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

Keywords: Multistage stimulation, seismic risk mitigation, portfolio of pilot projects
Conference: World Geothermal Congress Session: EGS - Enhanced Geothermal Systems
Year: 2015 Language: English
Abstract: 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 4,000 to 5,000 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-pilot-projects (petrothermal systems). 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 5,000 m depth, however, the unrequested side effect of induced seismicity occurred. The Basel results and the ongoing research and development show that the magnitude of such seismic events can be controlled by a series of measures. Therefore, especially the approved techniques from the oil and gas industry shall be included, like the combination of horizontal well with a multitude of stimulated zones. The stimulation of an Enhanced Geothermal System (EGS) involves the injection of a fluid at high pressure, which induces seismicity and enhances permeability. Although to date there is no recorded instance of a significant danger or damage associated with induced seismicity related to geothermal energy production, induced seismicity is an important topic that needs to be coped with because the occurrence of a large magnitude event causing structural damages cannot be fully excluded. This presentation reports on the tasks undertaken by Geo-Energie Suisse AG. It will be shown that the three key-points for the reduction of seismic risk based on the lessons learnt from the Basel data are: (1) the stimulated area must not exceed a certain threshold surface which lead to the development of a quasi-horizontal multistage stimulation system at a depth of about 4500 m, 2) the site selection by avoiding areas that are densely populated, that show an increased natural seismicity and that are located within or next to regional fault zones (3) the application of a probabilistic traffic light system , e.g., the injection is stopped if the exceedance probability of a certain magnitude is high. Such an Advanced Traffic Light System (ATLS) has been developed by the Swiss Seismological Service and has been calibrated using the data from the Basel project and adapted to our multistage stimulation system.
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