Effective temperature management is a well-known challenge in high-temperature oil and gas reservoirs, but geothermal wells can take that challenge to the extreme. Operators in those areas routinely encounter bottomhole temperatures that push the limits of what drilling tools in the bottomhole assembly (BHA) can tolerate, making drilling fluid temperature management critical.
For geothermal drilling, keeping BHA tools below about 150°C (302°F) is often the difference between consistent performance and expensive downtime. Without adequate cooling, electronics fail, elastomer seals degrade, and premature trips downhole drive up costs. At the same time, geothermal projects aim to preserve heat in the produced fluids to maximise the energy delivered to the surface.
This dual challenge – protecting downhole tools while retaining reservoir heat in the produced fluids – has pushed operators to seek new thermal management strategies. One promising approach combines insulated coatings for drillpipe with a surface mud chiller, creating a closed-loop system that helps drilling fluids start cooler, stay cooler, and safeguard tools throughout the well.
Managing downhole heat with insulation advances
Drillpipe coatings have a long history in oilfield operations. For more than 80 years, TuboscopeTM has advanced coating technologies to extend tubular life and improve operating efficiencies downhole. As drilling environments became harsher, coating advances kept evolving to protect tubulars from corrosion, chemicals, wear, and deposit build-up while improving hydraulic efficiencies to reduce pumping horsepower and frictional losses.
The latest challenge is thermal management. Ultra-high-temperature wells – both geothermal and unconventional oil and gas – demand drillpipe that slows heat transfer between the fluid inside and the rock outside. Even modest improvements in thermal insulation can protect sensitive tools downstream.
Developing a low-conductivity coating
To meet this need, NOV Tuboscope developed its Tube-KoteTM (TKTM)-Drakon insulating coating, designed specifically to minimise thermal conductivity while maintaining the protective features of earlier coatings. The development began in consulta-tion with operators in the Haynesville shale, who required solutions to keep mud cooler and extend the life of BHA electronics while drilling long oil and gas laterals in high-temperature reservoirs. Achieving these goals required a coating with a thermal conductivity (k) of 0.5 W/mK or lower.
The development process began by using current coatings as a starting point. Thermal conductivity testing was performed according to the ASTM E1530-19 test method, an industry standard for measuring the thermal conductivity of coatings in a temperature range of 20°C – 310°C (68°F – 590°F). Per this test method, the current coating class had an average thermal conductivity that was significantly higher than the target value.
A series of iterative formulation adjustments was conducted to decrease the coating’s thermal conductivity to below the target. The modified coatings were also evaluated in industry-standard laboratory tests to measure their physical properties and resistance to chemical and corrosive attack.
This work produced a coating with a k value of 0.1620 W/mK, three times lower than the target. The new coating not only lowers thermal conductivity to keep colder fluids (i.e. the drilling mud) cool and hotter fluids (i.e. the produced fluids) hot, but it also retains the same features of previous coatings – reliable protection against corrosion, wear, and deposit build-up – while improving hydraulic efficiencies.
The TK-Drakon insulating coating’s combined low thermal conductivity and enhanced chemical and abrasion resistance help keep mud cooler while boosting drilling efficiency. The coated drillpipe helps extend tool life and, on average, reduces BHA-related issues by almost 50%, lowering costs, minimising unplanned trips downhole, and delivering the well in fewer days.
Starting with a cooler mud at surface
Starting with a cooler drilling mud at the surface is critical to keeping the BHA tools within safe operating limits for longer and reaching deeper into the reservoir before picking up excessive heat.
Operators have long relied on three types of surface mud cooling systems:
- Evaporative mud coolers use water sprayed across coils carrying the hot mud. Heat is removed as the water evaporates. While effective in drier climates, these coolers lose efficiency in high-temperature, high-humidity, ambient conditions. Since water evaporates during heat extraction, a large, continuous water supply is required at all times. As a result, these units are typically placed near a large water source, or the necessary water is trucked in through the drilling process.
- Air blast coolers use air as the primary cooling medium, which passes across a series of finned heat exchangers that carry the heated drilling mud, cooling the mud in the process. While air blast coolers do not require a water source, they are also less efficient in higher ambient temperature conditions.
- Mud chillers use HVAC refrigeration systems to extract the high-temperature heat from the drilling mud. Chillers do not require a dedicated water source to operate and are effective in high-temperature ambient environments. They also provide precise temperature control, making them uniquely capable of targeting a specific temperature.
Among these options, mud chillers stand out as the most reliable cooling option in extreme geothermal environments.
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Energy Global's Winter 2025 issue
Don’t miss out on our final issue of Energy Global this year! The Winter issue begins with a regional report on Africa’s energy future, with articles on topics such as wind turbine components, geothermal drilling & operations and energy storage technology. With contributors including Magnomatics, Flyability, Bachmann electronic GmbH, NOV, and more, don’t miss out!