Monobore Well Architecture in Supercritical Geothermal Systems

Promise and Pitfalls at >400–500°C

Prepared by: Damir Horvat, Principal International D&W Engineering Expert
Organization: Asian Development Bank (ADB)
Date: 26th January 2025
Executive Summary ▼

As interest grows in harvesting supercritical geothermal energy from deep, high-temperature reservoirs (>400–500°C), the concept of monobore well construction has been proposed as a method to simplify well architecture, reduce drilling time, and maximize production bore access.

Key Finding: Monobore designs—while proven in certain oil and gas contexts—face significant engineering challenges in supercritical geothermal environments.

This paper evaluates the technical feasibility, risks, and readiness of monobore systems at 500°C and presents a comparative analysis against conventional multi-casing well construction.

Introduction ▼

The advent of supercritical geothermal systems presents a step change in geothermal energy potential, offering 5–10× energy density compared to traditional hydrothermal systems.

Achieving reliable and safe access to these reservoirs at temperatures up to 500°C and depths of 4–7 km requires radically rethinking well design and materials.

Emerging Concept: Monobore architecture—a near-constant inner diameter well from surface to reservoir.
Monobore Well Architecture Defined ▼

Monobore well construction maintains an almost constant internal diameter throughout the depth of the well, avoiding the traditional telescoping design that reduces diameter with each successive casing string.

This is typically achieved through:

Expandable Casing Systems: Advanced materials that can expand downhole
High-Collapse Slim Tubulars: Specialized pipe designs for extreme conditions
Advanced Sealing and Expansion Joints: Critical for pressure and temperature management
High-Performance Alloys: Ni-alloy or ceramic-metal hybrids for extreme temperatures
Potential Benefits in Geothermal Context ▼
Reduced Drilling Complexity: Fewer casing strings lower operational time and logistics load
Larger Flow Path: Enables higher mass flow rates and improved thermal output
Enhanced Downhole Access: Simplifies deployment of instruments, logging tools, and intervention systems
Material Efficiency: May reduce casing overlap and material waste, especially in shallower zones
Critical Engineering Challenges at 500°C ▼
Extreme Thermal Expansion: Uniform casing diameter without staged anchors introduces axial stress, risking buckling, plastic deformation, or rupture
Lack of Zoned Pressure Barriers: Absence of intermediate casing makes zonal pressure isolation and well control more difficult—particularly during kicks or fluid transitions
Cement Degradation: Long continuous cement sheaths may crack or debond due to differential expansion
Limited Material Availability: Continuous 3–5 km monobore in Ni-alloy, ceramic-metal, or high-entropy alloy is economically and logistically prohibitive
Elastomer Elimination: Elastomer-based seals are unusable above 300–350°C; alternatives (graphite, metal-to-metal, ceramic seals) are still low TRL at scale
Comparative Assessment: Monobore vs. Multi-Casing ▼
Metric Monobore Multi-Casing Advantage
Thermal Stress Tolerance Low High Multi-Casing
Zonal Isolation Capability Low High Multi-Casing
Flow Efficiency High Moderate Monobore
Flexibility of Material Deployment Low High Multi-Casing
TRL (>450°C deployment) 4–6 8–9 Multi-Casing
Lifecycle Integrity Unproven Proven in IDDP-1/2, Kakkonda Multi-Casing
Field Evidence ▼
Critical Finding: To date, no fully monobore geothermal wells have been constructed or sustained at 500°C.

Notable high-temperature wells:

IDDP-2 (Iceland): Used telescoping casing with heavy-wall steel and Ni-alloy liners
JBBP-1 (Japan): Proposed hybrid expandable liners but used conventional casings
DEEPEN (EU): Studying monobore materials but no field implementation yet
SuperHot Rock Project (Hawaii, Utah, NZ): Exclusively uses multi-string designs for structural and regulatory reasons
Recommendations and Outlook ▼

While monobore wells offer conceptual efficiency gains, their use in 500°C geothermal systems is currently technically immature and high risk.

Until the following conditions are met:

TRL >7 is reached for expandable Ni-alloy and ceramic-metal systems
Field trials validate thermal fatigue life and zonal integrity
Thermal strain compensation technologies are proven
Recommendation: Monobore designs should be considered only as part of hybrid architectures or for lower-temperature (≤350°C) geothermal plays.
Conclusion ▼

Monobore geothermal wells remain an ambitious innovation with future promise. For now, multi-casing architecture remains the only field-proven solution for 400–500°C supercritical geothermal development, offering superior safety, integrity, and long-term performance.

Investment Priority: TRL advancement, material science, and thermal strain management is essential before monobore systems can become viable in extreme geothermal conditions.