Controlling Induced Seismicity During Hydraulic Stimulation of a 6 Km Deep Enhanced Geothermal System in Finland

Near-realtime seismic monitoring of fluid injection allowed control of induced earthquakes during the stimulation of a geothermal well near Helsinki, Finland. The injection well was drilled down to 6.1 km-depth into Precambrian crystalline rocks. The well was deviated 45° from vertical and an open hole section at the bottom was divided into several injection intervals. A total of 18,160 m³ of fresh water was pumped into crystalline basement over a period of 49 days in June and July, 2018. The stimulation was monitored in near-real time using a deep seismic borehole array and series of borehole stations. Earthquakes were processed within a few minutes and results informed a Traffic Light System (TLS). Using near-realtime information on induced-earthquake rates, locations, magnitudes, and evolution of seismic and hydraulic energy, pumping was either stopped or varied between wellhead-pressures of 60-90 MPa and flow rates of 400-800 l/min. This procedure avoided the nucleation of a project-stopping red alert at magnitude M2.1 induced earthquake, a limit set by the TLS and local authorities.