The work, in which the ICM-CSIC has participated, proposes mechanisms to explain the cause of the earthquakes and why the worst ones occurred two weeks after the cessation of injection.
An interdisciplinary study involving the Mediterranean Institute for Advanced Studies (IMEDEA), the Institute for Environmental Diagnosis and Water Studies (IDAEA) and the Institut de Ciències del Mar (ICM), all belonging to the Spanish National Research Council (CSIC), as well as the Institute of Geosciences of Rennes (France), has revealed that insufficient monitoring at the Castor gas storage facility prevented them from understanding what was happening and anticipating the earthquakes that led to the closure of the project.
The paper, published in the journal Nature Communications, shows that inaccurate localisation of induced earthquakes, as well as failure to take into account all the processes that can induce them, can jeopardise the development of future geoenergy projects. These, including geothermal energy, geological carbon storage and underground hydrogen storage, have been identified as key to achieving decarbonisation and mitigating the effects of climate change.
Jesús Carrera, researcher at IDAEA and one of the authors of the study, explains:
"Having an adequate monitoring network and a protocol to manage induced seismicity is essential for the success of these projects. Since the seismometers on the seabed were not working, the gas injection at Castor should not have been carried out, as it was not possible to accurately locate the induced earthquakes in real time and understand what was happening".
"There is still uncertainty about the depth of the earthquakes, even though almost 9 years have passed since they were induced and various techniques have been used to improve their location. The lack of installation of operational seismometers around the Castor platform, including deep seismometers in some of the 14 boreholes drilled for the project, greatly complicates the analysis. In projects of this type, it is necessary to be able to locate induced earthquakes in real time, which requires an adequate monitoring system," specifies the ICM researcher Antonio Villaseñor, also an author of the study.
The study also explains that in addition to the increase in pressure in the subsurface caused by gas injection, there are other mechanisms that can produce earthquakes.
"In gas reservoirs, such as Castor, buoyancy plays a very important role, as the density of the gas is 10 times lower than that of the water that fills the pores of the rock where the gas is stored. According to Archimedes' principle, the vertical buoyancy force is equal to the volume of fluid displaced multiplied by the difference in densities between the displaced fluid and the fluid displacing it. Therefore, the vertical buoyancy force played an important role in the destabilisation of the Amposta fault, which triggered the Castor earthquakes," adds Silvia De Simone, from the Institute of Geosciences in Rennes (France), also a participant in the study.
The integration of the measurement network with physical processes is the key to the prediction and management of induced seismicity. In this sense, Víctor Vilarrasa, researcher at IMEDEA and IDAEA and co-author of the study points out:
"It is necessary to reverse the damage that the Castor earthquakes did on public perception, as without geoenergy projects climate neutrality cannot be achieved. Combining field measurements - injection flow, pressure and temperature evolution, deformations, induced seismicity - with models that take into account the relevant physical processes, could have managed the earthquakes and saved Castor."