Hydraulic stimulation experiments were conducted in a re-used Rotliegend well situated in the eastern part of the North German Basin. The well is used as an "in-situ geothermal laboratory" and as a reference location for several ongoing research projects. The aim of the projects and experiments is to develop technologies that will put primary low-productive aquifer structures into use for geothermal power generation. The frac operations in 2002 were designed to enhance the inflow performance by connecting the well to productive reservoir zones. Two consecutive zones within the Rotliegend sandstones were selected. Here core measurements show the most promising petrophysical reservoir properties with respect to a productivity increase. The stimulation treatments were performed as hydraulic proppant fracturing operations. Proppants were used to support the fractures and to guarantee a long-term fracture aperture. The treatment intervals are located in the open-hole section of the well at depths between 4080 m and 4190 m and at temperatures of about 140degreesC. The technical demands were therefore unprecedented in these challenging conditions. An open-hole-packer at the top and a sand plug at the bottom of each interval were used as hydraulic barriers. Applying this configuration the intervals were fracture-treated by injecting about 11 tonnes of proppant (high-strength ceramic grains) and over 200 m(3) of frac fluid (highly viscous gel) into the formation. The fracture treatments were conducted by means of two successive operations in each interval: first, a diagnostic treatment (datafrac), followed by the main treatment (mainfrac) with the proppant stages. The frac operations were successful. Propped fractures were created in both intervals and the inflow behaviour of the reservoir was decisively enhanced. The effective pressures applied for fracture initiation and propagation were only slightly above the in-situ pore pressures. The stimulation ratio predicted by modelling, however, could not be achieved. There were a number of reasons for this mismatch. Probably chemical and mechanical processes during closure led to a reduced fracture conductivity. The insights gained from the experiments are important for future fracture treatment projects on this site and in comparable locations. (C) 2003 Published by Elsevier Ltd on behalf of CNR.