Stress field observations from hydraulic fracturing and focal mechanism inversion at the STIMTEC underground research lab, Reiche Zeche mine, Germany

Between 2018 and 2021, the STIMTEC and STIMTEC-X hydraulic stimulation experiments were conducted at 130 m depth in the Reiche Zeche underground research laboratory in Freiberg/Germany. The STIMTEC experiment was designed to investigate the rock damage resulting from hydraulic stimulation and to link seismic activity and enhancement of hydraulic properties in anisotropic metamorphic gneiss. The following STIMTEC-X experiment aimed at better constraining the stress field in the rock volume to investigate the mechanisms leading to induced acoustic emission (AE) activity. Here, we present results from focal mechanism analysis of high-frequency (>1 kHz) AE events, associated with brittle deformation at the cm- to dm-scale induced by hydraulic stimulations. Focal mechanisms are calculated using full moment tensor inversion of first P-wave amplitudes using the hybridMT package. We use polarity and amplitude data from a (near) real-time seismic monitoring network, consisting of AE sensors, AE-hydrophones, accelerometers, and one broadband sensor. We observe changes in the predominant type of faulting from reverse faulting focal mechanisms during the frac and refrac cycles to oblique strike-slip focal mechanisms observed during subsequent high-volume fluid-injections performed during periodic pumping test. The observed differences in dominant focal mechanisms are consistent with the activation of less favourably oriented faults at increased pore fluid pressure during extended periodic pumping. We observe a reverse-faulting stress regime from focal mechanism inversion of low-volume injection stages for different boreholes, representative for the rock volume (typically ~5 m radially) surrounding the injection intervals. In contrast, stress field estimates obtained from analysing the instantaneous shut-in pressures of hydraulic stimulations in different boreholes indicate a regime change from thrust to strike-slip faulting in the investigated rock volume. The reservoir complexity seen at the scale of the experiment (30m x 30m x 20m) is large and is reflected by the significant variations in AE event activity in response to stimulation as well as small-scale rock, stress and structural heterogeneities.