The geothermal deep drilling St. Gallen GT-1

According to the 2008 energy concept of the city of St. Gallen deep geothermal energy should play a major role. Therefore the city commissioned a feasibility study including a regional 3D-Seismics survey. This study identified potential aquifer-units within the mesozoic sediments and in combination with a regional fault zone. The well GT-1 has been drilled between March and July 2013 and reached a final depth of 4.250 m below ground level or 4.450 m along hole respectively. Below almost 4 km of Tertiary Molasse sediments the Top Mesozoic was reached, followed by 400 m of Malm and 50 m of Dogger formations. In order to reach the fault zone from the drill site near the Sitter it was necessary to deflect the borehole. In a depth of 950 m the borehole was deflected by about 20° heading NW. The final borehole consists of four sections that have been accomplished mainly with PDC-bits and modern directional drilling equipment. Possible high pressure areas within the triangle zone or gas kicks have been postulated but could not be found. The Malm formations consist of typical swabian facies with spongial limestones, that show partly fractured sections. The fractured sections are interpreted as part of the St. Gallen Fault Zone. These have been stimulated successfully, causing a sudden gas blowout, accompanied by microseismic activity and a distinct seimic event of magnitude 3.5, urging rigorous well contrail activity. After several weeks of controlling and securing the borehole, aquifer testing was carried out. The extent of the gas reservoir is unkown and the tests could not detect an aquifer zone with sufficient productivity to realize a geothermal power plant.