Hydro-mechanical processes and their influence on the stimulation effected volume: Observations from a decameter-scale hydraulic stimulation project

Abstract. Six hydraulic shearing experiments have been conducted in the framework of the In-situ Stimulation and Circulation experiment within a decameter-scale crystalline rock volume at the Grimsel Test Site, Switzerland. During each experiment one out of two different shear zone types were hydraulically reactivated. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in eleven boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to three orders in magnitude, while jacking pressures of the stimulated structures reduced during most of the experiments. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone characteristics govern the seismo-hydro-mechanical response. The elevated fracture-fluid-pressures associated with the stimulations propagated mostly along the stimulated shear zones. The absence of high-pressure signals away from the injection point for most experiments (except two out of six experiments) is interpreted as channelized flow within the shear zones. The observed deformation field within 15 m–20 m from the injection point is characterized by variable extensional and compressive strain, produced by fracture normal opening and/or slip dislocation, as well as stress redistribution related to these processes. At greater distance from the injection location, strain measurements indicate a volumetric compressive zone, in which the strain magnitude decreases with increasing distance. This compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. The exceptional hydro-mechanical data reveal that the overall stimulation effected volume is significantly larger than implied by the seismicity cloud, and can be subdivided into a primary stimulated and secondary effected zone.