The risk of seismicity in obtaining geothermal energy

New research predicts the risk of seismicity in obtaining clean energy under the Earth’s surface.

This study has been led by the Mediterranean Institute for Advanced Studies (IMEDEA), dependent on the University of the Balearic Islands and the Higher Council for Scientific Research (CSIC) in Spain. It has been carried out in collaboration with the Institute for Environmental Diagnosis and Water Studies (IDAEA) of the CSIC) and the University of Colorado in the United States.

The study identifies the causes of the seismicity that occurred in 2006 in Basel (Switzerland) from an EGS (Enhanced Geothermal System) plant. This technology consists of drilling into the earth’s crust to use the heat that exists at great depths with the purpose of producing renewable energy.

Specifically, the scientific team has developed a numerical tool that makes it possible to reproduce the reactivation of the faults that occurred in the case of the Basel EGS plant in response to hydraulic stimulation in the injection well. This opens the way to the development of methodologies that allow geothermal energy to be used safely and cleanly to produce electricity continuously 24 hours a day, seven days a week and with zero carbon dioxide (CO2) emissions.

Among the new methods of obtaining energy, the EGS uses hydraulic stimulation to be able to produce electricity efficiently. Through a circuit in which cold water is injected into a well at high pressure, the existing fractures in the rock open permanently as a result of small landslides. As explained by Aurégan Boyet, an IMEDEA researcher and co-author of the study, “this process is necessary to increase the permeability of rock formations, normally fractured granites, that are between 4 and 5 kilometers deep. There the temperatures are higher than 150 degrees Celsius, so by pumping the hot water through another well, steam is generated on the surface, which drives the turbines that produce electricity.” Boyet states that the heat inside the Earth provides constant energy, which has a great advantage over other renewable energies that fluctuate over time.

However, hydraulic stimulation can cause problems. As specified by Silvia De Simone, a researcher at IDAEA and co-author of the study, “the slippage of the fractures generates micro-seisms. What is disconcerting,” he stresses, “is that once pressurized water is no longer injected, earthquakes of greater magnitude than those that occurred during the injection are frequently produced, generating earthquakes that are felt on the surface and can to cause damage, such as what happened in Basel or Pohang (South Korea). This phenomenon goes against logic, which would lead one to think that, once the injection stops, the pressure decreases and the stability of the faults should increase”.

But this is not what is observed, and the reasons are poorly understood. In the case of Basel, the largest earthquake occurred a few hours after the injection stopped. In order to accurately reproduce the temporal evolution and spatial distribution of the earthquakes, which reached a magnitude of 3.4, the scientific team incorporated into their numerical models processes that, until now, had been overlooked but are essential for to understand and predict the phenomenon. “Historically, changes in the pressure of the water that fills the fractures have been considered to be the cause of seismicity. However, pressure changes can only explain part of it, but they fail to explain post-injection seismicity”, says Víctor Vilarrasa, IMEDEA researcher and co-author of the study. “Our model combines the changes in rock strain and stress as a result of water injection, which in turn influence fault stability. Furthermore, when a fracture moves, stresses in its surroundings are redistributed, which can affect the stability of other fractures. Lastly, we have considered reducing the resistance of faults once they reactivate.”

Geothermal power station. (Photo: Don Follows/NPS)

Thanks to the new model, it has been possible to identify the causes of the seismicity in Basel during and after the injection. The authors of the study emphasize that this predictive tool will serve to adapt the parameters with which a geothermal energy project is operated (such as the circulation flow or the injection pressure) and to mitigate the risk of inducing earthquakes that can be perceived. . In this way, the security in obtaining clean energy is reinforced, which is intended to be an alternative to face the problems related to global warming caused by the use of fossil fuels.

Compliance with the Paris agreements to reduce greenhouse gas emissions comes with challenges. The goal is clear: limit the global temperature rise to below 2 degrees Celsius, compared to pre-industrial levels, and continue efforts to limit it to 1.5 degrees Celsius. The use of different types of renewable energies, such as geothermal energy, is essential to achieve this. Used since Roman times in thermal baths, geothermal energy takes advantage of the heat coming from the interior of the Earth. It is an energy that is respectful of the environment, does not produce polluting emissions and also guarantees the constant supply of electricity.

Scientific research in geoenergy developed at IMEDEA, within the framework of the European Research Council (ERC), works to minimize the risks that the use of the subsoil may entail in decarbonization. “Geological resources, as the origin of the problem, must also form part of the solution” concludes Vilarrasa.

The study is titled “Poroelastic stress relaxation, slip stress transfer and friction weakening controlled post-injection seismicity at the Basel Enhanced Geothermal System”. And it has been published in the academic journal Communications Earth & Environment. (Source: Ana Bonilla / IMEDEA / CSIC)