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Induced Acoustic Emission Activity Associated With the Stimtec-X In-Situ Hydraulic-Fracturing Experiment at the URL Reiche Zeche Mine, Germany

Session: Mechanisms of Induced Seismicity: Pressure Diffusion, Elastic Stressing and Aseismic Slip III

Type: Oral

Date: 4/20/2021

Presentation Time: 03:15 PM Pacific

Description: 

In 2018-2019, the STIMTEC hydraulic stimulation experiment was conducted at the Reiche Zeche underground laboratory in Freiberg, Saxony/Germany, to investigate the role of hydro-mechanical processes for enhancement of hydraulic properties in deep geothermal projects. We applied the same injection protocol to each of the ten stimulated intervals in the 63m-long injection borehole. Yet, we observed significant small-scale variability in the seismic and hydraulic responses to stimulation and stress field heterogeneity on the meter scale. While acoustic emission (AE) activity was high in the upper part of the injection borehole, no AE events were detected in its deepest part, penetrating a high-permeability damage zone. To investigate the stress and seismic variability as well as their interrelation further, we started the follow-on experiment STIMTEC-X. It involved eleven local stress measurements in October 2020 in three existing monitoring boreholes, with varying orientations and lengths. We used an adaptive, high-resolution, real-time seismic monitoring network comprising six AE-type hydrophones, six regular AE sensors and four accelerometers. The hydrophones were installed in combination with hydraulic gauges to make best use of existing infrastructure. Hydrophones were optimally placed for each measurement configuration anew with at least one deployed within ~3-4 m of the injection interval. We detected low-magnitude AE activity (M<-3.5) at high resolution, spatially distributed between distinct clusters identified previously, doubling of the seismically active volume. We also performed eight dilatometer tests to determine deformation characteristics of induced hydrofracs and pre-existing fractures. A circulation experiment between the injection borehole and two newly drilled boreholes of 23 m and 30 m depth is pending. Here, we present the seismicity associated with the STIMTEC and STIMTEC-X stimulation campaigns and focal mechanism solutions. We focus on how they contribute to 3-D volumetric stress field characterisation between local stress measurement points.

Presenting Author: Carolin M. Boese

Student Presenter: No

Authors