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Much has been written about the Newdigate swarm in recent weeks and months, but we feel that whilst it cannot be proven that there is defined  causal link between the earthquakes and oil exploration activities, this link cannot be disproven either.  And the fact that there was no observed seismic activity before drilling for hydrocarbons began at Horse Hill is, in itself, probably the strongest indicator that there is indeed a causal link.  This blog sets out the evidence.  We invite you to come to your own conclusions.

But note that many of the arguments put forward in support of the claim that the cause of the swarm is entirely natural are potentially flawed.  It is plain for all to see that a conclusion that the cause of the swarm may not be entirely natural, could add support to claims for a moratorium on all hydrocarbon exploration and extraction activity in the area.  And this could seriously harm the financial prospects of several oil and gas companies.  Then it is not surprising that the general consensus of the geological science community, most of whom derive financial support for their research from the industry (see Channel 4 report on this issue), is in support of the claim that the cause is entirely natural.  The questions are: are they truly independent and can you believe them?


As of 4th May 2019, 33 earthquakes have been registered in Newdigate, and they are continuing. The swarm started on 1st of April 2018. When a 3.0 tremor hit a few months later, in early July 2018, the level of public interest and suspicions over links with the oil & gas activities at nearby Horse Hill and Brockham sites increased dramatically. After our initial skepticism, the brief blog we published on 6th July 2018 on this topic remains our most read post ever. The strongest tremor measuring 3.1 ML was recorded on 27th February 2019. The earthquakes are causing damage to property and there is concern over the possibility of even stronger tremors.

The Times Open Letter & OGA Workshop

Following the summer 2018 sequence the Times published an open letter from four leading academic geologists on both sides of the “fracking” debate, including Professor Richard Selley – a Brockham resident. The letter called for a moratorium on all drilling, re-injection and flow-testing in the affected area.

This demand was ignored, but the Oil & Gas Authority (OGA) held a workshop on 3rd October 2018, including the letter signatories, other academics, regulators, and industry people, and chaired by the British Geological Survey (BGS). Participants concluded that, based on the evidence presented, there was no causal link between the seismic events and oil and gas activity. One person disagreed.

BGS / OGA Workshop Conclusions

The BGS used the Davis and Frohlich (1993) test to determine whether the tremors were induced. They concluded:

The case that the events were induced is primarily supported by the temporal correlation between the restarting of production at Brockham and the start of the earthquake sequence on 1 April. However, the very limited period and volume of production seems unlikely to cause a significant stress perturbation at a distance of 8 km. Oil production has been ongoing for many years in the region, without any previous seismicity. Finally, although this is a low seismicity region, even by UK standards, earthquakes are not unprecedented here.

Regarding Horse Hill, the BGS said:

Information provided to OGA by the UK Oil and Gas PLC [UKOG] and the Horse Hill operator, Horse Hill Developments Ltd [HHDL], states that pumping to increase wellbore pressure to try to get the well to flow was carried out in HH-1 on 9 July 2018. The earthquake sequence was already underway at this time. UK Oil and Gas PLC states that no fluids have been injected into the HH-1 well in 2018. This appears to rule out operations at HH-1 as a possible causative factor for the earthquake sequence.

The BGS only applied the test to fluid injection and hydrocarbon extraction activities. It said:

Strictly speaking, these [Davis and Frohlich criteria] are only applicable for seismicity induced by fluid injection. However, they can also be used to assess the evidence that the Newdigate earthquake sequence may have been triggered by fluid injection or extraction.

The BGS limited its analysis to fluid injections and production activity. They concluded the causes were natural because 1) injection at Brockham was only of low volumes and too far away, and 2) there had been no injection or production at Horse Hill prior to the onset of earthquakes on 1st April 2018.

But could other oil & gas activities be responsible for inducing the earthquakes through a mechanism that’s not yet fully understood? And would this change the timeline to correlate with the original onset of earthquakes on 1st April 2018?

One such mechanism is proposed by Stuart Haszeldine, Professor of Carbon Capture and Storage at the University of Edinburgh, who using the same Davis Frohlich criteria, concluded that the earthquakes were likely induced by oil and gas operations at the Horse Hill site.

Building on the work of David Smythe, Emeritus Professor of Geophysics in the University of Glasgow, who demonstrated that the HH-1 well has been drilled through a fault zone, Prof Haszeldine and his team propose that tremors are induced by a rapid depressurisation of the critically stressed fault zone after a period of gradual pressurization through a poor cement job. The issue is periodically addressed by draining off (bleeding off) the annulus after a long period of shut-in. This alters the distribution of pressure and fluid in a connected fault zone and causes a shallow earthquake.

Freedom of Information Act (FOIA) Requests

We filed a number of Freedom of Information Act requests with the various regulators. The disclosure provided by the Health & Safety Executive (HSE) includes weekly reports for HH-1 well interventions during weeks ending 1st, 8th and 15th July 2018. These timings appear to correlate with the Summer 2018 resurgence of the earthquakes on 27th June. According to the BGS and OGA, extended well test (EWT) didn’t start until 9th July; however, what is meant by this as explained by the BGS is “pumping to increase wellbore pressure to try to get the well to flow”. This leaves out the preparatory well interventions (such as bleeding off the annulus or pressure testing) ahead of pumping, which by the oil company’s own schedule started in late June.[1]

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Part of weekly report provided to HSE for week ending 1st July 2018


Well testing operations at Horse Hill planned for early April 2018

The activities described in the 3 weekly reports in July 2018 correlate very well with the schedule of work submitted to the HSE in early March 2018 as part of the notification for HH-1 Rig-less Intervention and Well Testing Programme included in the HSE disclosure. According to this document, well testing operations were planned to start in early April 2018. Given that 1st April 2018 is the date of the 1st recorded earthquake in the Newdigate swarm, this is significant.

Screen Shot 2019-05-30 at 01.32.28.pngScreen Shot 2019-05-29 at 21.49.04

This intended timing of the flow test is supported by UKOG announcements in late 2017 and early 2018 (UKOG holds a 50.64% interest in the Horse Hill licences, and a 77.9% interest in HHDL – the Horse Hill operator). On 19th Dec 2017, UKOG reported that the start was planned “directly upon the discharge of SCC’s [Surrey County Council’s] pre-commencement planning conditions, expected by the end of winter 2017/18”. A letter from Zetland Group to SCC representing HHDL dated 1st March 2018 stated that phase 1 .[2] of the works permitted under permission RE/16/02556/CON would be implemented from 6th March 2018 (even if not all planning conditions were discharged by that date), and on 23rd March 2018, the firm announced that all conditions were discharged and that it was now “awaiting approval from the Oil and Gas Authority.” It appears the company was ready and eager to start the EWT without delay.

However, from another FOIA request, we find out that on 29th March 2018, Tom Wheeler, the OGA’s Director of Regulation wrote to Stephen Sanderson (Chairman of both UKOG and HHDL) requesting a number of clarifications on HHDL’s operational, governance and financial arrangements. This could arguably have been the reason why the OGA’s consents for the EWT were not issued until 14th June 2018 (link here and here), despite the application submission date of 19th March 2018.

HHDL say that they didn’t do any work on well HH-1 until early July 2018. We’ve asked for HSE weekly reports (which is how well activity is monitored) for the period from 1st March to 15th April 2018, but no documents were provided for this period. It is important to note that these weekly reports are self-reported by the operator. There were no visits to the Horse Hill site by any of the regulators (OGA, HSE, SCC or the Environment Agency) in the period from 1st March to 15th April 2018 and, based on responses to our FOI requests, none of the regulators has records documenting the status of well HH-1 in the period around early April 2018.

The only operational information available is the already mentioned daily activity schedule provided by the operator – hardly an impartial party. This schedule was supplied ahead of the OGA workshop on 3rd October 2018 (long after the Newdigate earthquakes started and raised questions about links with oil exploration) and it was already shown as incomplete, omitting surface work taking place in March 2018.[1]

We see a serious issue in the lack of transparency and the fact that this inherently dangerous industry is allowed to self-monitor and mark its own homework, while the regulators rely on what operators tell them.

Until more documents are released, questions will remain. However, in addition to late June 2018 apparent correlation between tremors and activity at Horse Hill, there have been at least three other recent events, which appear to support the hypothesis of Stuart Haszeldine that every time a zone is re entered after a period of shut in, the annulus is bled off and this operation causes small seismic events:

  1. On 18th February 2019 UKOG reported that it re-established production from the Portland reservoir after a 6-months shut-in period (according to Haszeldine’s hypothesis, this would have been preceded by bleeding off the well annulus). Four days earlier, on 14th February 2019, the earthquakes resumed after several months of no activity.
  2. On 11th April 2019 UKOG reported pressure testing of the HH-1 well, which the announcement said was shut in for a 60-hr pressure build up. An earthquake was recorded on the same day, and another one 4 days later.
  3. On 17th May UKOG reported that it resumed production from the Portland following a week-long pressure build-up test. A tremor of 0.6ML magnitude was recorded on the same day.

Stuart Haszeldine and his team, Crispin Blunt MP, Keith Taylor MEP, the Weald Action Group and other groups called for another detailed independent examination of the evidence. We fully support this request. In order for this to happen, all wellbore activity data should be disclosed for the period since 1st March 2018 to present, and it should be made openly available to anyone. We also support a call for a moratorium on activities at the Horse Hill site until results of such investigation are known.

Secondly, we are calling for a change in the regulation of pressurised fluid injections, which are known to cause seismicity. At the moment, there is no monitoring or reporting on pressurised injections of acid-based fluids at Horse Hill or Brockham, although such injections are permitted at both sites. The traffic light system [regulation on fracking-induced seismicity] doesn’t apply here, meaning there is no protocol for evaluating potential links between injections and seismicity (see here our blog on this topic).


[1] The daily schedule provided by HHDL to the OGA reports the start of pressure testing on 29th of June, but this data comes from the operator and does not appear to have been verified. The date of 9 July 2018 start of EWT was supplied in “discussions” between the OGA and the oil company, and there is no record of this before or on that date (see FOI here). The schedule is also demonstrably incomplete, having for example entirely skipped the preparatory surface work taking place in March and April 2018 (including the excavation of another well cellar starting on 21 March 2018 using a JCB) as reported in the summary of the OGA workshop held on 3rd October 2018 (see Note 1, p.3).

[2] See summary of works planned under SCC Ref 2016/0189 / RE16/02556/CON here.


Residents’ petitions calling for independent inquiry:

Further reading:

British Geological Survey: Earthquakes near Newdigate, Surrey




(First published 6 May 2019).