A report on the swarm of earthquakes in Surrey earlier this year finds no evidence they were triggered by oil and gas operations. But an academic who contributed to the official investigation into the cause of the quakes disagrees with the conclusion and critics of onshore drilling have described the findings as a “whitewash”.

The earthquakes happened mostly between 1 April and 18 July in the area around Newdigate near Gatwick. The nearest oil and gas sites to the earthquake epicentres were Horse Hill, near Horley, and Brockham, near Dorking. The operators of both sites denied any connection between their activities and the seismic events.

The likely cause of the earthquakes was discussed at a workshop of more than 20 academics and regulators last month. The report, published today by the government’s Oil & Gas Authority, stated:

“Participants concluded that based on the evidence presented, there was no causal link between the seismic events and oil and gas activity.”

But Stuart Haszeldine, Professor of Carbon Capture and Storage at Edinburgh University, who took part in the workshop, told DrillOrDrop tonight that the earthquakes passed almost all the tests used to decide whether they were induced by human activity.

The workshop discussions centred on the tests, known as the Davis & Frohlich Criteria, developed in the United States. They ask six questions, which if answered yes, suggest the earthquakes were likely to be induced, rather than natural. Most of the participants said the Newdigate earthquakes passed only one of the tests but Professor Haszeldine said they passed five and possibly all six.

Earthquake criteria

Are the earthquakes unusual?

The Davis & Frohlich Criteria ask whether the earthquakes were exceptional in the region. The majority of participants pointed to three small earthquakes in Billingshurst, about 20km away, in 2005, as evidence that there had been earthquakes in the Weald basin.

Professor Haszeldine said in his presentation that the 14 Newdigate earthquakes in a three-month period were without precedent in the Weald Basin. He said there had been only eight shallow earthquakes in the entire regional area in the previous 40 years. He said the Billingshurst earthquakes were at a depth of 5km, much deeper than the Newdigate series, most of which were at an estimated depth of 1.5-2km.

Did the events happen around the same time as oil and gas activity?

Richard Luckett, of the British Geological Survey, another workshop participant, said there was a link between the earthquakes and local oil and gas operations. He said production work had started at the Brockham oil field, near Dorking, on 29 March, three days before the first earthquake on 1 April.

The operator of the Horse Hill well provided evidence to the workshop that there was no activity at Horse Hill between March 2016 and 25 June 2018. It said flow testing did not start until July 2018, long after the start of the earthquake series. DrillOrDrop previously reported statements by the company that it was not to blame (here and here).

But Professor Haszeldine described the timeline of activity at Horse Hill as “compelling”.

Groundwork began at Horse Hill on 12 March 2018, before the earthquake sequence began. A new cellar was excavated, starting on 21 March. Campaigners have provided evidence that there were staff onsite on the Horse Hill site on 1 April 2018, the day of the first earthquake. Well integrity tests were carried out on 5-6 April 2018 by checking annular pressure.

Professor Haszeline’s timeline showed that the deployment of a perforation gun on 13-17 August was followed by a seismic event on 18 August.

Are the epicentres within 5km of oil and gas wells?

Richard Luckett, of the BGS, answered ‘no’ to this question. He argued in his presentation that seismicity can be induced at distances of 10km or more – but only where there is large volumes of injected liquid. He said the Brockham well, at 8km from the epicentres, was too far away to meet this test because the volume of liquid was very small .

But Professor Haszeldine said the Horse Hill site met this test. The cluster of earthquakes and a magnitude 3 event were only 3km away from Horse Hill, he said.

Were the seismic events at or near exploration target depths?

Workshop participants had argued that the Newdigate earthquakes probably did not meet this condition. The workshop heard that production at the Brockham site was at a depth of about 600m, while the earthquakes were at about 2.1km.

But Professor Haszeldine said the Portland and Kimmeridge targets of the Horse Hill well were at the same depths of some of the seismic events.

Do geological structures connect the earthquakes to oil and gas operations?

Richard Luckett answered ‘probably no’ to this question. But Professor Haszeldine said a normal fault passes NE-SW through the cluster and the Horse Hill well. He also said there had been different interpretations of the epicentre of the earthquakes and that individual interpretations had changed.

Are subsurface changes in fluid pressure sufficient to cause seismicity?

Workshop participants answered ‘probably no’ to this question. Angus Energy had restarted producing oil at Brockham just before the first earthquake, after a two-year break. Produced water was being reinjected into the Portland sandstone reservoir. But at the time of the first earthquake, 3m3 would have been injected, the workshope heard. This compared with approximately 100m3-1,000m3/day at other sites which are known to have induced seismicity.

Professor Haszeldine answered ‘maybe’ to this question. He proposed a hypothesis of how the Horse Hill well could be linked to the earthquakes through fluid pressure changes.

According to his hypothesis, gas pressure built up in the well during the time it was shut in between 2016-2018. Gas moved up between the cement and the formation, because of a less than perfect cement bond. In late March 2018, as part of preparations to work on the well, the pressure was released at the surface. The sudden reduction in pressure below ground caused the fault to slip, triggering the earthquakes.

More research

The Oil & Gas Authority report called for a UK-specific scheme for assessing the likelihood of induced seismicity. Professor Haszeline said he was working with colleagues at Edinburgh on an academic paper on this subject, based on the Newdigate earthquakes. He also said:

“There needs to be a record of activity on oil and gas sites. We need evidence of all the activity on the surface which should be verified and archived at the time so that it can be gone back to in the future.”

The report also said

“There was a general desire for additional seismometers to be added to the BGS national network but it was agreed that the current government funding for micro-seismic baseline monitoring is appropriately limited to areas where hydraulic fracturing is proposed.”

Professor Haszeldine said additional seismometers could be installed at oil and gas sites for about £10,000.

Reaction

James Knapp of the Weald Action Group said;

“The Newdigate Swarm of earthquakes have been located to a small fault running NE from the cluster to Horse Hill exploratory oil well, just 3 kms away, which is bored straight through it. “It speaks volumes that the only way to reach the finding that the earthquake swarm is natural, is to disregard the existence of the Horse Hill well site altogether, and this is exactly what has been done for this report. “A fully independent investigation by Edinburgh University came to the opposite conclusion, that the earthquakes were induced by exploration activity, and they have produced a hypothesis which shows how Horse Hill well could have triggered the earthquakes. “The OGA report obscures the danger of the planned expansion at Horse Hill, from a single exploration well to seven production and injection wells. It also highlights the contradiction at the heart of oil and gas regulation: How can the Oil and Gas Authority, whose job it is to maximise oil and gas production, simultaneously offer the required checks and balances of a regulator? This flawed regulatory structure undermines public trust in the Government’s energy policy at a time when the IPCC tell us we have just 12 years to stop run away climate change. “Just like the troubled fracking at Preston New Road and the abandoned Preese Hall well, these tremors are happening because we don’t have a legally imposed respect distance for drilling near faults. This is most relevant to the deviated and horizontal wells needed for tight oil and gas production, to mitigate risk both of inducing earthquakes and of water pollution. “This blinkered report manages to avoid conflict with the government’s policy for a new era of damaging onshore fossil fuel extraction, and can best be described as a whitewash.”

Keith Taylor, MEP for South East England said:

“The OGA’s findings do not represent a scientific consensus. Some independent experts have assessed the same available data and reached a very different conclusion; finding that human activity was likely to be linked to the unprecedented seismic activity in Surrey. The OGA exists to promote and facilitate oil and gas drilling; their conclusion, and certainty with which they present it, may be disappointing but it is hardly surprising. “Oil and gas firms are desperate to continue their climate-destructive operations unimpeded by concerns over their seismic impact. But, however much the oil and gas industry may wish to sweep them away, questions about the links between the Surrey earthquake swarm and oil and gas drilling in the region still loom large. “While those questions remain unanswered, I will continue to support experts’ calls for a moratorium on all oil and gas drilling in the area. “The case for employing the precautionary principle is obvious. That we have witnessed unprecedented seismic activity in an area where unconventional fossil fuel drilling sites are active is, clearly, extremely concerning. Until the experts are able to form a consensus on the possible cause, a moratorium remains the most sensible course of action for all involved.”