摘要：Six hydraulic shearing experiments have been conducted in the framework ofthe In-situ Stimulation and Circulation experiment within a decameter-scalecrystalline rock volume at the Grimsel Test Site, Switzerland. During eachexperiment fractures associated with one out of two shear zone types werehydraulically reactivated. The two shear zone types differ in terms oftectonic genesis and architecture. An extensive monitoring system of sensorsrecording seismicity, pressure and strain was spatially distributed in11 boreholes around the injection locations. As a result of thestimulation, the near-wellbore transmissivity increased up to 3 ordersin magnitude. With one exception, jacking pressures were unchanged by thestimulations. Transmissivity change, jacking pressure and seismic activitywere different for the two shear zone types, suggesting that the shear zonearchitectures govern the seismo-hydromechanical response. The elevatedfracture fluid pressures associated with the stimulations propagated mostlyalong the stimulated shear zones. The absence of high-pressure signals awayfrom the injection point for most experiments (except two out of sixexperiments) is interpreted as channelized flow within the shear zones. Theobserved deformation field within 15–20 m from the injection point ischaracterized by variable extensional and compressive strain produced byfracture normal opening and/or slip dislocation, as well as stressredistribution related to these processes. At greater distance from theinjection location, strain measurements indicate a volumetric compressivezone, in which strain magnitudes decrease with increasing distance. Thesecompressive strain signals are interpreted as a poro-elastic far-fieldresponse to the emplacement of fluid volume around the injection interval.Our hydromechanical data reveal that the overall stimulation effectedvolume is significantly larger than implied by the seismicity cloud and canbe subdivided into a primary stimulated and secondary effected zone.
