All information on activity at a Surrey oil well should be published following another local earth tremor, residents and a community group have said.
The British Geological Survey (BGS) received more than 100 reports of a tremor early on Saturday morning (4 May 2019), centred near the village of Newdigate, about four miles from Gatwick airport.
The tremor measured 2.5 on the local magnitude scale and was one of the larger events in a swarm affecting the area in the past 13 months. Based on local reports, the BGS has updated the intensity of the latest tremor from 3 to 4.
People in the area said they were woken by “a loud bang” and shaking buildings. Others likened it to “some sort of explosion”.
Two investigations have said there is no evidence of a causal link between the earth tremors and oil and gas operations at Horse Hill, near Horley, or Brockham, near Dorking. The operators of the sites have strongly and repeatedly said their work did not cause any seismic activity.
The Brockham site has been discounted because it was considered too far away from the epicentre of the tremors. On the other site, the BGS told DrillOrDrop today:
“We conclude that the available evidence suggests that the events were not related to activities at Horse Hill.”
The group said it supported calls – made by residents, the local MP Crispin Blunt, MEP Keith Taylor and the Edinburgh University geologist Stuart Haszeldine – for another detailed independent examination.
Brockham Oil Watch said:
“In order for this to happen, all wellbore activity data [for the Horse Hill well] 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.”
To date, the BGS has recorded 34 tremors in the Newdigate area in the past 13 months on its online database.
Long-term Newdigate residents have said there had previously been no felt earth tremors in the area for at least 45 years. Analysis of the BGS database by DrillOrDrop shows there have been no earth tremors in this part of Surrey for at least 50 years. The recent tremors have all been relatively shallow compared with most other seismic activity in the UK.
The Newdigate swarm began on 1 April 2018 and continued until October 2018. There was then a gap until 14 February 2019. That month, the BGS recorded a total of eight tremors in a fortnight, the strongest of which was 3.1ML. Six tremors were recorded in March, three in April and two so far in May.
Many of the tremors have been tiny and were not be felt. But at least 11 were felt and assigned an intensity rating based on descriptions given by local people.
For the strongest tremor, the BGS recorded nearly 400 observations at intensity level 6, where effects included slight damage to buildings, such as fine cracks in plaster.
DrillOrDrop has reported on the two previous investigations into the cause of the earthquakes. One was based on a workshop led by the BGS and Oil & Gas Authority (OGA); the other was a peer-reviewed paper co-authored by Dr Brian Baptie, of the BGS, Dr James Verdon, of Bristol University and Professor Julian Bommer, of Imperial College.
Both investigations concluded there was no causal link, based on the evidence available, between the seismic events and the Horse Hill site. One reason given for its exclusion was no fluid injection or flow testing work being carried out before the first earth tremor on 1 April 2018.
The investigations referred to documentary evidence for Horse Hill, which showed that preparations for flow testing did not begin at the site until late June 2018, after the swarm started.
But residents and campaigners have argued that what happened at the site in the months before the flow test was more significant.
As part of the flow test preparations, the operator, Horse Hill Developments Ltd (HHD), said it planned to carry out checks on the annular pressure. This assessed the pressure exerted by fluid or hydrocarbons between two sets of casings or tubing in the well.
A document submitted by HHDL to the Health and Safety Executive (HSE) also referred to an operation to “bleed off pressure and top up the annuli as required”.
The BGS/OGA workshop noted there had been an annular pressure test on 5-6 April 2018, after the start of the swarm. The test was said to be satisfactory and no pressure was detected in the annuli (the space between casing and tubing or casing and the wellbore).
James Knapp, of the campaign network, Weald Action Group, said the well pressure would have been expected to be zero only after the pressure test and bleed-off described in the HSE document.
‘We would expect there to have been pressure present in March 2018 because there had been continuous build up and venting of gas pressure during the earlier test in 2016, before the well was shut in.”
There was also work going on at Horse Hill before the annular pressure test referred to by the BGS/OGA workshop.
A partner at Horse Hill, Doriemus, reported that in the three months up to 31 March 2018 there had been a final technical review and:
“ongoing operational planning, preparation and site works in preparation for the 150 day extended flow testing programme”.
This has prompted questions about whether operations other than injection and flow testing could have induced the earth tremors. If so, could these other operations be linked to the start of the swarm in April 2018?
Brockham Oil Watch said:
“We think it is wrong that the BGS and the OGA limited their analysis to injection or pumping activities only. This leaves out other wellbore interventions, which might have the potential to cause seismicity.”
Another team of academics has suggested that the tremors could have been induced by rapid depressurisation of a critically-stressed fault zone below the Horse Hill site after a period of gradual pressurisation.
Professor Stuart Haszeldine, heading a team at Edinburgh University, hypothesized that annular pressure in the Horse Hill well was vented after the long period of shut-in. This changed the pressure and fluid in the fault zone, inducing shallow earth tremors, the team said. Deep gas later recharged the annulus and fault zone and the cycle repeated, it suggested.
The hypothesis was that every time a zone of the well was re-entered after a period of shut-in, the annulus was bled-off and seismic events were induced.
James Knapp said:
“The work scheduled to be done prior to operations commencing is consistent with venting that Edinburgh University said could be the trigger for the swarm. The fact that there was no pressure detected on 5-6 April 2018 also supports this, as the gas pressure would have been bled off before the start of the swarm on April 1st.
“We know workers were on site in March 2018 preparing for the arrival of the well testing package and so all the dates fit.
“There is no proof that the agreed work schedule was followed, because the work logs haven’t been released.”
Brockham Oil Watch asked the HSE for weekly reports for the period from 1 March to 15 April 2018 to find out what activity took place at Horse Hill. It wanted to know whether any gas or fluids had been bled off from the annulus.
According to the group, the HSE said no reports were provided for this period. There were also no site visits during these weeks by any regulators.
Brockham Oil Watch said:
“The regulators rely on what the operators tell them, and there seems to be little or no verification of the information provided.
“In a system where operators largely check their own homework, it seems quite possible that the Horse Hill operator opened up the well in late March 2018, and in so doing changed the fluid pressure in the well, but that they never reported it to the regulators.”
Newdigate resident, Peter Mitchell, said:
“Since the OGA workshop there have been many further earthquakes. Yet there has been no initiative on the part of the OGA to investigate these or to call for a moratorium while possible causal links are re-evaluated.
“This is particularly relevant given the current application to significantly expand the Horse Hill site to include more production wells and water re-injection.
“The OGA has a responsibility of duty of care towards the people in the region of Newdigate and our properties.”
Brockham Oil Watch has also pointed to two coincidences this year.
The tremors resumed on 14 February 2019 (2.4ML and 0.2ML) after a break since 19 October 2018. Four days later, on 18 February 2019, UKOG issued a statement that production had been re-established from the Portland reservoir after a six-month shut-in.
On 11 April 2019, UKOG reported that the Horse Hill well was currently shut in for a 60-hour pressure build up test. A 0.7ML tremor was recorded that day and another (-0.3ML) four days later.
Brockham Oil Watch called for a change in the regulation of pressurised fluid injections. It said:
“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, which regulates seismicity induced by fracking, applies only to shale gas sites. It does not cover oil and gas sites in southern England. There is no protocol for evaluating potential links between injections and seismicity, the group said.
Seismologist, Dr Stephen Hicks, who has been monitoring the Surrey earth tremors said today:
“Analysis of the available seismic data is ongoing, and at this stage, we are still keeping an open scientific mind.
“With the local seismic network in place and with our advanced seismological analysis techniques, we are now able to detect many more micro-seismic events than standardised operational procedures. This is to be expected as the BGS’s seismic monitoring network of five stations now provides a very low detection threshold, and we expect to detect tens of aftershocks following each magnitude 2 to 3 sized event.
“Using the waveform “fingerprint” of each seismic event, we are able to efficiently scan the database for similar events. Detecting more seismic events provides us with better information on the evolution of the swarm in space and time, and to help interpret its cause.
“It appears that most events occur in a single cluster, and there are no obvious changes in the character of the earthquakes with time. Without the local monitoring network in place, we would only have been able to consistently detect events with a magnitude of greater than 2.
“We continue to work closely with the BGS, academic colleagues, and we are also actively engaging with the operators and policymakers.”
DrillOrDrop invited UK Oil & Gas, the parent company of Horse Hill Developments Ltd, to comment. This post will be updated with any comment.