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Surrey earthquakes: full investigation needed

Following the 3.1 magnitude earthquake on Thursday today, the seventh and biggest of a series in recent months, many people have contacted us asking us whether the oil drilling activities at Horse Hill or Brockham could be a cause or contributing factor.

It is impossible to be certain about this. Earthquakes of this size in Surrey are a new phenomenon, according to the British Geological Survey (BGS), so there aren’t monitoring stations nearby. The closest is more than 50 km away. Consequently BGS is not able to locate the epicentres precisely, however they calculate it is approximately 4.5km from the very deep Horse Hill well. We were pleased to read that temporary monitors are to be installed next week, which should allow the experts to get more accurate locations should there be any more earthquakes.

We believe this needs a proper investigation and are calling for all drilling and flow testing to be halted now to allow proper monitoring to take place. (Of course there are a lot of other good reasons why we think it should stop quite apart from earthquakes!)

Thank you to Brockham Oil Watch for the following article:

There had been no earthquakes in Surrey in the last 50 years, but since April 1 2018, a series of seven tremors (including three of magnitude well above 2 ML and one of magnitude 3.1 ML) has surprised and worried residents around Newdigate and wider area between Dorking and Crawley. This is at a time of increased oil and gas activity in the county, and many people are making a connection between the two.

We think that serious questions should be asked and investigated, especially in connection with the two oil and gas sites nearest to the epicentres: Brockham and Horse Hill.

In a statement issued following the earthquakes, the British Geological Survey (BGS) say that they are unable to categorically say if these earthquakes are related to hydrocarbon operations though they do not rule out that possibility.

The statement also says that, “it is well known that hydrocarbon exploration and production can result in man-made or “induced” earthquakes” and that “such events usually result from either long term hydrocarbon extraction, or the injection of fluids (e.g. hydraulic fracturing) during production.”  The announcement mentions that it seems unlikely that any flow testing at the Horse Hill site would result in induced seismicity.


We have written to the BGS to point out that what the statement fails to mention is that there is a re-injection well at Brockham, where produced water had been re-injected for years from operations at the Brockham and Lidsey sites. Both sites were closed for most of 2016 and 2017, but resumed production in March and February 2018 respectively, and water reinjection also restarted at Brockham in March according to the Oil and Gas Authority (OGA) website.

Based on analysis and studies done in US, the strongest relationship to seismicity has been found where wells re-injected waste water underground for permanent disposal. Over time pressure can start to build up on geologic faults causing them to slip. Earthquake risk can spread miles away from the disposal wells and persist for more than a decade after re-injection stops. This presents a challenge in analysing a possible link, exacerbated by a lack of reliable earthquake data due to the fact that BGS’s closest monitoring station is located more than 50km away from the estimated epicentres.

Horse Hill

With respect to the Horse Hill site, UK Oil & Gas (UKOG) announced on 27th June that they commenced “production flow test operations.” However, since then Mr. Sanderson, UKOG’s executive chairman said that “there’s been no sub-surface activity since March 2016” on the site. The OGA have confirmed this, but have not provided any clear evidence to support it.

Given that according to expert analysis, the Horse Hill-1 well was drilled into a fault zone, and that UKOG’s Environment Agency permit allows for injection of acid and chemicals under pressure high enough to squeeze it into the pores of the rock, we think that the activities at Horse Hill should be investigated and closely monitored.

Unfortunately, the type of flow test UKOG are intending to perform will not be caught by the OGA’s requirement to monitor seismicity in real time through a “traffic light protocol”, which is only required for sites where hydraulic fracturing is proposed. Therefore, there is no oversight mechanism from the OGA to monitor induced seismicity at Horse Hill. [1]


The convergence of the Horse Hill flow test, resumed operations at Brockham and the earthquakes is at the very least puzzling.

We think that there are too many unknowns and that the link should be thoroughly investigated before the operations can continue. There is an urgent need for monitoring equipment positioned locally to produce better data on earthquake epicentres and depths. Detailed injection information is needed from the regulators or industry to allow for analysis.

Well integrity should be tested as well to check if the earthquakes didn’t cause damage that could lead to environmental pollution.

This is critically important at a time when Surrey is facing a proliferation of applications for hydrocarbon exploration and production, including in some of its most precious areas of outstanding natural beauty.

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