Controlling induced seismicity during hydraulic stimulation of a 6-km deep EGS in Finland
Abstract
We show that near-real-time seismic observations allowed control of a hydraulic stimulation in a 6.1 km deep Enhanced Geothermal System project in Finland. The injection well, OTN3c, was drilled into Precambrian crystalline rocks. The last uncased 1000 m and 45° inclined part of OTN3c was divided into several injection intervals. A total of 18,159 m3 of water was injected during a period of 49 days in Jun-Jul 2018. Based on seismicity rates and magnitudes, injection wellhead pressures and flow rates were controlled between 60-90 MPa and 400-800 l/min. The stimulation was monitored in near-real time using (1) a 12-level seismometer array at 2.20-2.65 km depth in another well located 10 m from OTN3c and (2) a 12-station network installed in 0.3-1.15 km wells surrounding the project site. Events were processed within a few minutes and also used in a Traffic Light System (TLS). The stimulation resulted in detection of >43,000 earthquakes with -1.2<ML<1.9 - all below a TLS Red of ML 2.1. The relocated catalog of 4032 earthquakes was used to investigate the spatio-temporal evolution of seismicity and seismic energy release in response to injection.
The temporal distribution of seismicity correlates with injection rate time pressure (hydraulic energy). Between ML -1.2<ML<1.5 the Gutenberg-Richter (GR) b-value is 1.3 and is nearly time-invariant. There is a distinct drop-off in the number of events above ML>1.5. Regardless of injection interval, the activity mainly concentrated in a large cluster located at the bottom of OTN3c. Four other clusters appear at various depths around the inclined part of OTN3c. These strike SE-NW in agreement with the direction of maximum horizontal stress and tectonic features. Activated at different times, the clusters progressively expanded away from the stimulated well-section. Seismic injection efficiency (radiated/hydraulic energy) was low (10-4), and together with spatial distribution and temporal evolution of events suggests reactivation of pre-existing fracture network, the geometry of which likely constrained magnitudes to the maximum observed ML 1.9. Moreover, event magnitudes increased in a forecastable way with hydraulic energy, indicating that fracture propagation was stable and runaway on extended faults did not occur, in accordance with recent model of Galis et al., (2017).- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.S21A..01K
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGYDE: 7223 Earthquake interaction;
- forecasting;
- and prediction;
- SEISMOLOGYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8168 Stresses: general;
- TECTONOPHYSICS