Abstract
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 fractures associated with one out of two shear zone types were hydraulically reactivated. The two shear zone types differ in terms of tectonic genesis and architecture. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in 11 boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to 3 orders in magnitude. With one exception, jacking pressures were unchanged by the stimulations. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone architectures govern the seismo-hydromechanical 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 15–20 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 strain magnitudes decrease with increasing distance. These compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. Our hydromechanical 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.
Highlights
The need for CO2-neutral and nuclear-free energy production has led to global interest in the extraction of deep geothermal energy
We present direct hydraulic and mechanical observations that were made during six hydraulic shearing experiments conducted in February 2016 at the Grimsel Test Site (GTS), Switzerland
Given the large volume of data recorded, we will for the most part restrict ourselves to illustrating the hydraulic and mechanical observations using the figures for two stimulation experiments as representative for all six hydraulic shearing experiments, which are documented in the Appendix
Summary
The need for CO2-neutral and nuclear-free energy production has led to global interest in the extraction of deep geothermal energy. At the depths at which temperatures are high enough for industrial-scale electricity production (> 150 ◦C), the natural transmissivities of interconnected fractures are too small to establish sufficient fluid circulation for effective heat extraction (Manning and Ingebritsen, 1999) in many regions of the world. In these regions, the geothermal reservoirs need to be engineered by high-pressure hydraulic stimulation treatments that aim to increase the reservoir transmissivity (Brown et al, 2012). Krietsch et al.: Hydromechanical processes and their influence on simulation effected volume
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