Insulated drill pipe offers new tool to enable efficient drilling of ultra-deep geothermal wells

By Stephen Whitfield, Senior Editor

To drill the ultradeep, high-temperature wells needed for geothermal applications, it’s critical to make sure that downhole tools are protected. Insulated drill pipes (IDP) have been used in oil and gas drilling to maintain sufficiently low temperatures for the drilling fluid as it moves downhole, and thanks to a recently launched effort from Eavor Technologies, they can now help drillers and operators better navigate the ultra-high temperatures often seen in geothermal wells.

“Insulation is really important when you’re drilling hot resources,” said Alex Vetsak, Well Construction Advisor at Eavor. “The geothermal business requires you to drill as hot as possible – we’re talking temperatures downhole exceeding 500°F. When we’re talking about these numbers, it’s critical to be able to deliver cold drilling fluid from the surface to the downhole tools. You don’t want to burn the sensitive electronics at the bottom, and if your fluids are exceeding the temperature limit for those tools, they will burn and fail.”

The temperature ratings of different bottomhole assembly (BHA) components, such as the measurement-while-drilling (MWD) and rotary steerable systems (RSS), are particularly significant limiting factors, Mr Vetsak said at the IADC Drilling Engineers Committee’s Technology Forum in Houston on 1 November. To address this issue, Eavor developed its IDP as part of a suite of tools aimed at enabling high-temperature drilling. By insulating both on the outside and inside of the pipe with several layers of a proprietary coating solution, the heat transfer between the drilling fluid traveling down the drill pipe and the hot fluid traveling up the annulus is reduced. This results in cooler fluid being delivered to the BHA.

When used in conjunction with Eavor’s other technologies – the Rock-Pipe drilling fluid, its turbogenerator, and its magnetic ranging technology – the IDP helps create what the company calls an “Eavor-Loop,” a closed-loop geothermal system.

An Eavor-Loop is the connection of two vertical wells with horizontal multilateral wellbores that create a closed-loop system. The company’s proprietary working fluid – the fluid used to generate heat from a geothermal reservoir – is selected and added at surface, then circulated to harvest heat. Eavor-Loops can also be directionally drilled from centralized surface pads.

The company first manufactured a full IDP string in 2022 and tested its performance in a geothermal formation in Q3-Q4 that year at the Eavor-Deep project in New Mexico. An 18,000-ft well was drilled, along with a sidetrack, in a granite formation with rock temperatures of around 480°F. The project aimed to demonstrate all the technical elements required to construct a commercial Eavor-Loop system in deep, high-temperature hard rock, and testing focused primarily on gathering temperature measurements from MWD tools.

In this well, the IDP was able to help the well maintain circulating temperatures below 300°F at the bottom. Compared with the circulating temperatures from using a standard drill pipe – which Eavor calculated would be approximately 370°F, according to a transient thermodynamic drilling model – the IDP reduced downhole circulating temperatures by as much as 194°F, with a median of 142°F. Drilling was, therefore, enabled by keeping the temperature below the 300°F tool limit.

In May 2023, the IDP was put to the test again, this time as part of the US Department of Energy’s FORGE project at the University of Utah. Eavor provided 350 joints of internally and externally coated IDP for two bit runs, with the aim to demonstrate the technology’s capacity to reduce BHA temperatures, reduce temperature-related equipment failures and observe any potential drilling performance improvement.

In the first run, a full string of the IDP was utilized, along with a new motor; a mud chiller was also brought online toward the end of the run, decreasing the inlet temperatures from approximately 130°F to 110°F. An average MWD temperature of 149°F was seen while drilling, a reduction of 31° compared with the previous run in the well.

In the second run, a partial IDP string was used, with approximately 70% IDP and 30% regular non-insulated drill pipe. An average MWD temperature of 164°F was seen while drilling, which FORGE and Eavor attributed to the positioning of non-insulated pipe near the BHA. Further, it was observed that the average MWD temperature rose to 220°F while drilling in the next run, when no IDP was used. The formation reached a maximum temperature of 450°F throughout all runs.

After the IDP was pulled and inspected onsite, Eavor noted that the external coating showed negligible wear from the trial. There was minor damage to the internal coating on a small number of joints, but no damage was found on the MWD tool or the drill bit nozzles.

“This learning cycle of finding out how the pipe behaves can be done in the lab, but it’s really hard to compete with field deployment. What we’ve seen with our testing so far is that the thermal output, and the performance of the pipe, is matching our expectations,” Mr Vetsak said.

He also noted the impact of the IDP in protecting the MWD tools overall. In a previous run without using IDP, one of the MWD batteries – rated for up to 300°F – failed at a recorded temperature of 327°F. This was because the BHA faced heightened vulnerability during the process of tripping in the hole and at the beginning of drilling a new section due to a lack of circulation. This lack of circulation could lead to the column warming up close to the temperature of the surrounding formation.

While running in hole, the BHA encounters hot fluid, and without adequate circulation it can warm up quickly, requiring frequent pauses to circulate and cool down the BHA. If the cool-downs are not performed in time, the BHA is at risk of exceeding the maximum design temperatures.

As the IDP reduces heat transfer between the hot annulus and the cold fluid in the drill pipe, the risk of exceeding the BHA maximum temperatures is eliminated. During the two runs utilizing the IDP, Eavor and Forge reported no failures of the MWD tools. DC