Induced Seismicity in the Crystalline Basement - What Parameters are Important?

Abstract

Although induced seismicity in the context of reinjection wells is long known, the geothermal reinjection well Unterhaching Gt2 near Munich, Germany shows a surprising behavior. The injection pressures are quite low (below 10 bar), but seismic events up to magnitude 2.4 have been recorded. The most striking aspect of these seismic events, however, is their location. All of them are located below the open hole section of the borehole and occur within the crystalline basement well below the geothermal reservoir. The necessary hydrological connection between reservoir and basement, which might explain this behavior, is given by a steeply inclined fault zone through which the well passes. The results of this numerical model show that neither the injection rate nor the injected temperature can explain the observed seismicity and its location. The pore pressure changes caused by the injection rate are much too low (< 1000 Pa) and the induced thermal stresses are confined to the immediate surroundings of the well, so that neither can lead to the induced seismicity within the crystalline basement without the interaction with other parameters which so far have not been considered.