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.
Near-realtime monitoring system
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
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.
The resulting industrial data were post-processed at Section 4.2: Geomechanics and Scientific Drilling of GFZ German Research Centre for Geosciences together with industrial and university partners participating in the St1 Deep Heat project. The results of this 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 ] [ Download open-access article ]
The first step consisted of enhancement of the original industrial seismic data in attempt to search smaller seismic events. This resulted in detection of over 50,000 very small seismic events by using simple pattern matching algorithm applied to original waveform data. The completeness of the seismic catalog was shifted down from initial ML -0.5 to the ML -1.2.
The selected over 2,100 high quality and largest earthquakes were manually reviewed and extensively reprocessed. This included review of the waveform data and improvement of hypocenter locations using double-difference relocation algorithm. This increased the precision of earthquake hypocenter locations to <60 m for the 95% of the catalog and allowed to investigate the details and spatial and temporal evolution of seismic activity in response to injection operations.
Seismicity and injection operations
The seismicity overall appeared concurrently in three four major zones, regardless of the stimulated part of the open hole section of OTN-3 well. This is an interesting feature by itself giving a hint on the strength of rocks at the reservoir depth. As the packer failure hypothesis was rejected (packer is a device that isolates some section of the well in order to concentrate the injection in the specific area), the project team concluded that the injected fluids were bypassing the stage packers entering formation at certain discrete intervals. This would mean that the vicinity of the injection well is seriously damaged, likely due to the existence of damage zones and fractures induced by drilling of the ONT-3 well.
The seismicity was observed to propagate outside of injection well OTN-3 along south-east to north-west direction, which was expected as it this is direction subparallel to the direction of maximum horizontal stress. Also, the downward migration of earthquakes was observed in the largest zone of seismicity with the progress of stimulation campaign.
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 ]