Characterising induced acoustic emission activity observed during a mine-scale hydraulic-fracturing experiment in anisotropic crystalline rock
Authors: Boese Carolin , Kwiatek Grzegorz , Dresen Georg , Renner Joerg , Fischer Thomas , Plenkers Katrin ,   STIMTEC Team
Published: March 2020

&lt;p&gt;&lt;span&gt;Between early 2018 and late 2019 the STIMTEC hydraulic stimulation experiment was performed at ca.~130 m below surface at the Reiche Zeche research mine in Freiberg, Saxony/Germany. The project aims at gaining insight into the creation and growth of fractures in anisotropic and heterogeneous crystalline rock units, to develop and optimise hydraulic stimulation techniques &lt;/span&gt;&lt;span&gt;for EGS applications&lt;/span&gt;&lt;span&gt; and to control the associated induced seismicity under in situ conditions. A series of ten hydro-frac experiments w&lt;/span&gt;&lt;span&gt;ere&lt;/span&gt;&lt;span&gt; performed in a 63 m-long, 15&amp;#176;-inclined injection borehole and five mini-fracs for stress measurements in a sub-vertical borehole. &lt;/span&gt;&lt;span&gt;These were monitored using a &lt;/span&gt;&lt;span&gt;seismic monitoring system of twelve high-sensitivity Acoustic emission &lt;/span&gt;&lt;span&gt;(AE) &lt;/span&gt;&lt;span&gt;sensors, three accelerometers and one broadband sensor.&lt;/span&gt; &lt;span&gt;More than 11,000 high-frequency AE events with source sizes on the cm-to-dm scale accompanied the hydraulic stimulation in five of ten stimulat&lt;/span&gt;&lt;span&gt;ed&lt;/span&gt;&lt;span&gt; intervals in the injection borehole. Several hundred AE events were recorded during the mini-fracs in the vertical borehole. We investigate the characteristics of induced AE events by combining information obtained from high-accuracy&lt;/span&gt; &lt;span&gt;event locations using a transversely isotropic P-wave velocity model per station with station corrections, relative hypocentre locations, and focal mechanism solutions of selected events. The &lt;/span&gt;&lt;span&gt;AE &lt;/span&gt;&lt;span&gt;event clouds extend ca. 5 m radially from the injection point&lt;/span&gt;&lt;span&gt;s and show&lt;/span&gt; &lt;span&gt;vari&lt;/span&gt;&lt;span&gt;ying &lt;/span&gt;&lt;span&gt;orientations and dips. The &lt;/span&gt;&lt;span&gt;ca. &lt;/span&gt;&lt;span&gt;150 focal mechanism s&lt;/span&gt;&lt;span&gt;olutions&lt;/span&gt;&lt;span&gt; obtained using P-wave polarisation&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; display mixed-mode failure with a significant portion of them showing compaction. &lt;/span&gt;&lt;span&gt;The orientation &lt;/span&gt;&lt;span&gt;of the &lt;/span&gt;&lt;span&gt;maximum principal stress inferred from the hydro-fracs in the injection and vertical boreholes &lt;/span&gt;&lt;span&gt;has a trend &lt;/span&gt;&lt;span&gt;of &lt;/span&gt;&lt;span&gt;N&lt;/span&gt;&lt;span&gt;348&amp;#176;&lt;/span&gt;&lt;span&gt;E&lt;/span&gt;&lt;span&gt; and &lt;/span&gt;&lt;span&gt;a &lt;/span&gt;&lt;span&gt;plunge &lt;/span&gt;&lt;span&gt;of&lt;/span&gt;&lt;span&gt; 20&amp;#176;, as typical for southeast Germany. However, 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 orientations obtained from inverting focal mechanism solutions to provide additional information for interpreting stress-characterisation measurements.&lt;/span&gt;&lt;/p&gt;