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Source: Fraunhofer-Gesellschaft In times of the coal phase-out, geothermal energy is also moving more and more into focus. The thermal energy stored in the earth’s crust in rock layers or water reservoirs is available in virtually inexhaustible quantities. Completely independent of the weather or time of day, it can be used to obtain heat or to generate electricity. But the so-called deep geothermal energy requires drilling a few thousand meters deep. At 3000 meters, the temperature can be a good 100 degrees Celsius. There are also different types of stone such as granite, quartz sand, sandstone, limestone or claystone. Their respective properties such as hardness and density allow each rock to react differently when the drill bit hits it. All these factors make the drilling and extraction complex and require careful planning.i.BOGS simulates extreme conditions For this purpose, the Fraunhofer Institute for Energy Infrastructures and Geothermal Energy IEG has now developed the match.BOGS solution and put it into operation. It is an in-situ test stand consisting of three modules: the i.BOGS autoclave, the drill.BOGS drilling module and the fluid.BOGS module for the production of synthetic fluids. The system can physically simulate and examine all processes of a well up to a depth of 5000 meters. A number of sensors, for example acoustic, thermal and optical, measure the processes inside the autoclave and provide numerous data, whose analysis in turn enables conclusions to be drawn about the optimal setting and control of the drilling tools. “The holes can be better planned and settings such as the choice of drilling tool, number of revolutions or pressure can be optimized in advance,” explains Volker Wittig, Head of Advanced Drilling Technologies at Fraunhofer IEG. The i.BOGS autoclave was developed and built exclusively for the research team at the IEG according to their specifications. It takes 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, the temperature rises to 180 degrees Celsius. The pressure vessel thus simulates conditions that also exist at a depth of 5000 meters. A total of 25 screws, each weighing 9 kilograms, and a wall thickness of 20 centimeters ensure that the container can withstand the enormous loads. If necessary, 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 runs fully automatically. The module can be equipped with different tools. In addition to conventional chisels that work with mechanical destruction, the researchers at Fraunhofer IEG can also test new types of tools. 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 Fraunhofer IEG thus make a contribution to the further development of drilling tools.Synthetic fluids support the drilling processThe water from underground reservoirs is pumped to the surface in a closed circuit in geothermal energy, where it serves as hot water for heat generation or drives steam turbines to generate electricity. The cooled liquid then flows back into an underground reservoir where it heats up again. “That is why the behavior of liquids when pumping up must also be simulated in the test,” explains Tilman Cremer, research associate at Fraunhofer IEG. In parallel to geothermal application, these geofluids can potentially be used to separate and extract valuable raw materials such as (heavy) metals or rare earths. Accordingly, the fluid.BOGS module ensures the supply of synthetic fluids. Their flow behavior in interaction with the rock samples is also examined in the i.BOGs, where the experts at Fraunhofer IEG examine authentic fluid samples from reservoirs of choice, or even 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 enables the team to study and evaluate the flow processes in the autoclave. Special fluids, so-called drill muds, are also essential for the actual drilling operation. “The fluids are used on the one hand to lubricate, rinse and cool the tools, but on the other hand serve as a central means of transport for the loosened rock,” explains Volker Wittig. The three modules i.BOGS, drill.BOGS and fluid.BOGS and the many configuration options make the connection the test stand match.BOGS is unique. Jascha Börner, team member and research associate at Fraunhofer IEG says: “Pressure, temperature, flow rate, the composition of the rock samples, the mixing ratio of the fluids – we can set each parameter independently of the others.” In this way, a wide variety of conditions can be simulated and based on them Generate exact planning data for real drilling projects. Conclusion: Boost for geothermal energy and energy transition For the research team at Fraunhofer IEG, the work for the test operation is complex. First of all, the autoclave has to be filled with rock samples. Then pressure and temperature are gradually increased, the drilling tools are adjusted and the fluids are prepared. It usually takes a whole day for the simulation to start. But the effort is worth it, because there are many advantages for the drilling industry. Once the specific conditions at a location have been tested in the simulation, the operators receive greater planning security. The drilling operation becomes more efficient because all tools are optimally set from the start. The operators can ultimately save millions of euros. These optimization measures in geothermal energy help to make the energy turnaround more economical and efficient overall.


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