Geothermal energy has the potential to play an important role in increasing the production of energy from renewable sources in the U.S. and a new drilling tool could help the nation meet renewable energy goals quicker.

A percussive down-the-hole (DTH) hammer has been developed with the ability to withstand the high temperatures of geothermal drilling. The unique drilling tool is the result of a Department of Energy (DoE) funded research and development project that aims to lower the costs associated with bringing geothermal energy to consumers. The DoE contracted Atlas Copco, which subcontracted Sandia National Laboratories, based in Albuquerque, N.M.

President Barack Obama signed a memorandum in 2013 directing the federal government to double its renewable energy generation. If that rule were applied to geothermal, its power generation capacity would need to grow from approximately 3.8 GWe to 7.6 GWe in 2020, according to a January 2016 National Renewable Energy Laboratory (NREL) report.

A key step in expanding the presence of geothermal energy is reducing the price tag associated with it and part of reducing the cost is lowering exploration and development costs, which the new DTH hammer helps tackle, says Jiann Su, mechanical engineer with Sandia National Laboratories and principal investigator for the project. Atlas Copco designed and manufactured the drilling tools, and Sandia developed the capability to test them at high temperatures using its high operating temperature (HOT) test facility.

Unlike geothermal heat pumps (GHPs), which take advantage of the shallow ground tens of feet below Earth’s surface to produce heating and cooling for buildings, boreholes for geothermal electricity production can reach thousands of feet deep. While GHPs benefit from the consistent temperature range of around 50 to 60 degrees Fahrenheit, geothermal energy benefits from hot water found a few miles or more below Earth’s surface, which can reach hundreds of degrees Fahrenheit.

How it Works

Percussive DTH hammers are used extensively in mining and construction, Su says, pointing out that their ability to drill very fast through hard rock makes them useful for geothermal drilling as well. “DTH are able to drill up to multiple times faster than tri-cone or rotary bits in hard rock. They efficiently transfer energy directly to the rock, which leads to higher penetration rates. They also require lower weight-on-bit and torque compared to other drilling methods.”

However, he says oil mist and elastomer/plastic parts traditional DTH hammers use internally are unable to withstand the high temperatures in geothermal formations. Elastomers, which provide seals, will melt and, “As temperatures increase, the oils essentially cook and you get this sooty mess inside. It’s like running your car too long without changing your oil.”

“We developed a tool that can be used in high-temperature environments that can help increase the drilling rates and the rate of penetration to maybe five to 10 times that of conventional drilling operations.”

Developing tough lubricious coatings, which help reduce friction between parts, was critical for the research project’s success. DTH hammers encompass internal moving components that require lubrication, similar to a piston in a car engine. Su’s team worked with Sandia’s Materials Science and Engineering Center on a multilayer solid lubricant capable of operating at high temperatures.

“We have tested at temperatures up to 572 degrees Fahrenheit,” Su says.

The heightened heat tolerance enhances drilling rates of penetration, a very important aspect of the geothermal energy development process.

“We developed a tool that can be used in high-temperature environments that can help increase the drilling rates and the rate of penetration to maybe five to 10 times that of conventional drilling operations, so that’s a big plus for drillers,” he says. “It adds to the available options drillers have. This is not necessarily the final option for every drilling situation, but it does provide a good option for the right situation.”

Testing

Sandia’s HOT facility is an outdoor drilling test facility designed to test drilling tools in a controlled environment. It allows researchers to test 4-foot-by-4-foot-by-4-foot rock samples and is fully instrumented for data acquisition and remote operation for safety.

The hammer tested is 5 inches and made of metal alloys. It was exposed to high temperatures while drilling, which is a unique capability for the facility, Su says.

The HOT test facility is a three-sided open concrete structure that houses a 20-foot-tall drill rig, heating chamber and process gas heater. Researchers can simulate conditions deep underground and the elevated temperatures affecting the hammer. They can drill into different types of rock, including the granite often found in geothermal-rich locations.

While Atlas Copco was responsible for designing a hammer without plastic parts, Su says that in addition to lubrication development, he also played a role in creating the HOT facility — a large task initiated specifically for this research project.

“The work required integrating multiple subsystems, including electrical, mechanical, pneumatic and control systems. Sandia also worked with Atlas Copco on what instrumentation was required to collect the necessary data,” he says.

In the end, Su says the project, which started in 2011 and was completed in 2015, was a success. The tools are well beyond prototype and proof of concept, and the team is now looking for opportunities to field test.

Looking ahead, Sandia is using the HOT facility for other activities including the development of drilling automation.