Last summer, many people in Espoo and Helsinki noticed a strange noise and felt subtle ground shaking. These phenomena could be traced back to Otaniemi in Espoo, where a geothermal heating plant was being constructed by the St1 Deep Heat company.
The acoustic signals and the ground shaking were caused by small earthquakes that occurred about six kilometres beneath the construction site.
This is a new situation for Finnish officials and residents because this is the first project of its kind in this country. This lack of experience also means the effects of the water injection on the induced seismic activity were difficult to assess beforehand.
The existing official supervision and permission procedures do not consider engineering projects that may lead to earthquakes and related ground shaking.
The Ministry for the Environment has called on the Institute for Seismology at the University of Helsinki and the Geological Survey of Finland to report on the risks associated with geothermal heat production.
First in the world
Associate Professor Gregor Hillers from the Institute of Seismology has studied data from the St1 Deep Heat project as an independent researcher. Among other things, he has been involved in collecting seismic data to analyze earthquakes caused by the engineering activities.
He thinks the project can yield important societal benefits as well as interesting scientific insights on earthquake behavior.
– This is a significant project on a global scale. Nowhere else has anyone drilled so deep in similar geothermal projects. Previous attempts have been interrupted or abandoned due to induced earthquakes or other operational difficulties. So far, St1 has managed to keep the size of the earthquakes small enough, he says.
The Otaniemi heating plant is also exceptional considering its location in a densely populated area, and its vicinity to the infrastructure and facilities of Aalto University and the Meilahti hospital.
– Informing the public is also important. If we are not open about the project and its implications, people may start to speculate, he says.
What has been happening in Otaniemi?
The company has drilled a 6.1 kilometer deep hole that is 6.4 kilometer long into the bedrock for a heating plant; the temperature in the hole is up to 120 degrees celsius. Drilling was completed in spring 2018.
– The hole itself is not sufficient, the bedrock at depth also has to be broken up so that water can flow into and circulate in the cracks in the rock to heat up efficiently. The bedrock has been broken up by pumping pressurized water down into the lower parts of the hole where no casing allows a contact with the surrounding rock. Breaking up or shearing the bedrock along cracks causes the earthquakes that can be felt at the surface, Hillers explains.
When the plant is ready, cold water will be pumped into the ground, where the hot bedrock will heat it. The hot water will be pumped up through another planned hole to the surface, and the heat will be used for local district heating.
– The company controlled the water pumping to prevent the earthquakes from reaching a predefined size and an acceptable limit of shaking, says Hillers.
However, it is not possible to completely control all aspects of the induced seismicity. Earthquakes tend to continue to occur even though the pumping of water has been stopped after the initial stimulation phase, although at a much reduced rate. Earthquakes may also be part of the regular operation of the plant once it is established. The Institute of Seismology has recommended that the earthquakes should not exceed the magnitude of 2.1. So far, they have remained smaller than that. The strongest events were in the 1.7-1.9 magnitude range.
– There is a small probability that earthquakes that exceed the recommended size occur during and after the life cycle of the plant.
So far, the engineering activities do not seem to have had any damaging effects. However, people from Espoo and Helsinki have reported disturbing noise and shaking to the Institute of Seismology during the 2-month stimulation period in 2018. There has been feedback especially from Munkkiniemi in Helsinki and Niittykumpu and Laajalahti in Espoo.
According to Hillers, the drilling does not directly impact the groundwater quality. However, the circulating water contains higher concentrations of minerals and should therefore be treated before it can be released into the natural water system.
Geothermal heat among other heat-production methods
All in all, Hillers says he is following this geothermal project with interest.
– This is one way to “produce” energy. It has to be compared to other ways; nuclear, water, sun, wind, coal… We have to evaluate the advantages and disadvantages of the different technologies. An operating geothermal plant is practically emission-free. In the future, an increasing number of buildings may be heated with geothermal energy, here and elsewhere, Hillers says.
If the project proceeds as planned the Otaniemi heating plant may start to operate in 2020.
Plans for geothermal plants elsewhere in Finland are currently discussed. All following projects will benefit from the experiences made with the Otaniemi stimulation.
Open lecture by Gregor Hillers: KumpulaNYT: Man made earthquakes and geothermal energy use - Generalities and peculiarities associated with the Espoo deep heat project. Tuesday 7.5.2019 at 15:30-17:00. Kumpula Campus, Exactum building, Room B123, Gustaf Hällströmin katu 2B, Helsinki
Head of Department of Geosciences and Geography Annakaisa Korja