By Stephen Beech
Fracking may have triggered more than 100 earthquakes in Surrey, according to a new study.
The controversial oil extraction technique is believed to have caused a series of more than 100 small quakes in the commuter belt in 2018 and 2019, suggest the findings.
The earthquakes, which occurred in Newdigate and surrounding areas from April 2018 until early 2019, were recorded as being between 1.34 and 3.18 magnitude.
The tremors were linked to cracks in walls and ceilings and other damage to people’s homes, with reports of houses and beds shaking.
Geologists have been divided over whether the earthquakes could have been triggered by extraction at the Horse Hill well in Horley, less than 6.5 miles away.
Some researchers had previously argued that the pattern of earthquakes did not correspond to the oil extraction, with periods of increased extraction not followed directly by an increase in seismic activity.
The new study involved running more than a million simulations estimating the frequency of earthquakes based on the timing and volume of oil extraction.
The findings, published in Geological Magazine, showed that the model predictions roughly matched what occurred, suggesting a link between the extraction of the oil and the earthquakes.
Study lead author Dr. Matthew Fox, of University College London (UCL), said: “Our study suggests there is a link between the earthquakes and oil extraction at Horse Hill, but we cannot rule out that this link is a coincidence.
“More work needs to be done to understand if this is cause and effect. However, our findings indicate it is plausible that oil extraction triggered the earthquakes.”
Oil extraction at Horse Hill was stopped in October after planning permission was quashed in a landmark legal case.
Supreme Court judges ruled that the full climate impact of the project should have been considered before Surrey County Council approved it.
However, another project has received council approval to conduct exploratory oil drilling in Balcombe, West Sussex.
That is subject to a legal challenge by a residents’ group, with the case expected to be heard by the Court of Appeal later this month.
Extracting oil changes the fluid pressure in the rock from which the oil is extracted from.
Scientists say the change in pressure can potentially diffuse through tens of miles underground to intersect a fault.
The speed that the change in pressure moves depends on the rock’s permeability, how quickly fluids can pass through it.
As faults are often under considerable strain, even a small change in stress can cause the rapid movement of rock that leads to an earthquake.
For the new study, researchers used a mathematical model to predict the number of earthquakes that might occur based on how much oil had been extracted, accounting for the two different rock types that oil was extracted from.
Building on the work of the late University of Glasgow geologist Dr Rob Westerway, the research team were able to account for oil extraction switching between two different rock types, with one, Portland rock, being a million times more permeable than the other, Kimmeridge.
The team said that means the lag time between extraction and a potential induced earthquake would be a matter of days rather than weeks.
Although the earthquakes began just before oil extraction started, the researchers said the earthquakes could have been triggered by preparatory works – for example, checking the pressure in the well – which could have led to a similar pressure change travelling underground to the fault line.
The research team said the picture was “complicated” by the fact that seismometers were installed in the area only after the first larger earthquakes had occurred.
Any smaller earthquakes that might have occurred prior to the instruments being installed were not recorded.
The research team says that highlights the need for detailed seismic monitoring before oil extraction begins at proposed sites.
Co-author Professor Philip Meredith, also of UCL, said: “Our study highlights the importance of monitoring the seismicity of areas where oil extraction might occur before any works start.
“Caution should be the byword. It is no good saying you don’t have a problem when you potentially do.”
He added: “There has been no significant seismic activity in Surrey for decades, so these earthquakes were unusual events.
“However, unusual events do happen in nature, so we can’t rule out the possibility that the timing associated with the oil extraction was a coincidence.”
