Lessons Learned from Basel: New EGS Projects in Switzerland Using Multistage Stimulation and a Probabilistic Traffic Light System for the Reduction of Seismic Risk

In Switzerland the large geothermal potential for electrical power production can only be used if Enhanced Geothermal Systems (EGS) will be technically feasible because 90% of the deep underground consist of low permeable crystalline rocks. Based on the data of the Deep Heat Mining Project Basel a new multi-stage stimulation concept has been developed which reduces the risks of induced seismicity and promises a better return of energy. The mitigation plan for induced seismicity includes three major points. First, induced seismicity will be reduced by limiting the areas of the hydraulically stimulated fracture planes. This can only be achieved with a borehole completion allowing the hydraulic isolation of up to 30 individual borehole sections within the reservoir. Secondly, the site selection criteria lead to avoid densely populated areas, areas with high natural seismic activity and placing the geothermal reservoir at a safety distance of at least one kilometer from regional or major fault zones. Third, in addition to deterministic risk studies we have applied a probabilistic approach recently developed by the Swiss Seismological Service to show that the multistage stimulation of smaller fracture zones has a better risk profile than the massive stimulation concept of the Basel project. 1. INTRODUCTION In Switzerland the large geothermal potential for electrical power production can only be used if Enhanced Geothermal Systems (EGS) will be technically feasible. Based on the data of the Deep Heat Mining Project Basel a new concept has been developed which reduces the risks of induced seismicity and promises a better return of energy. In addition the acceptance by the local population and the authorities play an important role. Geo-Energie Suisse Ltd. the following organization of Geopower Basel Ltd. has started permitting procedures for EGS pilot projects at five sites in Switzerland in order to create a portfolio of projects sites. The sites are located in the communities of Haute-Sorne (Canton Jura), Avenches (Canton Vaud), Etzwilen (Cantons Thurgau and Zurich), Pfaffnau and Triengen (both canton Luzern). Hydrothermal systems for an electricity production from deep geothermal energy in a larger extent are in Switzerland exceptional areas like fault zones and permeable aquifers in depth of 4000 to 5000 m. A large-scale extension of such systems like in the Munich region could not be proven up to now. In Switzerland for sedimentary systems in greater depths, whose temperatures also allow an electricity production, only a small zone in the foreland of the Alps can be taken into account. Additionally the situation complicates, as the very good aquifers in the Bavarian Molasse Basin become to the West less permeable with lower flow rates of the groundwater. Based on these considerations the Geo-Energie Suisse Ltd. decided to focus their activities on the development of EGS-pilotprojects (petrothermal systems) because the great potential for electrical power production lies within the crystalline basement (Fig. 1). If it is possible to install in the crystalline rocks of the basement an artificial heat-exchanger, that technology can be used nearly everywhere in Switzerland. For the first time such a big permeable system could be created in the well Basel-1 in 5000 m depth, however, the unrequested side effect of induced seismicity occurred.