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Aerial view of the St1 Deep Heat project site in Espoo, Finland (Photo: Tero Saarno)

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.

Aerial view of the St1 Deep Heat site in Espoo, Finland (Photo: Tero Saarno)

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.

2018 stimulation campaign

Near-realtime monitoring system

Dashboard system used by fastloc.REEL-AI application to present catalog of seismicity processed in near-real-time, as well as evolution of seismicity during the project (the screenshot taken from project not related to St1 Deep Heat stimulation campaing in 2018)

For near-realtime seismic monitoring and reporting, the fastloc.REEL system was used, which I developed for fastloc GmbH. This system detected and located more than 8,000 earthquakes in the vicinity of project site during the stimulation campaign and shortly afterwards. The resulting first-break information included 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 from fastloc GmbH team working remotely and on-site. 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 pushover alert system, and 3) email alert messages.

During the stimulation, the seismicity evolution was used to decide in ample time on how to change stimulation parameters. Our fastloc GmbH team also served their expertise while discussing the modifications in injection program in response to the development of induced seismicity together with other partners. By the end of the project at 18,500 m3 of fresh water injected, the maximum observed local magnitude was ML 1.9, which was just below the red alert level of ML 2.1 set up by the local authorities.

Controlling induced seismicity

Spatial distribution of 2018 and 2020 seismicity as visible from east direction. The time of occurrence is color-coded, with warm colors corresponding to 2018 stimulation (dominating the seismicity) and deep blue color corresponding to 2020 stimulation (events around the OTN-2 well). The size of spheres correspond to actual source size estimation assuming constant stress drop of 8 MPa

Already at the begin of stimulation (Phase 1-2), the project team quickly realized that the seismic energy release is proportional to the hydraulic energy, or equivalently product of pressure and volume. It was also identified that any stop in injection leads to quick reduction in seismic activity. In Phase 2 of injection, the maximum well head pressure of around 90 MPa was applied, and the injection was performed with long intervals lasting up to 3 days. This resulted in acceleration of seismic energy release and led ultimately to a series of relatively “amber” earthquakes that put the injection temporarily on hold. It was obvious for the project team that a redefinition of the injection strategy is required.

The project team realized that the maximum observable magnitude increase with cumulative injected fluid volume. Surprisingly to the team, the trend of this magnitude evolution with injection already seemed to nicely follow that predicted from the fracture mechanics-based of Galis et al. (2017). While extending the observed trend to the planned total volume of 20,000m3, it was a concern of project team that by the end of the red alert ML 2.1 event may occur.

Galis model relates maximum magnitude of the so-called arrested rupture to the amount of energy available stored for rupture propagation. The project team decided to reduce somehow the amount of hydraulic energy stored in the geothermal system. At first, the well head pressure was reduced to values below 90 MPa in attempt to reduce the hydraulic input energy rate into the developed geothermal system in Phases 3 of injection. In following phases 4  and 5 injection plan was progressively changed. The injection was still performed at lower injection pressure, but the injection times were progressively reduced and the resting periods in-between stimulations were enhanced. This strategy visibly stabilized the seismic energy release with respect to hydraulic energy input. Till the end of the project, the red alert threshold was not exceeded, as the maximum observed magnitude reached M1.9 in the stimulation stage 4.

2020 Stimulation Campaing

Between May 4t, 2020 and May 20th, 2020, 2875 m3 of water has been injected into the open-hole section spanning between depths 4856-5765 m of the deepened OTN-2 well, to establish the communication between OTN-2 and OTN-3 wells. Similarly to 2018 stimulation campaing in OTN-3, the 2020 stimulation was injection-rate controlled. The maximum well head pressure did not exceed 72 MPa and the injection rate was kept at 400 l/min. This led to occurrence of much smaller seismicity, that is currently being analyzed (see some electronic materials below).

Publications and other electronic materials

Our team at Section 4.2: Geomechanics and Scientific Drilling of GFZ German Research Centre for Geosciences is continuously working on seismic data from both stimulation campaings, together with industrial and university partners participating in the St1 Deep Heat project. So far two papers has been published related to the project:

  • 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 ] [ Download open-access article ]
  • Leonhardt, M., G. Kwiatek, P. Martínez-Garzón, M. Bohnhoff, T. Saarno, P. Heikkinen, and G. Dresen (2021). Seismicity during and after stimulation of a 6.1 km deep enhanced geothermal system in Helsinki, Finland. Solid Earth 12, 581–594, DOI: 10.5194/se-12-581-2021. [ Article Page ] [ Link to associated data publication ]

You can also have at the video presentation related to the analysis of St1 seismicity from 2020 stimulation that was presented during 2020 annual meeting of the Seismological Society of America and at the EGU 2020 virtual conference:

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, published in the Journal of Seismology in 2019:

  • 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 ]