Mechanical characterization of Freiberger Gneiss (Reiche Zeche, Germany) from laboratory to field scale

Reservoir rock properties play an important role in the overall efficiency of petrothermal systems. Potentially interesting target formations for such systems include igneous and metamorphic rocks. For such lithologies with typically low porosity values, pre-existing fractures and foliation emerge as possible controlling factors of their hydraulic and mechanical behavior, respectively. Understanding the hydromechanical behavior of the reservoir rock is vital for the planning, execution and monitoring of hydraulic stimulation and exploitation, and continued safe operation. We inferred the elastic behavior of Freiberger gneiss from millimetre to tens of meters scale from laboratory and field measurements. Laboratory measurements were performed on samples prepared from blocks collected in the Reiche Zeche mine in Freiberg, Germany, and on cores retrieved from boreholes, drilled to perform hydraulic stimulation experiments as part of the STIMTEC (STIMulation TEChnologies) project. Controlled-source, P- and S-wave ultrasonic measurements were performed on samples and cores at room pressure and temperature conditions, covering a wide range of angles between wave-propagation direction and foliation, to characterize the degree of mechanical anisotropy of the gneiss. Magnitude and anisotropy of P-wave velocities determined from the laboratory measurements on the cores are in broad agreement with velocities calculated from in-situ sonic log measurements and active ultrasonic transmission experiments  (Boese et al., 2022) representing travel path lengths from meter to decameters in the rock mass, with a mean fracture distance of decimeters. At elevated pressures, ultrasonic measurements on cylindrical samples suggest the dominance of foliation over microcracks in determining the elastic behavior. The lack of a substantial reduction in velocities, deduced from in-situ active and passive microseismic analyses (Boese et al., 2022), except in highly deformed volumes, constrains the stiffness of the in-situ fractures.

Reference
Boese, C. M., Kwiatek, G., Fischer, T., Plenkers, K., Starke, J., Blümle, F., Janssen, C., and Dresen, G.: Seismic monitoring of the STIMTEC hydraulic stimulation experiment in anisotropic metamorphic gneiss, Solid Earth, 13, 323–346, https://doi.org/10.5194/se-13-323-2022, 2022.