Induced Acoustic Emission Activity Associated with the STIMTEC In-Situ Hydraulic-Fracturing Experiment

Session: Mechanisms of Induced Seismicity: Pressure Diffusion, Elastic Stressing and Aseismic Slip [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

In 2018/2019 a mine-scale hydraulic stimulation experiment was performed at ca.~130 m below surface at the STIMTEC site in Freiberg, Germany. The STIMTEC project aims at gaining insight into the creation and growth of fractures in anisotropic and heterogeneous crystalline rock, to optimise hydraulic stimulation techniques and to control induced seismicity under in situ conditions. Ten hydro-frac experiments were performed in a 15^o-inclined, 63 m-long injection borehole and five mini-fracs for stress measurements in a sub-vertical borehole. More than 11,000 high-frequency acoustic emission (AE) events accompanied the hydraulic stimulation in five of ten stimulated intervals in the injection borehole. Several hundreds of AE events were recorded during the mini-fracs in the vertical borehole. We investigate the induced AE events by combining absolute event locations using a transversely isotropic P-wave velocity model per station with station corrections, hypocentre relocations, and focal mechanisms of selected events. The event clouds of three stimulated intervals show a variety of orientations and dips, extending ca. 5 m from the injection point. About 150 focal mechanism solutions derived using P-wave polarisations, display mixed-mode failure with a significant portion of them showing compaction. The maximum principal stress orientation obtained from the hydro-fracs in the injection and vertical boreholes has a strike of 348^o and plunge of 20^o, as typical for southeast Germany. Significant discrepancies in the magnitudes of the principal stresses were measured between these boreholes ca. 15 m apart, resulting in different faulting regimes. We present stress inversions from focal mechanism solutions to provide additional information for interpreting the stress-characterisation measurements.

Presenting Author: Grzegorz Kwiatek

Authors