St1 Deep Heat project
The St1 Deep Heat project site is located in Helsinki suburbian area at the Aalto University in Espoo, approximately 6 km away from Helsinki city center. The project aims to provide a sustainable baseload for the campus area heating network by extracting the hot water from the depth of over 6 km. The intended configuration is to have a well doublet, where one well will be used for (cold) water injection and second well for (hot) water extraction. In the first stage of the drilling, a 6.4 km measured length well OTN-3 was drilled, with the last 1000 meters of the inclined open-hole section. The last part of the well targeted permeable geological formations at 5-6 km depth with temperatures up to 120C.
The designed Enhanced geothermal systems (EGS) hold the promise of using the ubiquitous heat energy of Earth. However, enhanced geothermal systems typically requires opening – so-called “stimulation” – of fluid flow channels to enhance the permeability of the new reservoir. The natural by-products of this engineered process are earthquakes. Seismic activity related to enhanced geothermal systems have seriously affected or even terminated some geothermal projects such as Basel, Switzerland in 2006 or Pohang, South Korea in 2018. Therefore, implementing of safe stimulation strategies was critical for public acceptance of designed enhanced geothermal system.
Before this could happen, it was necessary to hydraulically stimulate the future geothermal reservoir by injecting fluids into the OTN-3 well in order to enhance its permeability. The stimulation was performed in Summer 2018. In the following stage, a new well will be drilled into the created damage zone in order to establish the well doublet and to start the commercial exploitation of the heat.
Preparation for reservoir stimulation
The stimulation monitoring network, designed by ASIR LLC, was composed of 12 sensors located at depth 2.6 km in adjacent borehole OTN-2, approximately 3 km above the injection area. This was completed by additional 12 borehole sensors located up to 11 km away from the project site. A separate seismic network of 17 geophones was installed in locations critical from perspective of ground motions, and it was used by ARUP Geohazards to support the traffic light system operations.
fastloc GmbH involvement
The system for processing and interpretation of passive seismic data in near-realtime fastloc.REEL detected and located more than 6000 earthquakes in the vicinity of project site during the stimulation campaign and shortly afterwards. The resulting first-break information included the initial hypocenter location and magnitude estimate which were both available up to 5 minutes after the earthquake occurrence. In case of larger events, the magnitude estimate and hypocenter location was manually verified by the seismologist working remotely. The catalog of seismic events was updated continuously during injection operations and provided information both to the Traffic Light System (TLS) operator as well as injection engineers, both located at the project site in Espoo. All parties were informed through three independent channels: 1) The dedicated website (“dashboard”), where catalog of seismicity and its evolution was presented in near-real time, 2) the SMS-like alert system, and 3) email alert messages. The seismicity evolution was used to decide in ample time on how to change stimulation parameters. fastloc GmbH team also served their expertise while discussing the modifications in injection program in response to the development of induced seismicity. By the end of the project at 18,500 m3 of fresh water injected, the maximum observed local magnitude was M1.9, which was just below the red alert level of 2.1 set up by the local authorities.
The resulting industrail data were post-processed by Section 4.2: Geomechanics and Scientific Drilling from GFZ German Research Centre for Geosciences together with industrial and university partners that took part in the first stage of the St1 Deep Heat project. The results of analysis were presented in the article published in 2019 in Science Advances:
Kwiatek, G., T. Saarno, T. Ader, F. Bluemle, M. Bohnhoff, M. Chendorain, G. Dresen, P. Heikkinen, I. Kukkonen, P. Leary, M. Leonhardt, P. Malin, P. Martínez-Garzón, K. Passmore, P. Passmore, S. Valenzuela, and C. Wollin (2019). Controlling fluid-induced seismicity during a 6.1-km-deep geothermal stimulation in Finland, Sci Adv 5, no. 5, eaav7224, doi 10.1126/sciadv.aav7224. [ Article Page ]
The development and operation of the Traffic Light System implemented for the St1 Deep Heat project was presented in another publication by Thomas Ader and co-authors in the Journal of Seismology:
Ader, T., M. Chendorain, M. Free, T. Saarno, P. Heikkinen, P.E. Malin, P. Leary, G. Kwiatek, G. Dresen, F. Bluemle, and T. Vuorinen (2019). Design and implementation of a traffic light system for deep geothermal well stimulation in Finland, J. Seismol. DOI: 10.1007/s10950-019-09853-y. [ Article Page ]