People living in the area of Surrey hit by earthquakes last summer felt another tremor this morning.
The quake, measuring 2.4ML (local magnitude) happened at 7.43am and came as geologists from Edinburgh University pointed the figure at the Horse Hill oil site as a cause of 15 tremors in 2018.
The leading investor in Horse Hill, UK Oil and Gas plc, said there was “no reasonable scientific basis for the claims” made by the Edinburgh geologists and described one of the authors as a “well-known opponent of the oil and gas industry”.
But Friends of the Earth said the Edinburgh conclusions should “give the government pause for thought when considering calls by the industry to relax regulations on fracking-induced seismicity.
Today’s tremors shook furniture and was described as a “large bang”, “strong impact” and a “general rumble”. A second smaller tremor, measuring 0.2ML, was recorded 15 minutes later.
The epicentres were between Charlwood and Newdigate, north west of Gatwick Airport – the same area as the previous swarm. It was also estimated to be at the same depth, of 1-2km.
The 2018 earthquakes, which were unprecedented in this part of Surrey, began on 1 April and continued until 19 October. The strength varied from minus 0.4ML to 3.0ML.
In November last year, the Oil & Gas Authority ruled out human cause for the 2018 earthquakes. It said no work was being undertaken at the nearby Horse Hill oil exploration site and so it should be excluded from its analysis.
Horse Hill Developments Ltd (HHDL), which operates the Horse Hill well, consistently said its work did not induce the earthquakes.
But a paper by three Edinburgh geologists, released today, concluded:
“Our assessment supports the concern that Horse Hill oil exploration triggered the  earthquakes.
“We infer that future oil exploration and production close to critically-stressed faults in the Weald is likely to result in similar earthquake events.”
The authors of today’s report, Dr Andrew Cavanagh, Dr Stuart Gilfillan and Professor Stuart Haszeldine, said they had evidence that there was work at the Horse Hill site when the quakes started in April 2018.
They said social media posts and responses to Freedom of Information requests indicated that well preparations for flow testing immediately preceded the Newdigate earthquakes.
One possible cause of the quakes could be the release of pressure at the Horse Hill Well site, they said:
“Prior to flow testing in April and July, Horse Hill appears to have encountered a natural source of overpressure in the gas-rich oils of the Kimmeridge, as observed in the ‘gas lift’ reported for the well.
“We infer that management of this pressure (probably by bleeding the well annulus prior to testing) likely altered the Horse Hill fault stress balance, which then impacted on the Newdigate fault, causing the earthquakes”.
They said the well and site engineering logs – which have not been made public – could provide more information on well intervention pressure changes as the trigger for the 2018 quakes.
The geologists said the 2018 earthquakes clustered around 1km below the surface, close to the intersection of the two faults.
The Horse Hill fault intersects the oil well at the same depth as the exploration targets and continues towards the Newdigate fault, they said. The authors described this intersection as “a likely conduit for pressure changes between Horse Hill and the Newdigate cluster”.
They added that later earthquakes happened when flow testing work was underway on the Horse Hill site:
“the earthquakes in August and September coincide exactly with the deployment of a perforation gun downhole to improve flow.
“The pattern appears to be months of inactivity prior to well intervention and preparation for flow testing. The well interventions then precede earthquake activity by hours to days.”
The authors have sent their paper to Surrey County Council, which is currently consulting on proposals to drill four more oil wells and carry out long-term oil production at Horse Hill.
Stephen Sanderson, the chief executive of UKOG plc, said today Professor Haszeldine had attended a workshop on the Surrey seismic events, organised by the OGA.
“Professor Haszeldine’s speculations for the cause of the Newdigate cluster were dismissed by all of the other 18 expert attendees at the OGA workshop. As well as the OGA’s workshop’s finding that there was no direct causal link to oil and gas activity, further scientific papers are to be published by experts in seismicity supporting the same conclusions.
“Even though they were provided with specific data by the OGA, Professor Haszeldine and his two colleagues have provided a completely incorrect timeline of HHDL’s operational activities. There was no ‘flow testing in April’. Flow testing at Horse Hill only began in July 2018.”
Mr Sanderson dismissed the Edinburgh statement that earthquakes in August and September coincided with the deployment of a perforation gun.
“Re-perforation of one of the oil reservoirs took place on 17 August 2018.
“Firing of a perforating gun is an event lasting milliseconds, not two months.”
Geolgist Dr Stephen Hicks, of Imperial College London, also dismissed the Edinburgh report:
“The report by Cavanagh, Gilfillan and Haszeldine presents a working hypothesis that lacks analysis of one of the key pieces of seismic evidence available: seismic waveform data from the earthquakes. This is available to everyone, including fellow scientists at: http://www.earthquakes.bgs.ac.uk/monitoring/data_archive.html“
He said the earthquake locations relied on by the Edinburgh geologists were “out-of-date” and should be updated to reflect the data from local seismic stations installed in July and August last year.
“As a rigorous and independent scientist, I am open-minded, but at present, there remains no significant physical explanation based on substantial evidence that could explain how these earthquakes could have been induced by human activities.
“Although a surprise to those living in the area and to us seismologists, the one silver lining of today’s magnitude 2.4 earthquake is that it is the largest earthquake which was recorded by all 5 local seismic stations, and the data from it can give us more confidence in our interpretations of the earthquake mechanisms. Analysis of the data is ongoing and this work will be submitted for formal, peer-reviewed scientific publication in the near future.”
“Pause for thought”
Friends of the Earth clean energy campaigner Tony Bosworth said:
“Today’s report linking earthquakes in Surrey last year to oil exploration should give the government pause for thought when considering demands from oil and gas companies to relax regulations.
“The government has said that the current system is fit for purpose. They must keep to this and not make further concessions at the first sign of complaints from a dirty, unwanted and unnecessary industry.”
“Company should make site information available”
Weald Action Group, a network of community groups opposed to oil and gas extraction in south east England, called on HHDL to provide the well and site engineering logs for the period of the 2018 quakes.
James Knapp, of the group, said:
“The Oil and Gas Authority appears to have been less than rigorous in researching what caused these quakes.
“Horse Hill Developments Ltd hasn’t provided the information that’s needed for a proper assessment and what information there is available clearly points the finger at their well site.
“With plans afoot for a massive increase in oil extraction at this site, this matter needs to be thoroughly investigated before any decisions are made.
“We support calls for a moratorium on drilling and on decisions on planning applications which might cause quakes. They have caused a lot of concern locally and the process of analysing the cause of the quakes has been far from transparent”.
“Caution needed in linking seismicity to oil operations”
Ken Cronin, Chief Executive of UK Onshore Oil and Gas (UKOOG), said:
“Last year Newdigate saw a number of similar shallow seismic events, leading the Oil and Gas Authority to launch an expert panel of geoscientists to review the evidence and determine the cause. They concluded that there was no causal link between the seismic events and oil and gas activity. In addition, the British Geological Survey prepared a report for the panel, noting that earthquakes are not unprecedented in the area and the events were unlikely to have been induced by oil and gas activity.
“Those drawing links between onshore oil development in the region and the latest seismic event should exercise caution in doing so. There will be a number of factors that have contributed to the recent seismicity in Newdigate, but to date, experts have concluded that oil exploration has not been one.”