Demonstration of An Egs Multi-Zonal Stimulation System At The Bedretto Underground Rock Laboratory

The multi-stage hydraulic stimulation concept proposed by Geo-Energie Suisse, based on lessons learned from the Basel EGS project, was successfully demonstrated in 2020 and 2021 at the Bedretto underground rock laboratory in the granite of the Swiss Alps at a depth of 1100 m. The three main success criteria were met: first, a significant increase in flow rates and thus an increase in the energy output of an EGS project; second, detailed monitoring and mitigation of induced seismicity; and third, validation and qualification of advanced multi-stage stimulation devices from the oil and gas industry for geothermal applications.

The increase in flow rates predicted in 2016, using numerical modeling by factors between 3 and 6 for the multi-stage stimulation concept compared to single-stage stimulation was demonstrated in Bedretto in-situ at a reservoir scale of 1:3. Scaling the results from the field work to a deep EGS project such as Haute-Sorne in the Swiss canton of Jura yields a range of flow rates from 78 to 160 l/s. This value significantly exceeds the success criteria set at 65 l/s for the Haute-Sorne project. This is also a positive result given the inherent uncertainties. Applied to practice, this means that the electrical power of an EGS project can be increased from about 1.5 MW - if a single stimulation stage is carried out over the entire borehole reservoir section - to about 4.5 to 6 MW if a multi-stage stimulation approach is used.

Real-time monitoring and mitigation of induced seismicity and associated risks - the second main criterion for proving success - was also successfully demonstrated. The maximum moment magnitude of -1.6 within the stimulated rock volume was lower than naturally occurring seismicity (up to a magnitude of 0.0) at greater distances of about 1000 m from the stimulated reservoir.

Finally, the oil and gas industry's advanced multi-packer system for zonal isolation along the 400 m borehole for multi-stage stimulation of a crystalline geothermal reservoir worked flawlessly. The operation of the valves between the packers also worked reliably. Instrumentation of the multi-packer system with fiber optic lines for DAS, DTS and HPM (Heat Pulse Measurement) provided very useful data for identifying flowing fractures between packers before, during and after stimulation. Fiber optic instrumentation also provided important information on operational aspects such as valve opening or closing and possible packer bypass. This is important for a deep EGS project where the valves are located at a depth of 3000 to 7000 m and the mechanical valve movement of only a few decimeters at the surface cannot be controlled.

A worldwide unique multilevel EGS reservoir has been created in the last few years with great personnel, technical and financial effort, equipped with high-resolution and state-of-the-art measurement technology. This now offers the unique opportunity to investigate the complex behavior of coupled processes in an EGS reservoir in a long-term test, and thus to validate and optimize existing engineering approaches that have so far only been derived from uncalibrated numerical models.