Traffic Light System Design and Implementation for Geothermal Stimulation in Helsinki, Finland

Abstract

St1 Deep Heat is developing a geothermal doublet for the purpose of delivering deep geothermal heat to local district heat networks. As part of the project, a first well was drilled to a depth of 6.5 km west of Helsinki, Finland, and was stimulated in June to July 2018 to improve rock permeability in contact with the well. To complete the geothermal doublet, a nearby well is planned to be completed in the future. Given that the stimulation took place in a densely populated area with multiple sensitive receptors, a seismic Traffic Light System (TLS) was required before the start of well stimulation. The TLS thresholds were established in a probabilistic way to account for uncertainties in the available data. The thresholds were based on a combination of the surface expression of induced seismicity and associated magnitudes, so that false alarms related to surface expression not due to an induced seismic event could be avoided. The final TLS thresholds included a peak ground velocity (PGV) of 1 mm/s associated with a ML ≥ 1 event to trigger an Amber alert, and a PGV of 7.5 mm/s associated with a ML ≥ 2.1 event to trigger a Red alert. Specific thresholds based on PGV and peak ground acceleration (PGA) were gathered for sensitive receptors and related to earthquake magnitudes in a probabilistic way. The thresholds were then revisited after the well stimulation, making use of the acquired data, using the same methodology.