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How should drill pipe be selected for geothermal drilling? What are the technical requirements in high-temperature environments?
Author:
WHH
Source:
LONGWAY
Published:
2026-09-30
Geothermal energy development is accelerating worldwide. Unlike conventional oil and gas drilling, geothermal wells face three major challenges—high temperatures, hard rock formations, and corrosive fluids—placing unique demands on drill‑pipe selection. Choosing the wrong drill pipe can result in reduced service life and frequent replacements at best, or catastrophic failure under high temperatures and downhole breakage at worst.
This paper systematically reviews the operational characteristics of geothermal drilling, the technical requirements for drill pipes, and recommendations for their selection, thereby providing a technical reference for drill‑tool management in geothermal projects.
I. The Three Major Operational Challenges of Geothermal Drilling
| Challenge | Specific manifestations | Impact on the drill pipe |
| High temperature | Well-bottom temperatures can reach 150–350°C, with some enhanced geothermal systems exceeding 400°C. | Material strength decreases, seals fail, and thread sealant performance degrades. |
| Hard rock | Hard rock formations such as granite and basalt are extremely abrasive. | Drill rods wear out quickly, drill bits have a short service life, and torque fluctuates significantly. |
| Corrosive fluid | Geothermal fluids contain H₂S, CO₂, Cl⁻, SiO₂, and other constituents. | Corrosion and pitting of the pipe body, as well as the risk of stress corrosion cracking. |
II. Core Technical Requirements for Drill Pipes in Geothermal Drilling
1. High-temperature resistance
High temperature poses the greatest technical challenge in geothermal drilling. As temperatures rise, the yield strength and tensile strength of drill pipe materials decline, thread lubricants may lose their effectiveness, and sealing performance deteriorates.
| Temperature class | Temperature range | Drill Pipe Selection Recommendations |
| Medium and low temperature | <150℃ | Standard API drill pipe (G105/S135) + high-temperature thread compound |
| Medium to high temperature | 150-250℃ | S135 + High-Temperature-Resistant Internal Coating + Special Heat Treatment |
| High temperature | 250-350℃ | V150 + High-Temperature-Resistant Alloy Steel + Special Thread Design |
| Ultra-high temperature | >350℃ | Special‑material drill rods (such as alloy steel or composite materials) |
Critical Control Point:
The heat treatment process must ensure microstructural stability at elevated temperatures.
Thread sealant must be a high-temperature‑resistant type, such as all‑metal thread sealant or high‑temperature synthetic grease.
The internal coating must be resistant to temperatures of ≥200°C (e.g., LPC5000P, with a temperature resistance of 204°C).
2. Hard-rock drilling capability
Geothermal wells frequently encounter hard formations such as granite and basalt, which are highly abrasive. Requirements for drill pipes:
High-strength steel grade: S135 or V150, designed to withstand the high torque of hard-rock drilling.
Wear-resistant strip : LW800 (hardness ≥ 60 HRC, with optimal wear resistance), protective joint
Anti-fatigue design: Hard-rock drilling generates significant vibration, so the NC series threads with excellent fatigue resistance should be selected.
3. Corrosion Resistance
H₂S, CO₂, and Cl⁻ in geothermal fluids pose a corrosion threat to drill pipes. Selection recommendations:
| Corrosive medium | Risk | Protective measures |
| H₂S | Sulfide Stress Cracking (SSC) | Select SS sulfur-resistant steel grade or apply an internal coating. |
| Carbon dioxide | Uniform corrosion, pitting corrosion | Internal Coating (LPC3000P/LPC5000P) |
| Cl⁻ | Pitting corrosion, crevice corrosion | Internal coating + corrosion-resistant alloy |
| SiO₂ | Scaling, affecting flow | Smooth inner coating, reducing scale deposition. |
III. Recommended Configuration of Drill Pipes for Geothermal Drilling
1. Steel Grade Selection
| Deep Well | Temperature | Recommended steel grade | Reason |
| <2000m | <150℃ | G105 | Conventional geothermal wells: economical and practical. |
| 2000-3500m | 150-200℃ | S135 | Medium-to-deep geothermal wells, balancing strength and temperature resistance. |
| >3500m | 200-250℃ | S135/V150 | Deep geothermal well, high load |
| Hot dry rock | >250℃ | V150 or special material | Ultra-high temperature; requires special design. |
2. Joints and Threads
Connector type: NC series (NC46, NC50), with excellent fatigue resistance.
Thread treatment: cold rolling plus phosphating, enhancing fatigue strength and galling resistance.
High-torque requirements: A double‑shoulder joint (DSTJ) can be selected, increasing torsional strength by 20–50%.
3. Surface Protection
| Protection Technology | Recommended model | Function |
| Wear-resistant strip | LW600/LW700 | Protect the joint and reduce wear. |
| Internal coating | LPC3000P/LPC5000P | Corrosion resistance, reduced flow resistance |
| Thread lubricant | High-temperature resistant type | Ensure lubrication and sealing under high-temperature conditions. |
4. Heavyweight Drill Pipe (HWDP) application of
HWDP in geothermal wells is used for:
Transition between the drill pipe and the drill collar to reduce stress concentration.
Provides flexible cushioning during hard-rock drilling to reduce vibration.
Replace part of the drill collar to reduce friction.
It is recommended to use an integral HWDP (AISI 4145H), which is weld-free and offers higher strength.
IV. Key Points for Inspection and Maintenance of Drilling Rigs in Geothermal Drilling
| Test Items | Frequency | Method |
| Thread Inspection | After pulling out of each well | Magnetic Particle Testing (MT) |
| Pipe wall thickness | Every 2–3 wells | Magnetic Flux Leakage (MFL) or Ultrasonic Thickness Measurement |
| Thickened transition zone | Annually or per three wells | Ultrasonic Testing (UT) |
| Wear-resistant strip | After each trip out of the hole | Visual inspection + hardness testing |
| Internal coating | After each trip out of the hole | Visual inspection + coating thickness measurement |
| Hardness Testing | Every year | Portable hardness tester |
Special reminder for high‑temperature wells: After pulling out of the hole, allow the drill pipe to cool naturally before inspection to prevent damage caused by operating at elevated temperatures. Thread compound may carbonize at high temperatures; therefore, promptly clean the threads and reapply thread compound after pulling out of the hole.
V. Procurement Considerations
When purchasing drill pipes for geothermal drilling, the contract should clearly specify:
| Procurement Item | Technical Requirements |
| Steel grade | Select G105/S135/V150 based on well temperature and well depth. |
| Temperature resistance requirements | Specify the maximum operating temperature. |
| Connector Type | NC series, with DSTJ optional |
| Wear-resistant strip | LW600/LW700/LW800 |
| Internal coating | LPC3000P/LPC5000P, temperature resistance ≥200℃ |
| Thread lubricant | High-temperature resistant type, offering a temperature range. |
| Nondestructive testing | 100% UT+MT, with MTC issued. |
| Traceability | The heat number is consistent with the MTC. |
| Third-party inspection | Supports on-site inspections by SGS/BV. |
VI. Frequently Asked Questions (FAQ)
Q1: Can standard oilfield drill pipes be used for geothermal drilling?
For medium- and low-temperature geothermal wells (<150°C), standard API drill pipes may be used, but they must be paired with high‑temperature‑resistant thread compound and an internal coating. For high‑temperature geothermal wells (>200°C), specially designed drill pipes are recommended.
Q2: What is the maximum operating temperature of geothermal drilling drill rods?
The recommended operating temperature for standard API drill pipe typically does not exceed 200°C. For temperatures above this limit, drill pipe made from special materials or subjected to specialized heat treatment must be used. The V150 steel grade can withstand even higher temperatures.
Q3: Why is an internal coating used in geothermal wells?
Geothermal fluids contain corrosive species such as H₂S, CO₂, and Cl⁻; an internal coating effectively isolates these corrosive agents, thereby extending the service life of drill pipes. Additionally, the internal coating reduces flow resistance and lowers pumping pressure.
Q4: How is the inspection interval for geothermal drilling drill pipes determined?
Determined based on the temperature and corrosivity of the geothermal well. For high-temperature wells (>200°C), it is recommended to conduct testing after each well is pulled out; for medium- and low-temperature wells, testing may be performed once every 2–3 wells.
Q5: What type of drill pipe is used for enhanced geothermal system (EGS) drilling?
Enhanced geothermal system (EGS) reservoirs typically exceed 250°C; it is recommended to use V150‑grade steel or drill pipes made from specialized materials, in conjunction with high‑temperature‑resistant thread compound and wear‑resistant tape. The specific selection should be determined based on an assessment of well temperature and depth.
VII. Conclusion
| Key points | Core content |
| Operating Condition Challenges | High temperature (150–350°C), hard rock, corrosive fluids |
| Steel Grade Selection | G105 (medium-low temperature), S135 (medium-high temperature), V150 (high temperature) |
| Connector Type | NC series, with DSTJ optional |
| Surface Protection | Wear-resistant belt (LW600/700/800) + Internal coating (LPC3000P/LPC5000P) |
| Thread lubricant | High-temperature-resistant type must be selected. |
| Key Testing Points | MT for thread inspection, MFL for pipe body inspection, UT for thickened sections |
| Procurement Requirements | Clearly specify the temperature rating, steel grade, coating, and thread‑sealant temperature range. |
Geothermal drilling places special demands on drill pipes in terms of high-temperature resistance, wear resistance, and corrosion resistance. Proper selection of equipment, the use of high‑temperature‑resistant thread compounds and internal coatings, and enhanced inspection and maintenance are essential to ensuring the safety and efficiency of geothermal drilling operations.
Do you need to procure drill rods suitable for geothermal drilling for your project?
Contact Hebei Longway Petroleum Equipment Co., Ltd.
Obtain professional technical consulting
Hebei Longway holds international certifications such as API 5DP and API 7-1, and offers drill pipes in S135 and V150 grades, along with high‑temperature‑resistant internal coating services.
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