Complexity of the Fracture Network Development During Stimulation of a 6.1-Km-Deep Enhanced Geothermal System in Finland

Session: Mechanisms of Induced Seismicity: Pressure Diffusion, Elastic Stressing and Aseismic Slip
Type: Oral
Date: 4/29/2020
Time: 11:45 AM
Room: 215 + 220
Description: 

In summer 2018, a hydraulic stimulation campaign was performed in Helsinki suburban area in the frame of St1 Deep Heat project. During a period of 50 days over 18,000 m3 of fresh water were injected into the reservoir located at 6,1 km depth. The stimulation resulted in nearly 44,000 earthquake detections with M>-1.2. 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 M<=1.9, just below the project-stopping M 2.1 red alert of the traffic light system.

In this study we investigate the spatial, temporal, triggering, clustering and seismic source properties of seismicity and hydraulic-radiated energy budget in response to past injection operations using refined seismic data. These parameters are used to characterize the complexity of fracture network and local stress state development in response to 2018 injection operations and used to understand the state of seismic hazard and draft possible scenarios of seismicity and maximum magnitude development ahead of second stimulation campaign planned for March 2020.

We reprocessed, enhanced and relocated the original seismic catalog using a new velocity model derived from VSP campaign. The polarity-based focal mechanisms were calculated using cross-correlation-aided techniques and used to calculate stress tensors. We found the stimulation activated a depth-constrained network of planar fractures oriented favorably with respect to the global stresses. The seismicity does not display signatures of inter-event triggering with the largest events generally underrepresented in the catalog and not accompanied by foreshock/aftershock sequences. Instead, the cumulative radiated energy is linearly related to the hydraulic energy. We observe two distinct time periods characterized by different slopes and short time lag between radiated energy rate and hydraulic energy change. The obtained results suggest that occurrence of seismicity is governed by elastic energy stored in the geothermal system.

Presenting Author: Grzegorz Kwiatek

Authors