Practical Guide to the IADC Geothermal Well Classification System Your Comments

The IADC has introduced a practical classification system for geothermal wells, expressed in familiar drilling terminology. This framework defines the key elements needed to design, mobilize, and execute a geothermal drilling project. It is focused on deep geothermal wells and is intended to form the basis of future Geothermal Well Drilling Guidelines. At present, offshore projects and repurposed wells are outside its scope, but the system still covers about 80% of geothermal well concepts currently in use.

This classification makes the planning process more systematic, transparent, and consistent across wells and projects. Its structured approach gives management and regulators a clear view of the project’s scope, risks, and requirements—supporting better planning, budgeting, and alignment among stakeholders.

The following description outlines the different levels and categories within the IADC classification system for geothermal wells and demonstrates how the system can be practically applied.

Three levels and eight categories

The framework has three levels, Project, Site, and Well, which are split into eight categories of Reservoir Dependency, Asset Purpose, Location Sensitivity, Rig Capacity, Well Design, Construction, Drilling Complexity and Well Control. Think of it as a compact checklist that captures purpose, constraints, and technical difficulty.

1) Project Level - Strategic Context

The project level defines the big picture as to why the well is being drilled and what type of reservoir it is meant to reach. This level aligns the well design with the end goal of the project. Understanding the project’s strategic context helps achieve alignment between the subsurface, drilling, and facilities teams on the well’s required deliverables.

A. Reservoir Dependency

This describes whether a geothermal project can rely on the natural properties of the reservoir (heat, fluid, and permeability) or if it needs engineered solutions (like stimulation or closed-loop systems) to enable production.

Reservoir-Dependent: In this case, the reservoir already contains heat, fluids, and natural permeability. Drilling simply provides access, and the well can produce without major subsurface intervention.
Reservoir-Independent: Here, the reservoir has sufficient heat but lacks permeability and/or fluid saturation for commercial flow. In this case, engineered solutions such as Enhanced Geothermal Systems (EGS) or Advanced Closed Loop (ACL) are required to create or improve flow pathways before production.

B. Asset Purpose (a project can serve more than one purpose)

Asset purpose explains what the well is drilled to achieve. Whether the goal is to provide heat, generate electricity, or extract valuable minerals. It links the well design and operation to the project’s end use.

Heat Production: Wells provide direct thermal energy for applications such as district heating, greenhouses, aquaculture, or industrial processes.
Power Production: Depending on reservoir enthalpy, fluids may be used in binary, flash, or dry-steam power plants.
Mineral Extraction: Some geothermal brines contain dissolved minerals such as lithium, silica, or rare metals. Wells may be drilled specifically to recover these valuable by-products, either as a primary or secondary goal.

2) Site Level - Environmental & Operational Context

The purpose of the second level is to describe the conditions and constraints at the drilling location. This helps planners anticipate regulatory, logistical, and rig availability issues to ensure that the project can be executed realistically given the site’s restrictions. The focus on the operational context at this level helps screen rigs and the drilling constraints before preparing cost estimates and tendering.

C. Location Sensitivity

It describes how the setting of a geothermal project affects drilling design, permitting, and operations. It highlights the level of restrictions and special considerations needed to manage community, environmental, or logistical impacts.

Rural: Remote sites with fewer restrictions on noise, lighting, or surface footprint.
Urban / Industrial: Populated or industrial areas with stricter controls on noise, light, and emissions; permitting is more complex.
Offshore: Still emerging but included for future readiness; offshore projects face higher costs, logistics challenges, and unique well design needs.
Flags: Any special considerations for the location.
Residential: Wells near neighborhoods require strict controls on noise, vibration, and lighting to minimize community impact.
Sensitive: Areas where environmental protection, water scarcity, or social acceptance are major concerns (e.g., deserts with limited water, regions with strong local activism).

D. Rig Capacity (based on required hook load)

When planning rig capacity for selecting a rig, consider not just the hook load but torque and drag as well. Torque and drag are often higher in geothermal wells because of large hole diameters and long casing strings. To plan realistically, rig classes are aligned with Rystad’s land rig market ranges.

Super Light: <100 metric tons (<221 klbf)
Light: 100–199 metric tons (221–440 klbf)
Medium: 200–399 metric tons (441–881 klbf)
Heavy: 400–599 metric tons (882–1,322 klbf)
Super Heavy: >600 metric tons (>1,323 klbf)

3) Well Level - Technical Execution

The purpose of the Well Level is to define the technical design and operational complexity of the well. This provides the engineering details needed to size equipment, select materials, plan drilling practices, and manage well control. This level provides a full technical risk register upfront and the temperature, pressure, drilling, and well control hazards are recognized early.

E. Design (what must be sized during planning)

It helps in establishing key design factors early. These design parameters govern the selection of casing grades, wellhead and BOP ratings, and cementing systems, ensuring the well can safely and reliably handle geothermal conditions throughout its operating life.

Final Hole Diameter: Geothermal wells are generally designed with larger diameters than typical oil and gas wells to accommodate high flow rates of steam or hot water. Standard designs often finish with a hole size around 12¼", though some projects and studies consider final diameters up to 14¾". The larger size ensures sufficient production capacity and reduces flow restrictions over the life of the well.
Number of Sections: The number of casing intervals (excluding the surface conductor installed for civil stability) defines the complexity of the well. Each section is set to isolate unstable formations, manage pressures, and protect casing and cement integrity under high-temperature conditions.
Vertical Depth (TVD): Depth is measured from ground level and represents the distance to the target reservoir. In geothermal drilling, TVD influences not only drilling time and cost but also the achievable temperature gradient and long-term productivity of the well.
Maximum Bottom-Hole Temperature (BHT): The expected reservoir temperature at total depth directly impacts the choice of casing, wellhead equipment, elastomers, drilling fluids, and cement systems. High BHT environments require materials rated for thermal cycling, scaling, and long-term durability.
Maximum Surface Pressure: Both drilling and possible stimulation pressures (particularly in Enhanced Geothermal Systems, EGS) must be accounted for. These pressures determine wellhead ratings, BOP requirements, and casing burst/collapse design margins.

F. Construction (what the well will endure during operations)

This category describes the conditions a well must withstand during its operational life. It guides the selection of materials, equipment, and drilling methods to ensure safe and reliable performance.

Temperature Bands: Temperature has a direct impact on tools, casing, cement, and completion materials.

Low: <120 °C (<250 °F)

Medium: 120–175 °C (250–350 °F)

High: >175 °C (>350 °F)

Pressure Regimes & Control Methods: Proper pressure management is critical in geothermal drilling.

UBO (Underbalanced Operations): The well is intentionally drilled with influx, which is controlled and handled safely at surface.

MPD (Managed Pressure Drilling): A technique for precise annular pressure control, often applied in fractured or over-pressured zones, or where productivity must be enhanced without uncontrolled influx.

G. Drilling Complexity (what the bit/BHA must withstand)

This category describes the challenges the bit and bottom-hole assembly (BHA) must withstand while drilling. It reflects the hardness of the formations, the nature of the well path, and any special design features that add difficulty.

Predominant UCS (Unconfined Compressive Strength): Rock strength is a key factor in bit and BHA selection.

Low (<58 MPa / <8 ksi): Soft formations such as shale, weak sandstone, or chalk.

Medium (58–110 MPa / 8–16 ksi): Stronger sandstone, limestone, siltstone, dolomite.

High (110–220 MPa / 16–32 ksi): Hard formations like granite, basalt, gneiss, quartzite.

Very High (>220 MPa / >32 ksi): Extremely dense rocks such as quartzite or certain basalts.

Flags (additional factors increasing complexity):

Interbedded formations: Alternating lithologies complicate bit/BHA selection and drilling parameters.

Multilateral wells: Multiple wellbore branches increase design and operational complexity.

Interception wells: Planned intersections between wells (e.g., for connecting EGS stimulation zones).

Directional Difficulty Index (DDI): A measure of wellbore tortuosity. For multilaterals, classification is based on the most deviated or most complex branch.

H. Well Control (what can flow, and in what form)

This category defines what fluids can enter the wellbore and in what form. It helps determine the equipment, procedures, and training needed to safely manage flow during drilling.

Liquid: Reservoir produces mainly water with little or no risk of flashing to steam. Conventional well control practices are sufficient.
Two-Phase: Reservoir contains both liquid and steam, or there is a credible risk of flashing during drilling. Requires specialized well control methods (e.g., quenching) and additional crew training.
Vapor: Superheated steam reservoirs or dry intervals drilled with air/foam. These require rotating head systems and specialized manifolds (such as banjo boxes).

Flags (special hazards):

Hydrocarbons: Possible oil or gas zones when drilling through sedimentary sections.
Toxic gases: Risks of H₂S, CO₂, or methane are common in geothermal wells and must be planned for.
Supercritical conditions: Reservoirs above 374 °C and 221 bar create extreme challenges for well control, materials, and safety systems. Classification should consider in-situ pressure–temperature conditions and flashing probability, in line with standards such as NZS 2403.

How to use the classification in practice

Define the project: Reservoir dependency (dependent vs. independent) and asset purpose(s). This frames stimulation needs, plant concept, and offtake.
Map site constraints early: Set noise/light/traffic mitigations, water sourcing/handling, and stakeholder plan; choose a rig class that covers hook load and torque with margin.
Freeze critical design inputs: Final hole size, sections, TD (TVD), Tmax, and Pmax (separate drilling vs. stimulation). These drive wellhead/BOP rating, casing design, cement, and logistics.
Select construction envelope: Circulating-temperature band, expected pressure regime, and whether MPD/UBO is beneficial (HSE and production/trip time trade-offs).
Quantify drilling difficulty: Predominant UCS, interbeds, DDI (or proxy until surveys exist), and any multilateral/intercept plan. Use this to tune bit/BHA, hydraulics, and parameters.
Set well control philosophy: Choose the Liquid / Two-Phase / Vapor class, add risk flags (hydrocarbons, toxic gases, supercritical), and align equipment, procedures, and training accordingly.