rarely exceed ~175°C; geothermal wells routinely pass 200°C. This ceiling, not the rock, is the sector's single biggest technology bottleneck. Hephae's 210°C-rated system was the 2026 milestone.

PDC bit

The dominant fixed-cutter bit type, adapted from shale for geothermal's harder, more abrasive rock. Cutters lose integrity above ~200°C, driving materials R&D distinct from O&G bit design.

Hybrid bit

Combines fixed-cutter (shearing) and roller-cone (crushing) elements in one bit for the variable-hardness formations common in geothermal targets.

Mud motor

Downhole motor used for directional control. Its rubber stator is a key temperature-limiting component, because the elastomer fails long before the metal does.

Rate of penetration (ROP)

Drilling speed. In granite or basalt it can run an order of magnitude slower than sedimentary rock, making drilling time the dominant capex driver. Fervo's ~70% drilling-time cut is the KPI to watch.

Millimeter-wave / plasma drilling

Non-mechanical approaches that vaporize or spall rock instead of grinding it, aimed at ultra-deep SHR wells where bits fail. Quaise (gyrotron) and GA Drilling (plasma) lead; both pre-commercial in 2026.

High-temperature cementing

Standard cement loses strength at geothermal temperatures; wells need silica-modified specialty systems. Cement failure under thermal cycling is a recurring wellbore-integrity issue.

Injectivity / productivity index

Flow rate per unit of pressure differential, the primary technical readout of whether a well can accept or deliver fluid at commercial rates.

Stimulation (hydraulic, thermal, chemical)

Techniques to enhance permeability. Hydraulic fracturing is EGS's core method, tuned for seismicity control via smaller, staged, pressure-managed "soft stimulation" treatments.

Zonal isolation

Keeping fluid in the intended intervals via cement and packers. It is harder at geothermal temperatures because elastomers and cement bonds degrade faster.

Multilateral / fishbone wells

Multiple lateral bores from a single trunk, used to expand EGS reservoir contact, and the literal architecture of Eavor's closed loop.

Doublet

A paired injection and production well, the minimum unit of a circulating geothermal system.

DAS / DTS fiber sensing

Fiber-optic distributed acoustic and temperature sensing, used to track fracture growth, flow allocation, and thermal breakthrough in real time, and for induced-seismicity early warning.

Thermal cycling / wellbore integrity

Repeated heat-up/cool-down cycles that O&G wells rarely see, causing fatigue-driven casing failures. That is a distinct long-term asset-integrity risk to underwrite.

Section 03

Power Generation & Surface Plant

Dry steam plant

Uses reservoir steam directly to spin the turbine. Simplest and most efficient, but it needs a rare vapor-phase resource (The Geysers, Larderello).

Flash steam plant

Drops the pressure of high-temperature water (~180°C+) to "flash" it to steam in one or more stages. The workhorse for high-temperature hydrothermal resources.

Binary cycle plant

Transfers geothermal heat to a secondary working fluid with a lower boiling point; the geothermal fluid never touches the turbine. Works from ~90–150°C+, and has been the dominant U.S. plant type since 2000.

Organic Rankine Cycle (ORC)

The standard binary cycle, using an organic working fluid (isopentane, isobutane, engineered refrigerants). Fluid selection is a core efficiency and regulatory decision.

Kalina cycle

An ammonia-water alternative to ORC, with modestly higher efficiency, more complexity, far less commonly deployed.

Supercritical CO2 (sCO2) cycle

An emerging cycle using CO2 above its critical point, using compact machinery, potentially higher efficiency for hot EGS/SHR resources. Early-commercial as of 2026.

Working fluid

The circulating fluid in a binary cycle. One of the highest-leverage design decisions in binary plant engineering, with thermodynamics traded against flammability, cost, and climate regulation.

Air-cooled vs. water-cooled condensers

Water cooling is more efficient but consumes water, a major constraint in the arid Great Basin. Air cooling is increasingly standard for water-stressed sites despite the efficiency penalty.

CHP / cogeneration & cascaded use

Extracting electricity plus useful heat from the same resource, then reusing the fluid at progressively lower temperatures, from power to district heat to greenhouses, to maximize project economics. It matters to a power thesis for two reasons: the heat offtake can be a material share of a project's revenue rather than a by-product, and heat-led economics are much of why Europe's geothermal buildout has favored district heating over power generation.

Reinjection

Returning spent fluid to the subsurface after heat extraction. Standard practice for reservoir pressure maintenance and environmental compliance.

Capacity factor

Actual output divided by theoretical maximum. Geothermal's headline advantage: the operating fleet runs at 70–90%, which is true firm, baseload-capable power, against roughly 25% for solar PV and 35% for onshore wind. The critical number when comparing against intermittent renewables.

Section 04

Economics, Finance & Investment

LCOE (Levelized Cost of Energy)

Total lifetime cost divided by lifetime output, the standard cross-technology comparison. As of 2025: conventional flash ~$63–74/MWh, binary ~$90–110/MWh; EGS higher (~$125–145/MWh) but falling fast with drilling costs.

PPA (Power Purchase Agreement)

A long-term contract to sell output at an agreed price, increasingly signed directly with hyperscalers (Google–Fervo, Meta–Sage/XGS) rather than only utilities. Newer deals price at a premium for firmness.

Capacity vs. energy payments

Contracts can pay for available MW regardless of dispatch, or for MWh actually delivered. Geothermal's always-on profile makes capacity-based and hybrid structures increasingly relevant to underwriting.

Resource / exploration risk

Drilling risk on temperature, permeability, and fluid chemistry at once. This is geothermal's version of dry-hole risk, historically less de-risked by seismic imaging than oil & gas.

"First 10%" strategy

Funding only the earliest project stages, meaning pre-survey, exploration and test drilling, which consume roughly 10% of total project capex (~$4–7M of a ~$50M program) but resolve most of the geological risk. Once a resource is proven the asset re-rates from speculative prospect to financeable infrastructure and can be sold forward. The return comes from riding the cost-of-capital curve down: in at 16–20% equity, out to infrastructure capital targeting 6–8%. Only works as a portfolio, since greenfield first-well success runs near 50%.

"Picks & shovels"

Investing in the companies that sell into geothermal projects: drilling technology, downhole tools, stimulation and turbines, rather than the developers who own them. The appeal is that they earn on industry capex regardless of which developer or architecture wins, and have clearer acquisition pathways via the oilfield service majors. The catch is market size: geothermal well budgets are structurally smaller than shale's, so the durable businesses are usually dual-use across geothermal, high-temperature oil & gas, and mining.

Firm / baseload power premium

The extra value the market assigns to always-available power versus intermittent sources. It is now explicit in data-center PPA pricing, and central to the geothermal thesis versus wind and solar.

Drilling cost as % of capex

Typically 40–60%+ of total project cost, which makes drilling improvements the single highest-leverage lever on project economics, exactly like well costs in a shale play.

Ramp-up / reservoir decline risk

The risk a field's productivity declines faster than modeled: pressure depletion, thermal breakthrough between well pairs, scaling. The geothermal analog of decline-curve risk.

Royalty / lease structure

U.S. geothermal resources are leased from federal (BLM), state, or private mineral owners much like oil & gas, a direct due-diligence analog to O&G land work.

Section 05

Policy, Regulatory & Incentives (U.S., 2026 status)

Section 45Y — Clean Electricity PTC

Technology-neutral production tax credit paying per kWh generated by qualifying zero-emission facilities, including geothermal.

Section 48E — Clean Electricity ITC

Technology-neutral investment tax credit worth up to 30% of qualifying capital investment, with labor and domestic-content adders, as an alternative to 45Y.

OBBBA (One Big Beautiful Bill Act, 2025)

The reconciliation law that sharply curtailed wind and solar credits but preserved 45Y/48E for geothermal (plus nuclear, hydro, storage). That is the central policy differentiation in the current investment narrative. Full credit for construction starts through 2033, then 75% / 50% / 0% by 2036.

Prohibited Foreign Entity (PFE) rules

OBBBA-era restrictions disqualifying projects that take above-threshold "material assistance" (equipment, components, IP) from prohibited foreign entities. An increasingly important supply-chain diligence item for developers sourcing specialized gear.

DOE Loan Programs Office (LPO)

Federal loan guarantees that have backstopped several geothermal demonstration and early-commercial projects, and a financing source to track in project capital stacks.

FORGE

DOE's dedicated EGS field laboratory in Utah, used to de-risk stimulation and monitoring techniques, and a good proxy for what's about to become commercially standard.

Enhanced Geothermal Shot

DOE's target of a 90% EGS cost reduction to ~$45/MWh by 2035, the public benchmark against which to compare any developer's claimed cost trajectory.

BLM leasing & categorical exclusions

The federal pathway for geothermal exploration on public land. BLM now mandates annual competitive lease sales (IM2026-004) and allows small-scale exploration without full NEPA review, which directly affects time-to-revenue.

Section 07

Key players, the 60-second version

Full profiles with funding, milestones, and analyst views live on the Market Map.

Fervo Energy

Leading EGS developer (NASDAQ: FRVO); Cape Station is the flagship; the benchmark for bankability and drilling-cost trends.

Eavor

Closed-loop (ACL) leader; first commercial closed-loop power at Geretsried, Germany; sells seismicity elimination. Loop 2 timeline withdrawn in July 2026 after drilling difficulties.

Sage Geosystems

Geopressured (GGS) developer, and its own category rather than an EGS variant. The only architecture that doubles as long-duration storage; Ormat co-led its Series B and licensed the technology.

Mazama Energy

Superhot-rock developer at Newberry, Oregon, holder of the 331°C EGS temperature record, reached with conventional directional drilling rather than new physics. A 2023 AltaRock/Blade Energy joint venture.

Quaise Energy

Millimeter-wave drilling for ultra-deep superhot access, and now a developer in its own right via Project Obsidian (Central Oregon, 250 MW phased). Pre-commercial; highest technical ambition in the sector.

Hephae Energy

High-temperature MWD tools. The bottleneck-technology supplier to watch rather than a power generator.

Ormat Technologies

The incumbent major (NYSE: ORA): vertically integrated IPP and ORC equipment OEM, and the baseline public-market geothermal exposure.

SLB, Baker Hughes, NOV, Halliburton

The oilfield-services majors, all now supplying geothermal-rated bits, tools, and integrated services. The O&G supply chain entering geothermal is itself an investable trend.