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Abstract The GeneSys project aims to develop low-permeability sediments of the Northern German Basin for geothermal energy supply in order to heat the BGR Geo Center in Hanover, Germany. In May 2011 a massive frac operation was carried out to create a large heat exchanger in sediments of the Middle Bunter at 3700m depth. Subsequent to the frac operation, two low-rate injection tests were performed below fracture-closure pressure at initial effective normal stresses of approximately 3.5MPa and 5MPa, respectively. The injection phases of the two tests provide evidence for a large and highly conductive fracture although proppants were not used during the entire frac operation. We calculated hydraulically active fracture areas of more than 0.5 km from the analysis of the pressure transient during the tests relying on matrix permeabilities determined in laboratory experiments. In principle, such areas are sufficient to yield geothermal power in the range of 2MW. The hydraulic fracture properties diminished between the two low-rate injection tests and hence, with increasing effective normal stress. We suppose this observation is due to pressure-dependent self-propping effects. The pressure records during the shut-in phases of the two tests could not be explained with conventional hydraulic models. The continuous increase in normal effective stress may affect the shut-in pressure transient to an extent that conventional analysis cannot account for but require extensions for hydro-mechanical coupling.
1. Introduction Profitable extraction of geothermal energy from lowpermeability formations requires hydraulic stimulation. The GeneSys project aims at the development of a low-permeability, sedimentary reservoir of the Northern German Basin for the provision of geothermal energy to heat the facilities of the Federal Institute for Geosciences and Natural Resources (BGR) in Hanover, Germany. At the geothermal test site, the BGR wants to demonstrate an innovative cyclic concept using one borehole, only (Krug et al., submitted for publication).
In May 2011 a massive frac operation was performed to create a large heat exchanger in the underground (Tischner et al., 2013). The target formations were stimulated by injecting 20,000m3 of freshwater, without the addition of proppants or any other additives.
Proppants are frequently used during frac operations with the aim to maintain hydraulic conductivity of fractures below closure pressure (Smith and Shlyapobersky, 2000, Zimmermann and Reinicke, 2010). Using proppants increases the costs of a fracture treatment and requires chemical additives in the frac fluid that present a potential environmental threat. At the geothermal test site Horstberg, Germany, Jung et al. (2005) demonstrated that unpropped water fractures in low-permeability sediments stayed permeable below closure pressure and that it is possible to extract geothermal energy by relying on a cyclic concept. Apart from the field experiments in Horstberg, only little experience has been collected with pure water fracs in low-permeability sediments.
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