Seismic monitoring of the STIMTEC hydraulic stimulation experiment in anisotropic metamorphic gneiss
Authors: Boese Carolin M. , Kwiatek Grzegorz , Fischer Thomas , Plenkers Katrin , Starke Juliane , Blümle Felix , Janssen Christoph , Dresen Georg 
Published: February 2022

Abstract. In 2018 and 2019, we performed STIMulation tests with characterising periodic pumping tests and high-resolution seismic monitoring for improving prognosis models and real-time monitoring TEChnologies for the creation of hydraulic conduits in crystalline rocks (STIMTEC). The STIMTEC underground research laboratory is located at 130 m depth in the Reiche Zeche mine in Freiberg, Germany. The experiment was designed to
investigate the rock damage resulting from hydraulic stimulation and to link
seismic activity and enhancement of hydraulic properties in strongly
foliated metamorphic gneiss. We present results from active and passive
seismic monitoring prior to and during hydraulic stimulations. We
characterise the structural anisotropy and heterogeneity of the reservoir
rocks at the STIMTEC site and the induced high-frequency (>1 kHz) acoustic emission (AE) activity, associated with brittle deformation at
the centimetre-to-decimetre scale. We derived the best velocity model per recording station
from over 300 active ultrasonic transmission measurements for high-accuracy
AE event location. The average P-wave anisotropy is 12 %, in agreement
with values derived from laboratory tests on core material. We use a
16-station seismic monitoring network comprising AE sensors,
accelerometers, one broadband sensor and one AE hydrophone. All
instrumentation was removable, providing us with the flexibility to use
existing boreholes for multiple purposes. This approach also allowed for
optimising the (near)-real-time passive monitoring system during the
experiment. To locate AE events, we tested the effect of different velocity
models and inferred their location accuracy. Based on the known active
ultrasonic transmission measurement points, we obtained an average
relocation error of 0.26±0.06 m where the AE events occurred using a transverse isotropic velocity
model per station. The uncertainty resulting from using a simplified
velocity model increased to 0.5–2.6 m, depending on whether anisotropy was
considered or not. Structural heterogeneity overprints anisotropy of the
host rock and has a significant influence on velocity and attenuation, with
up to 4 % and up to 50 % decrease on velocity and wave amplitude,
respectively. Significant variations in seismic responses to stimulation
were observed ranging from abundant AE events (several thousand per
stimulated interval) to no activity with breakdown pressure values ranging
between 6.4 and 15.6 MPa. Low-frequency seismic signals with varying
amplitudes were observed for all stimulated intervals that are more
correlated with the injection flow rate rather than the pressure curve. We
discuss the observations from STIMTEC in context of similar experiments
performed in underground research facilities to highlight the effect of
small-scale rock, stress and structural heterogeneity and/or anisotropy
observed at the decametre scale. The reservoir complexity at this scale
supports our conclusion that field-scale experiments benefit from
high-sensitivity, wide-bandwidth instrumentation and flexible monitoring
approaches to adapt to unexpected challenges during all stages of the
experiment.