GEOREAL: An adaptive stimulation experiment at 3.9 km depth at the KTB deep crustal lab, Germany

In 2023, the GEOREAL hydraulic stimulation experiment will be conducted at the KTB deep crustal lab in Windischeschenbach/Germany that originated from the Continental Deep Drilling Program of the Federal Republic of Germany (https://www.gfz-potsdam.de/ktb-tiefenlabor/). The two 4 and 9.1 km deep boreholes were drilled between 1987 and 1994, followed by a long-term experimental program between 1996 and 2005, providing key knowledge on in-situ geomechanical processes and the subsurface at the KTB site. This also led to the foundation of the International Continental Scientific Drilling Program (ICDP) in 1996.

GEOREAL aims at addressing research topics relevant for characterizing the geothermal potential of the metamorphic basement. The two KTB wells provide direct access to a petrothermal fluid reservoir at crustal depth and at temperatures ≥100°C at >3 km depth in the low-permeability rock, typical for large parts of the Earth's crust in Germany. These ambient pressure and temperature conditions are representative for a deep geothermal reservoir and have been extensively studied in the past. GEOREAL builds upon three major injection experiments between 1994 and 2005, during which 150–400 microearthquakes were located in close proximity to the stimulation intervals. The largest induced earthquake of M=1.2 occurred during the phase of highest flow rate. Most of the observed events had M∼0.

The GEOREAL hydraulic stimulation experiment aims at further refining the adaptive reservoir stimulation concept employing near-real-time microseismic monitoring with direct feedback on hydraulic parameters. It will include a series of hydraulic tests at depths ≥3.9 km to investigate the effect of pressure build-up and release, the role of continuous and periodically varying flow rates, the effect of relaxation phases and maximum injection pressure on the spatial propagation of induced earthquakes and the temporal evolution of their magnitudes. This procedure was successfully applied during the 2018 and 2020 geothermal stimulations in Helsinki, Finland. Using a double packer assembly, controlled injection in 15–20 m-long depth intervals, identified through logging will be performed. The goal of GEOREAL is to enhance hydraulic reservoir properties in the KTB pilot hole while avoiding noticeable seismic events. A unique seismic monitoring network will be set up with a 12-level geophone chain in the KTB main hole at only ~300 m distance to the stimulation interval to monitor the fluid injection with high precision. In addition, ≤40 seismometers will be installed surrounding the KTB, including several 45–150 m deep boreholes, and their data transmitted in real-time for rapid evaluation. With this setup, we expect a significantly higher number of locatable microearthquakes than observed during previous injection experiments at the KTB and thus more detailed information on the spatio-temporal propagation of the induced seismicity. A further goal of GEOREAL is to improve existing best practices for technical implementation and to reduce potential risks associated with the technology, thus improving the acceptance of deep geothermal energy in Germany.