Coda Q analysis of active UT measurements to monitor the dm-scale fracture network in response to the STIMTEC hydraulic stimulations, Reiche Zeche URL (Germany)

Abstract

The occurrence of induced microseismicity is commonly used to characterize a stimulated geo-reservoir, e.g. in terms of monitoring the damage and stress evolution in the rock mass. Studies in Underground Research Laboratories (URLs) have the advantage that a close by Acoustic Emission monitoring system (earthquake source – receiver distances < 50 meters) allows to investigate the detailed evolution of induced seismicity at the decimeter scale in response to injection operations. Controlled hydraulic stimulation experiments often benefit from additional active seismic measurements conducted near the source regions of the induced seismic events to better characterize the rock mass. The so-called active Ultrasonic Transmission (UT) measurements produce stronger signals covering a broad frequency range from the centimeter to decimeter scale which may be used to investigate space-time varying fracture network development and attenuation properties. The STIMTEC project in the Reiche Zeche URL in Freiberg (Germany) provides more than 300 active UT measurements that were performed before and after hydraulic stimulations in two boreholes in the targeted rock volume, an anisotropic metamorphic gneiss. To investigate spatio-temporal changes of frequency-dependent scattering attenuation and thus to monitor variations in the local fracture network, we analyzed S-coda waves of 88 spatially representative UT measurements covering a signal frequency content of 1 – 60 kHz. We grouped neighboring UT measurements to estimate frequency-dependent mean-Qc (coda quality factor) values for in total eight UT groups, covering different borehole depth intervals and time periods. Stable Qc estimates for center frequencies between 3 – 21 kHz of octave-width frequency bands were obtained. Our estimates show a characteristic frequency-dependence as observed at the field scale in geological reservoirs. Temporal variations of Qc values are strongly connected to hydraulic stimulation, and these variations are more significant than those resolved from estimated velocity changes. We find indication for healing processes of injection-induced small-scale fractures during a two-months post-stimulation phase indicated by attenuation changes at high frequencies not resolvable at the earth’s surface. The coda analysis further reveals spatial differences of attenuation characteristics supporting previous assumptions based on borehole televiewer logs and mapped structures of an existing fault with larger damage zone that crosses the stimulated rock volume. We conclude that the coda analysis of active UT measurements complements established imaging methods used during experiments in URLs. In particular, coda analysis is a powerful tool for the detection of damage zones and for monitoring changes in local fracture networks with immediate application for imaging geo-reservoirs considered for exploitation or underground storage of gases and liquids.