Multidisciplinary characterization of a complex fault zone in crystalline basement rock

Safe and controlled exploration of deep geothermal resources in future decades is a key pillar for successfully transitioning to a carbon-neutral economy. A largely untouched source of deep hot source rocks can be found in crystalline basement formations in many European regions. Hydraulic stimulations are required to harness this thermal energy. Past geothermal projects were not always publicly accepted due to unintended induced seismicity that accompanied the projects. Faults and entire fault systems are jointly responsible for these seismic events and must therefore be thoroughly understood before they may be stimulated, to minimize tremors and unintentional shaking.

The Bedretto Underground Laboratory for Geosciences and Geoenergies in Ticino, Switzerland allows for decameter scale stimulation experiments and access to deeply (> 1km) buried crystalline faults. A complex fault zone is hydraulically and petrophysically described as part of the FEAR project, using various field and laboratory techniques. Two sub-parallel boreholes obliquely intersecting the target fault are analyzed using geophysical image and sonic logs. Hydraulic tests on predefined, packered intervals in the form of pulse-, constant rate- and step-rate injection tests are implemented on field scale, deducing parameters such as hydraulic conductivity and injectivity. In addition, laboratory petrophysical experiments on samples retrieved from varying parts along the fault zone are performed to determine permeability under certain effective stresses, porosity, and p-wave velocity, among other properties. This allows for a cross-scale hydraulic and petrophysical comparison. Prior, structural geologists described and analyzed the target fault using core logging, outcrop, and fracture data. Correlations between structural, hydraulic, and petrophysical observations can be drawn.