Current status of the EGS gross schönebeck project: On the way to demonstrate sustainable brine production from deep sediments of the North German Basin

The Rotliegend of the North German basin is the target reservoir of an interdisciplinary investigation program to develop a technology for the generation of geothermal electricity from low-enthalpy reservoirs. An in-situ downhole laboratory was established at the site of the deep well GrSk 3/90 at Gros Schonebeck with the purpose of developing appropriate stimulation methods to increase permeability of deep aquifers by enhancing or creating secondary porosity and flow paths. The goal is to learn how to enhance the inflow performance of a well from a variety of rock types in low permeable geothermal reservoirs. At present, a research borehole has been drilled to 4.3 km depth and is completed in sedimentary rock formations bearing 150°C. This well (GrSk 3/90), originally completed in 1990, was re-entered in 2000, hydraulically stimulated in 2002 and 2003, and tested in 2003, 2004 and 2005. In April 2006, drilling started on a second well to enhance mass-flow rate from the reservoir using a doublet. The goal is to demonstrate sustainable production with a long term circulation experiment. At the conditions at Gros Schonebeck, a reservoir with some natural permeability, an arrangement of connecting line of both wells and perpendicular created fractures is most appropriate for minimizing both the auxiliary energy required to drive the thermal water loop, and the risk of a temperature short circuit of the system during a planned 30-year utilization period. In the first well, a series of stimulation experiments with different fracturing concepts were carried out. In a first attempt, open hole hydraulic proppant-gel fracturing treatments were conducted in two pre-selected sedimentary reservoir zones in Rotliegend sandstones at a depth of about 4 km. These proved on the one hand to be technically demanding, and on the other hand, less successful than expected due to a sub-optimal design. Nevertheless, the main inflow zones could be clearly identified. In a second step, the concept of zonal selection and proppant application was abandoned and massive waterfrac treatments were applied over the entire open hole interval of the well below 3874m to the final depth at 4309m. Evidence of the creation and properties of vertical fractures were retrieved from pressure response analyses and demonstrated a bilinear flow regime in the reservoir. Therefore, the stimulation effect in terms of a productivity increase can be determined for the described concepts and improvements can be recommended for similar field experiments.