St.Gallen

The geothermal project undertaken by the city of St.Gallen (Switzerland) and operated by the St.Galler Stadtwerke was based on a large-scale 3D seismic survey in 2010. The survey led to drilling of the geothermal well, “St.Gallen GT-1”, to a measured depth of 4450 m across Tertiary and Mesozoic rock formations as well as extensive scientific and technical analysis, and well completion aimed at geothermal use. The area has additionally been instrumented with a micro-seismic array operated since 2012 by the Swiss Seismological Service (SED).

After complications occurred during well stimulation and testing in July 2013 (a gas kick and seismic events up to ML 3.5 as a consequence of killing the well) the project was temporarily stopped. In October 2013 the project was resumed with a modified drill stem test producing gas and water and collection of a comprehensive data set on the developed jointed aquifer within the regionally prominent St.Gallen Fault Zone.

The well has since been temporarily shut-in, and is being pressure monitored from surface as well as seismically monitored. Because of very low hot water flow rates, yet high natural gas rates produced and associated high risks for felt induced seismicity, the geothermal project has been halted in May 2014 although the public acceptance of the project remained high and without having drilled a second well to complete the initially planned hydrothermal doublet.

St.Gallen has then started to search for an ultimately commercially viable alternative use of this well and define the conditions for a definitive well abandonment. A gas project was evaluated but stopped soon after Gas Initially In Place (GIIP) was estimated as too small although the gas was produced at high rates and very high quality over short time. In the course of this analysis, St.Gallen has invited E&P companies and science to engage in the discussion with a view towards collecting ideas for the future use of the well.

Within S4CE the focus as field site lies on providing a well characterised data set for correlating seismic activity to fluid injection and gas-water production that lead to increased seismic activity in the past. Therefore, simulation of the geomechanical system of St.Gallen has already been initiated by S4CE partner geomecon in an earlier project and will be carried on and intensified within S4CE. The proposed study will yield a superior understanding of the interaction of mechanisms in stabilising and destabilising of faults, which will be particularly important given that limestone is a prevalent rock in Europe. Besides measuring regional seismic activity this drill site in “off-mode” gives the chance for additional surface and downhole measurements at different times throughout S4CE since drill site and well will be conserved at least until the end of S4CE (see also pictures below).