Natural gas processing plant in Aderklaa, Lower Austria. Credit: Bwag.
The possible role of natural gas extraction in causing earthquakes has long been a concern for scientists. But a new study by Italian researchers looked at the problem from another angle: how do earthquakes affect the formation of gas fields? The results1 suggest that where there is methane reservoir above a seismic fault, the latter is unlikely to produce a strong earthquake. “Strong earthquakes cause surface fractures that prevent the formation of methane deposits” says Gianluca Valensise from the National Institute of Geophysics and Volcanology (INGV), Italy, lead author of the study.
In collaboration with researchers from the National Institute of Oceanography and Applied Geophysics (OGS), Valensise built on the results from a previous study that showed that the sources of the earthquake on 20 and 29 May 2012 in northern Italy (magnitude 6.1 and 5.9, respectively) were located over a cluster of non-productive gas wells, surrounded by productive wells at a few kilometres’ distance2. “That study was based on a nice intuition, shifting the attention from earthquakes triggered by hydrocarbon exploration, hence human-induced, to the reverse problem” says Valensise. “What role do active fractures play underneath potential productive gas deposits?’
Thanks to three unique geophysical databases and robust statistical analyses, the researchers looked at 18 seismogenic faults and 1651 wells drilled for gas exploitation in Italy, and studied the distance between the gas reservoir basin and fault plain. They were able to confirm that the shorter the distance, the less likely it is for a fault to cause a strong earthquake. “It is important to note that we studied shallow faults (2–6 km) which create superficial earthquakes and not very deep ones (15–20 Km) that can also trigger earthquakes, but because at higher depths they do not affect gas reservoirs”, adds Valensise.
This study could also help the search for underground energy and CO2 storage sites, which could be part of a gradual transition from fossil fuels to a carbon–neutral economy. Depleted natural reservoirs are safer, compared to man-made ones, as they are more likely to be intact, unaffected by shallow active faults, thus considerably decreasing the hazard of triggered seismicity. Selecting a reservoir whose past performance is known would also decrease the risk of dispersion of methane or carbon dioxide to the atmosphere.
