The aim of the GeneSys project is to exemplarily develop a low permeable sedimentary reservoir of the Northern German Basin to provide geothermal heat. In May 2011 a massive waterfrac operation was performed within this project in Hanover. The test site is located less than 100 m away from the next housing estate. A lot of efforts were taken to minimize the sound emissions while fracturing and to increase the acceptance in the neighborhood. The target formation is a sandstone of low permeability at about 3,700 m depth. Here, fracturing is the only option to create an artificial heat exchanger as basis for any geothermal utilization. About 20,000 m³ of filtered fresh water were injected at flow rates up to 90 l/s. The pressure response during fracturing indicated the creation of a high conductive fracture. Further the high pressure during fracturing approved that the formation is pressurised and the minimum principal stress component is close to the overburden. Different methods were applied to monitor the fracture development. But neither the surface seismic network, nor remote sensing (GPS-Monitoring and INSAR) provided any signals related to the fracture propagation. On the other hand the comprehensive monitoring demonstrated that under this geological setting massive fracturing does not cause any detectable ground motion. Low rate injection tests were performed two and five months after fracturing, respectively. These tests provided evidence for a high (infinite) conductive fracture and a fracture area in the range of 1 km2. It has to be emphasized that these tests were carried out at a pressure level significantly below the closure pressure. Thus, the fracture retained a high hydraulic conductivity even though no proppants were used to keep the fracture open. In November 2011, with a delay of six months, the artesian back flow of the well was commenced. The recovered water was oversaturated with respect to NaCl at surface conditions. Due to cooling induced scaling, a salt plug formed in the well. Now the focus in the project has been redirected to the question if the salt concentration in the water could be reduced after several cycles of fresh water injection and back production of formation water. Results of these hydraulic and geochemical considerations will be presented.

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