Geothermal activities at the KTB deep crustal lab (Windischeschenbach, Germany)

The site of the Kontinentale Tiefbohrung (KTB), Windischeschenbach, is one of the best characterised locations of continental upper crust worldwide. The two superdeep KTB boreholes represent a unique in situ underground research laboratory in Europe, allowing research into the sustainable utilisation of the crystalline subsurface for the energy and heat transition.The GEOREAL experiment aimed at hydraulically stimulating the fractured crystalline metamorphic basement rocks at 4 km depth while minimising the potential risk of induced seismicity by real-time control of injection parameters. The KTB infrastructure allowed to inject 600 m³ of water during 6-15 November 2023 into the 4 km deep KTB pilot hole, while the main borehole (200 m distant at the surface) was used for seismic monitoring.Fluid injection took place through/bypassing the stuck packer pressurising the open borehole section at the depth interval of 3.85–4 km. Flow rates were variable (10-220 l/min). The GEOREAL experiment had to be stopped prematurely due to a leak in the casing cement. No induced microseismic events were detected. Pressure data, monitored at the well head of both KTB borehole, was analysed in conjunction with recordings from a year-long injection experiment into the same formation performed in 2004-2005. The hydraulic parameters show similar values as previously obtained. The observations provide more detail on the hydraulic connection at depth between both boreholes. A new drawdown test is underway to confirm these findings, substantiating the potential of petrothermal research at the KTB superdeep crustal lab after 30 years of operation.