Geothermal energy is among the most sustainable sources of energy found on this planet. The hot water which can be found in the earth crust at a depth of about 5000 metres already has a temperature of about 200 degrees Celsius and can be pumped to the surface where it may be used to generate electricity via a steam turbine or to heat buildings with the help of heat pumps. The spent water is then returned to the earth crust through a well to be heated up again and start the whole cycle afresh. However, constructing the wells through which the water is pumped to the surface is expensive. Also, operators can never be certain that the well will indeed reach any hot-water resource at all, a risk which is estimated at 30 percent. Therefore, operators are relentlessly to make the process more efficient and sustainable, which is reflected in the patented inventions, including e.g. US9541309B2 issued by CONTROLLED THERMAL TECHNOLOGIES Pty Ltd or US20200217304A1 issued by HMFSF IP Holdings, LLC.
Now (2022), scientists at Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems IEG in Bochum have designed a method to create perforations around the borehole within 50 metres and hydraulically link the surrounding water-filled cracks and fractures to the borehole so that the water can flow into the production well to be pumped up from there. The technology which they employed for the process is called micro turbine drilling (MTD) technology and used a compact micro drilling turbine equipped with a special drill bit. It measured about 3.6 centimetres in diameter and 10 centimetres in length. The micro turbine was combined with a high-pressure hose, which powered it with 200 litres of water per minute at an inlet pressure of about 100 bar, facilitating the rotation of the drill bit. The bit was made up of a tungsten carbide matrix with incorporated diamond grains and able to grind into the rock at up to 80 000 rotations per minute.
The main challenge the scientists were faced with was to steer the micro drilling turbine out of the main well into the surrounding rock at a relatively large working angle. For this reason, a special device was created which could be steered out of the main well at an angle of about 45 degrees and enabled the scientists to create new fissures with hot water. The water could then be pumped into the main well using hydraulic pressure.
Improving geothermal systems has been given a lot of scientific attention in the past few years. In 2020, for example, the ThermoDrill project which was supported by the Community Research and Development Information Service (CORDIS) of the European Commission, designed a hybrid drilling technique which combined standard rotary drilling with water jet cutting. The high-pressure water jet, was focused directly above the drill bit which helped damage the rock on impact. This made it easier for the drill to penetrate the rock, thus increasing the overall efficiency of the drilling process. Final field tests confirmed that the ThermoDrill technique was able to at least double drilling speed.
Image: drill bits
In 2021, scientists started to develop a new generation of drilling system to better harness the potential of deep geothermal energy. To do so, the new technique will combine high-pressure water jets and downhole fluid-driven hammers. Rock fragmenting computer simulations, combining solid and fluid dynamics, will be employed to find the optimal jetting settings and to explain the breakage process. The new drilling technology will use high-pressure water jets that can cut the rock into particular profile shapes to prepare the rock to be easily broken by inbuilt fluid-powered percussive hammers. The drill prototype will be built by 2024 and will first be tested in the ARMINES laboratory by drilling horizontally through great thicknesses of rock before field testing can commence.
Example: US10253570B2 Extendable drilling tool
“Disclosed is an extendable drilling tool, the extendable tool moving in a drilling direction and including a body with a longitudinal axis and at least one arm that is mobile relative to the body in a direction of movement that forms a first angle with the longitudinal axis that is non-zero, less than 45°, and open upstream according to the direction of drilling. The extendable tool includes a piston that is mobile in a direction parallel to the longitudinal axis, the piston being located downstream according to the direction of drilling relative to the arm, the piston including at least one bearing surface that works with at least one arm and that forms with the longitudinal axis a second angle that is non-zero, less than 90°, and open downstream according to the direction of drilling”.
Using micro turbine drilling for geothermal installations has several advantages: drilling additional branches from the main well using MTD increases well productivity and decreases the exploration risk. The tungsten carbide matrix of the drill is especially suited for hard rocks like granite and also capable of drilling steel which is an important feature because wells are usually lined with a steel casing for better stability. The water that powers the micro turbine serves as both a coolant for the drill and also removes the drill cuttings. This technology is particularly suitable for geothermal energy because it can drill through the hard rock in which geothermal reservoirs are often found.
The next step in the research will be to record the drilling noises. The scientists believe that the characteristic pulse pattern of the drilling turbine can be used as a reference for the analysis. Thus, it will be possible to determine whether the drill is rotating at the right speed, stuck or is even running dry. The noise is transmitted to the steel pipes as structure-borne sound and recorded on the surface. Experts estimate that the number of geothermal power plants in Europe will increase significantly over the next five to eight years. Micro turbine drilling from the Fraunhofer IEG can play a significant role in making production wells less risky, less costly and more efficient.