Researchers working at a geothermal well near Helsinki, Finland have found a way to prevent some of the larger fluid-triggered earthquakes at the site, according to a report published in the May 1 issue of Science Advances.
Using a dense system of instruments deployed around the well, the team monitored nearby seismic activity so that they could recommend changes to the project's pumping operations before they triggered an earthquake large enough to shut down the well.
The findings offer a promising way to control earthquakes at enhanced geothermal systems, where water injected into artificially fractured rock layers heats up under the ground and is retrieved to provide power for electrical or heating and cooling systems.
Energy experts consider geothermal power a clean energy source along with wind and solar power, since it produces low levels of carbon emissions. In some enhanced geothermal systems, however, the pressure of the water pumped through the injection wells moves through faults and leads to earthquakes.
The wells have been linked to a series of earthquakes near St. Gallen, Switzerland, including a magnitude 3.5 quake in 2013, and may have caused a magnitude 5.4 earthquake in Pohang, South Korea in 2017.
The research team, led by Georg Dresen of GFZ German Research Centre for Geosciences in Potsdam, Germany, used a network of instruments to monitor seismic activity near the St1 Deep Heat Oy well on the urban campus of Aalto University, where the power generated by the well will be used to heat buildings on campus. The well is the world's deepest enhanced geothermal system, located 6.1 kilometers (3.8 miles) below the ground surface.
Dresen and colleagues collected information about earthquake rates, locations and magnitudes from a network of 24 seismometers near the well. Their analysis of these data allowed them to alert the well operators when they thought a "red light" earthquake of magnitude 2.0 was likely to occur.
The red-light limit was set by the Finnish national government as the largest acceptable earthquake that could be triggered by well operations. People usually do not feel a magnitude 2.0 earthquake, but the quake can be picked up by seismic instruments.
The operators used the information to temporarily stop pumping at the well or adjust pumping flow and pressure rates. Altogether, the operators pumped 18,160 cubic meters (641,314 cubic feet) of fresh water into the rock layers and 8,412 seismic events occurred during the study, which took place in June and July 2018. The monitoring plan prevented any magnitude 2.0 earthquakes that would have shut down the well.
Although the ability to deliver near-real time information to the operators was important to the plan's success, "equally important was organizing an action plan like we had in Helsinki, where a traffic light system defined which actions should be implemented once the seismic events exceeded a certain magnitude threshold," said Dresen.
Monitoring systems also work best when researchers know as much as possible about the local geology and natural faults around a well, Dresen said, since fluid injection "may potentially activate faults and generate seismic events."
Earthquakes can also be triggered by hydraulic fracturing or fracking, which breaks apart rock to release oil and gas stored underground. Like enhanced geothermal systems, fracking wells pump fluids over a short time period of weeks or months close to the wellhead. This makes it likely that the monitoring and traffic light system used at the Helsinki well could be applied to prevent earthquakes at fracking sites, Dresen said.
It would be less likely to work at sites where oil and gas companies inject large volumes of wastewater back into the ground after drilling operations, Dresen explained, noting that wastewater injection is the cause of most induced earthquakes in the United States.
A local network of seismometers like the one at the Finnish well could not adequately monitor "wastewater injection that is performed over extended time periods, where the earthquakes often occur at a large distance to the injection points and at variable depths," he said.