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

Date: 4/26/2019

Time: 01:30 PM

Room: Cascade I

We show that near-realtime seismic monitoring of fluid injection allowed control of induced earthquakes during the stimulation of a geothermal well near Helsinki, Finland. The OTN3 injection well was drilled down to 6.1 km-depth into crystalline rocks. Its open-hole section was deviated 45° from vertical and divided into several injection intervals. A total of 18,159 m³ of fresh water was pumped during 49 days in summer 2018. The stimulation was monitored in near-real time using (1) a 12-level seismometer array at 2.20-2.65 km depth in an observation well located ~10 m from OTN3 and (2) a 12-station network installed in 0.3-1.15 km deep boreholes surrounding the project site. 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.

The stimulation resulted in detection of >43,000 earthquakes with -1.2<ML<1.9. The 4032 events were double-difference relocated and used to investigate the spatio-temporal evolution of seismicity, seismic energy release, and maximum magnitude in response to injection. The Gutenberg-Richter (GR) distribution and relation between hydraulic and radiated energy suggest (re-)activation of size-limited network of distributed fractures. The temporal behavior of GR b-value, as well as a lack of temporal (Omori-type) correlations in a presence of spatial localization of earthquakes suggest very limited earthquake triggering and stress transfer. The maximum observed magnitudes scale with stored elastic (~hydraulic) energy, following a fracture-mechanics based model of Galis et al. (2017). Our results suggest a possible physics-based approach to controlling stimulation induced seismicity in geothermal projects.

Presenting Author: Peter E. A. Malin

Authors