Complexity of the fracture network development during stimulation of a 6.1-km-deep Enhanced Geothermal System in Finland
Abstract
In summer 2018, a hydraulic stimulation campaign was performed in Helsinki suburban area within the frame of St1 Deep Heat mining project. During a period of 50 days over 18,160 m3 of fresh water were injected in an effort to stimulate the reservoir located at 6,100 m depth. The stimulation resulted in 43,882 earthquake detections with MW>-0.6. Using near-real-time seismic monitoring and data analysis to adapt injection rates and injection schedule, the maximum magnitudes of induced seismic events were successfully constrained to MW<1.9. The procedure allowed avoiding a project-stopping M2.0 red alert of the traffic light system. In this study we analyze the spatial and temporal evolution of seismic locations, source mechanisms and statistical properties in response to the injection operations. We reprocessed and enhanced the original seismic catalog to refine hypocenters and source parameters. A new velocity model derived from Vertical Seismic Profiling campaign and an updated database of P- and S-wave arrival times was used to enhance the original catalog to 7,400 events for which absolute hypocenters were determined. The events were then relocated using the double-difference technique. In addition, a collapsing method was applied to sharpen details of spatial development of the recorded seismicity. To characterize the complexity of earthquake source mechanisms in the catalog, the P-wave waveforms of the strongest events were used as templates to derive polarities for smaller events using cross-correlation techniques. Using clustering analysis, the resulting polarity patterns form distinct focal mechanism families characterizing the fracture network around the injection well. The obtained results highlight development of a complex fracture network and the causal relation between the evolving seismic event pattern and the stimulation procedures.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S21B..01L
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7223 Earthquake interaction;
- forecasting;
- and prediction;
- SEISMOLOGY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8168 Stresses: general;
- TECTONOPHYSICS