However, deep geothermal energy requires drilling to a depth of several thousand meters. Even at 3000 meters, the temperature can be a good 100 degrees Celsius. In addition, there are different types of rock such as granite, quartz sand, sandstone, limestone or claystone. Their respective properties, such as hardness and density, cause each rock to react differently when the drill bit hits it. All these factors make drilling and extraction complex and require careful planning.
Simulation of extreme conditions
The Fraunhofer Research Institution for Energy Infrastructures and Geothermal Energy IEG has now developed and commissioned the match.BOGS solution for this purpose. It is an in-situ test rig consisting of three modules: the i.BOGS autoclave, the drill.BOGS drilling module and the fluid.BOGS module for producing synthetic fluids. The rig can physically simulate and examine all the processes involved in drilling to a depth of 5,000 meters. A series of sensors, such as acoustic, thermal and optical sensors, measure the processes inside the autoclave and provide a wealth of data.
Their analysis in turn enables conclusions to be drawn about the optimum setting and control of the drilling tools. "Drilling can be better planned and settings such as the choice of drilling tool, rotational speed or pressure can be optimized in advance," explains Volker Wittig, Head of Advanced Drilling Technologies at Fraunhofer IEG. Test drillings on site are no longer necessary.
The i.BOGS autoclave was developed and built exclusively for the research team at the IEG according to their specifications. It holds rock samples with a length of 3 meters and a diameter of up to 25 centimeters. Inside, a pressure of up to 1250 bar is built up and the temperature rises to 180 degrees Celsius. The pressure vessel thus simulates conditions that prevail at a depth of 5000 meters. A total of 25 bolts, each weighing 9 kilograms, and a wall thickness of 20 centimetres ensure that the container can withstand the enormous loads. If required, special borehole tools or pumps can also be integrated and tested.
Drilling tools with laser or high-voltage pulses
In the drill.BOGS drilling module, two hydraulic cylinders deliver a feed force of up to 400 kN. An electric motor drives the drill rod into the rock with a torque of 12 kNm. The measurement, control and regulation technology ensures that the process is fully automatic.
The module can be equipped with different tools. This allows the researchers at the Fraunhofer IEG to test new types of tools in addition to conventional chisels that work with mechanical destruction. For example, these remove the rock with high-voltage pulses, bombard it with lasers or heat the rock surface so that it breaks more easily. "Contactless drilling protects the high-quality tools and extends their service life," says Wittig. The tests at the Fraunhofer IEG thus contribute to the further development of drilling tools.
Synthetic fluids support the drilling process
In geothermal energy, the water from underground reservoirs is pumped to the surface in a closed circuit, where it is used as hot water to generate heat or drive steam turbines to generate electricity. The cooled liquid then flows back into an underground reservoir to be heated up again. "This is why the behavior of fluids during pumping up must also be simulated in the test," explains Tilman Cremer, research associate at Fraunhofer IEG. In parallel to the geothermal application, valuable raw materials such as (heavy) metals or rare earths can potentially be separated and extracted from these geofluids. Accordingly, the fluid.BOGS module ensures the supply of synthetic fluids. Their flow behavior in interaction with the rock samples is also investigated in the i.BOGs.
The experts at the Fraunhofer IEG examine authentic fluid samples from reservoirs of choice or produce these fluids themselves. For example, a precisely calculated mixture of water and components such as chlorine, calcium, magnesium and various minerals is fed into the i.BOGS. This allows the team to study and evaluate the flow processes in the autoclave.
Special fluids, known as drill muds, are also essential for the actual drilling operation. "The fluids are used to lubricate, rinse and cool the tools, but also act as a central means of transport for the dissolved rock," explains Volker Wittig.
The combination of the three modules i.BOGS, drill.BOGS and fluid.BOGS and the many configuration options make the match.BOGS test stand unique. Jascha Börner, team member and research associate at the Fraunhofer IEG, says: "Pressure, temperature, flow velocity, the composition of the rock samples, the mixing ratio of the fluids - we can set each parameter independently of the others." This makes it possible to simulate a wide variety of conditions and generate precise planning data for real drilling projects.
Conclusion:
Boost for geothermal energy and the energy transition
For the research team at the Fraunhofer IEG, the work for the test operation is complex. First, the autoclave has to be filled with rock samples. Then the pressure and temperature are gradually increased, the drilling tools adjusted and the fluids prepared. It usually takes a whole day for the simulation to start. But the effort is worth it, as there are many advantages for the drilling industry. Once the specific conditions at a site have been tested in the simulation, operators have greater planning certainty. Drilling operations become more efficient, as all tools are optimally adjusted from the outset. This can ultimately save operators millions of euros. These optimization measures for geothermal energy help to make the energy transition even more economical and efficient overall.
