First results on induced seismicity and its source parameters during hydraulic stimulations in the Bedretto Underground Laboratory, Switzerland

The Bedretto Underground Laboratory for Geoenergies and Geosciences (BedrettoLab) is a unique research environment in a tunnel in the Swiss Alps with an overburden of one kilometer. For a first experimental campaign in the context of enhanced geothermal reservoir creation (enhanced geothermal system, EGS), six 150-300 m long monitoring boreholes were drilled in the BedrettoLab and equipped with a multi-component geophysical monitoring system. The seismic sensors are composed of acoustic emission sensors, accelerometers and geophones to cover a magnitude range from nano to micro seismicity. An additional 400 m long stimulation borehole is equipped with a multi-packer system dividing it into 15 separate intervals of 8 to 55 m length.

In the first phase, we performed characterization stimulations in eight intervals using comparable small amounts of injected fluid (350-14000 l) dependent on the length of the interval and their initial transmissivity. In the second phase, we are restimulating the intervals with higher injection volumes. The objective of these experiments is to improve our understanding of the hydro-seismo-mechanical response of the surrounding rock mass to hydraulic injections. 

For each interval, the seismic activity varies in number and magnitude and in its spatial and temporal evolution although comparable injection volumes were used. We interpreted the spatio-temporal evolution of event clouds together with the preconditions of the intervals, such as fracture network and transmissivity. Those preconditions can have a big impact on how the rock mass behaves when pressurized during hydraulic stimulations. Depending on the interval, event clusters are distinguished, for which first moment tensors are estimated. The moment tensors show different mechanisms not only for different intervals but also for different clusters. Finally, we compare the seismicity to other observables in the volume like stress and temperature measurements performed in monitoring boreholes to recognize fluid pathways and the response of the rock mass.