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Abstract. We performed a series of 12 hydraulic stimulationexperiments in a 20 m × 20 m × 20 m foliated, crystalline rock volumeintersected by two distinct fault sets at the Grimsel Test Site,Switzerland. The goal of these experiments was to improve our understandingof stimulation processes associated with high-pressure fluid injection usedfor reservoir creation in enhanced or engineered geothermal systems. In thefirst six experiments, pre-existing fractures were stimulated to induceshear dilation and enhance permeability. Two types of shear zones weretargeted for these hydroshearing experiments: (i) ductile ones with intensefoliation and (ii) brittle–ductile ones associated with a fractured zone. Thesecond series of six stimulations were performed in borehole intervalswithout natural fractures to initiate and propagate hydraulic fractures thatconnect the wellbore to the existing fracture network. The same injectionprotocol was used for all experiments within each stimulation series so thatthe differences observed will give insights into the effect of geology onthe seismo-hydromechanical response rather than differences due to theinjection protocols. Deformations and fluid pressure were monitored using adense sensor network in boreholes surrounding the injection locations.Seismicity was recorded with sensitive in situ acoustic emission sensorsboth in boreholes and at the tunnel walls. We observed high variability inthe seismic response in terms of seismogenic indices, b values, and spatial andtemporal evolution during both hydroshearing and hydrofracturingexperiments, which we attribute to local geological heterogeneities.Seismicity was most pronounced for injections into the highly conductivebrittle–ductile shear zones, while the injectivity increase on thesestructures was only marginal. No significant differences between the seismicresponse of hydroshearing and hydrofracturing was identified, possiblybecause the hydrofractures interact with the same pre-existing fracturenetwork that is reactivated during the hydroshearing experiments. Fault slipduring the hydroshearing experiments was predominantly aseismic. The resultsof our hydraulic stimulations indicate that stimulation of short boreholeintervals with limited fluid volumes (i.e., the concept of zonal insulation)may be an effective approach to limit induced seismic hazard if highlyseismogenic structures can be avoided.

Influence of reservoir geology on seismic response during decameter-scale hydraulic stimulations in crystalline rock