Influence of fractures and their properties on hydraulic permeability in a fault zone-controlled fractured granite – basic scientific research for an EGS feasibility study, Northern Bavaria, Germany

Germany's geothermal potential outside the known geothermal provinces has been little investigated so far, as it does not involve deep sedimentary basins. The crystalline basement offers a yet untapped potential for producing geothermal energy with enhanced geothermal systems (EGS). In the research alliance "Geothermal Alliance Bavaria", which is funded by the Bavarian State Ministry of Science, we are investigating the potential for EGS in northern Bavaria.

A region in Franconia with a geothermal anomaly has already been delimited and is the focus of further investigations. A granite body covered by several kilometers of sediments was identified as the source of this geothermal anomaly. For a more detailed investigation of the hydraulic conditions in a fault zone-controlled granite reservoir, a surface analogue was found in a granite quarry in the Fichtelgebirge in northeastern Bavaria.

The quarry is transected by a fault zone and shows a narrow fracture network at the surface. A testfield consisting of 15 wells with depths between 15 and 25 m was set up in the quarry to analyse the influence of a fault zone-controlled fracture network on the hydraulic permeability. A photogrammetric model, surface geophysical measurements and borehole geophysics have been carried out to record the fracture network in detail. The influence of the fracture network on the hydraulic permeability is to be determined by various hydraulic tests.

Slug and pulse tests show variable, but overall low hydraulic permeabilities in the individual boreholes with values between 10^-7 – 10^-10 m/s. Slightly higher permeabilities assumingly correlate with more prominent fractures or fracture zones detected in image logs and several geophysical logs. Double packer tests on selected fractures/fracture zones will determine single fracture permeabilities in order to clarify which fractures and whether certain fracture properties mainly influence the hydraulic permeability. Furthermore, these double packer tests are intended to identify connectivity between individual wells through specific fractures or fracture zones.

In a further step, hydraulic packer tests will be used to determine fracture opening pressures and the stress field.

First results of hydraulic tests evaluated so far will be presented and the influence of recorded fracture properties of single fractures and fracture zones on the observed hydraulic permeability will be presented and discussed.