Weekend Long Read: Campaigners call for tougher rules on acid use at Weald oil sites

Weald map

Map: Woudloper [GFDL ( or CC BY-SA 3.0 (, from Wikimedia Commons

Campaigners in southern England have criticised what they say is a “gaping hole” in the regulations covering the use of acid on oil sites.

They describe the current rules as “vague, flawed and open to abuse”, risking induced seismicity and potential groundwater pollution. Oil companies in the Weald basin should be required to provide detailed plans and reports of their operations, they say.

Brockham Oil Watch, a residents’ group which monitors the industry in the region, said:

“This is another huge loophole in the “gold standard” regulatory system that needs to be urgently closed.”

Acid is used in the industry in a variety of techniques to improve the flow of oil. The techniques range from cleaning the well and voids in the rock, to stimulating oil flow by opening or creating fractures.

Brockham Oil Watch’s concern centres on techniques that pump acid into a well at pressures below that needed to fracture rocks – the level known as the fracture gradient or fracture pressure.

A series of requests for information by the group has confirmed that where the pressure is below the fracture gradient, operators do not have to comply with key regulations required for fracking.

The requests have also revealed that where operators say their purpose is solely to clean the wellbore, there is no requirement to send information to the regulators on the volume of acid or the pressure used at particular times and locations.

But without this information, the campaigners argue, regulators cannot independently confirm what techniques have been used.

Ada Zaffina, a member of Brockham Oil Watch, said:

“The regulators are not able to verify when fracture pressure has been met unless they are provided with adequate reporting, but reporting is not required so long as the operator says it is not intending to apply fracture pressure.”


Photo used with the owner’s consent

Acid and the Kimmeridge formation

The use of acid at oil sites had previously gone largely unnoticed by the public. But it is attracting increasing attention in the Weald now that oil companies are exploring in rocks in the Kimmeridge formation. Angus Energy is preparing to appraise a well drilled in the Kimmeridge at Brockham, while Horse Hill Developments Ltd (HHDL) has a flow test underway at its site near Horley.

The Kimmeridge rocks are dominated by shales and are known as “tight” or low permeability. One method for creating or enlarging fractures in the shale to improve the flow of oil is initially to dissolve the rocks with acid.

Ms Zaffina said:

“Acid stimulation can be done at below fracture pressure, and so entirely escape the regulations and reporting currently required for fracking.

“This also allows the drillers to call their operations ‘conventional’ and to avoid the local communities’ overwhelming disapproval of fracking or fracking-like operations.”

Volumes, pressure and timing

At Horse Hill, the oil site near Horley, HHDL said it intended to carry out an acid wash on its exploration well. This technique is used to remove debris from the wellbore or material clogging the natural voids in the rock formation.

Environment Agency logoThe Environment Agency (EA) told Brockham Oil Watch that an acid wash was not considered to be stimulation and was not covered by requirements under an environmental permit. The EA said:

“As a result, the environmental permit for Horse Hill does not require detailed information on the amount, pressure and volumes of acid used in specific formations at specific times. We do not require the operator to report the information.”

The EA confirmed that for Horse Hill it did not hold information on:

  • The length of the interval of the rock formation where acid treatment would be carried out
  • The number of intervals to be treated with acid
  • The total volume of acid and additives to be used in any multi-stage treatment
  • The number of times the treatment would be repeated
  • The timing of acid operations

The EA also confirmed there was no requirement in the environmental permit for regular reports on volumes of fluid and pressures or information on when a treatment was carried out.

The regulator said the intended acid wash was understood to be “occasional” and so it had not assessed the total cumulative volume of acid that could be used in repeated or multi-stage treatments.

Asked how the use of low pressure in an acid wash was enforced, the EA said:

“There is also no numerical limit or calculation that we use to determine those activities that do not need a permit. It is a determination based on ‘intent’ as well as volumes to be used.”

Asked how the distance that acid penetrated into the rock formation was verified, the EA said:

“We assess the activity based on the intended result the activity is required to produce.

“If it is to remove a build-up of precipitate or debris in the near-wellbore area then it may be acceptable to regard this as an acid wash. If the intention is, however, to enhance the natural porosity in the wider formation then this would clearly be stimulation.

“In the simplest terms an acid wash would be considered to be near-wellbore maintenance activity that would clearly carry a low environmental risk.”

The EA added that its staff checked, during site inspections, how often acid was used. They would confirm that equipment and chemicals were in line with what would be expected for acid wash activities. Acid wash sites would have relatively small volumes of acid and different equipment compared with sites which were using acid at higher pressures, the EA said.

Waste reinjection

Brockham Oil Watch asked the EA about what should happen under the rules to wastewater resulting from acid treatments. The EA replied:

“The fluids returned to surface from an acid wash type activity would be considered to be water containing substances resulting from the exploration and extraction of hydrocarbons so could legally be combined with produced water and injected in a permitted reinjection well.”

Asked whether this also applied to matrix acidising, which applies more pressure than an acid wash, the EA said: “the waste code would be the same”.



The Health and Safety Executive (HSE) told Brockham Oil Watch it should be notified by a site operator about:

“any well operation that could result in an accidental release of fluids from the well or reservoir to which it is attached. This would include well stimulation operations.”

But the list of information required for the notification does not include volumes of acid or other chemicals or the pressures used.

The notification of the well testing programme at Horse Hill, which included a contingency for acid treatment, did not contain information on volumes, pressures or other numeric limits on stimulation activities.

The HSE added that for the Horse Hill site:

“no notification has been sent to us regarding any acid treatments or any other activities at this well.”

Earth tremors

The Oil & Gas Authority (OGA), which regulates seismic activity induced by well operations, said:

“Acid washes and matrix acidisation and other completion treatments using acid that are not engineered to be above the fracture pressure would not routinely need to be described in the application for approval to complete the well unless we considered them to be of particular significance for the well in question.”


Part of OGA infographic on Traffic Light System

The OGA said operations using the injection of significant volumes of fluid above fracture pressure would be considered to be hydraulic fracturing. They would require a hydraulic fracture plan as part of the application for approval to complete the well.

Under the hydraulic fracture plan, the company would be required to report the injected volumes and pressures. It would also be expected to include a real-time traffic light scheme for monitoring local seismicity.

Operators of oil sites in the Weald say they are not using hydraulic fracturing. This means they are not subject to hydraulic fracture plans or the traffic light system. There is no protocol for monitoring seismicity.

The OGA and the HSE have both confirmed that they do not hold information on the total number of well stimulations onshore.

The OGA said:

“Until the recent increased public interest in hydraulic fracturing, well stimulation was regarded as a routine aspect of well operations, and was not subject to specific consent, and its use may not have been systematically recorded.”

The HSE said:

“We may hold some of this data, but as HSE does not collate this information, it would be manifestly unreasonable to check each individual onshore well file to see if we have any information within scope or not.”

“Tougher regulation needed”

Brockham Oil Watch has said the UK regulators should follow the example of California and treat all stimulation methods equally, regardless of how much pressure was used. The group also said no flowback from any form of stimulation should be reinjected.

Ms Zaffina said:

“Operators should be required to produce well stimulation plans, as opposed to hydraulic fracturing plans that we have currently. They should provide detailed reporting of operations that were actually carried out.”

She said the reports should include the total volume of water, acids and other chemicals.

“This is the only way to evaluate fully the impact, particularly on horizontal wells that will require multiple treatment stages. Currently, only vague proposals based purely on the operator’s intent are required.”

She described the exclusion of chemicals from controls under the environmental permit as “a paper-based exercise and open to abuse”.

“There is no monitoring of what actually takes place other than site visits and on-site document check.”

On induced earth tremors, she said:

 “The traffic light system for monitoring of seismicity should be extended to all pressurised injections and reinjections, because it’s been proven that earthquakes can be caused by increased pore fluid pressure.

“We know that the Weald is a seismically active area. Yet, currently there is no requirement for even the basic reporting of when injections at oil sites in the region are taking place, making any analysis of causal links very difficult.”

On waste from acid treatments, she said:

“We believe that waste water from acid stimulation should be treated in the same way as flowback from hydraulic fracturing, which is not allowed to be reinjected, and needs to be handled by a specialist treatment facility.

“But currently, waste produced from acid wash or matrix acidising can legally be combined with produced water and reinjected. We’re very worried about this, given that Brockham has a permitted reinjection well.”

  • Brockham Oil Watch is a residents’ group which is concerned about the use of acid in a three-year appraisal due to start soon at the Brockham oil site in Surrey. The group is part of the wider campaign network, Weald Action Group

Links to Freedom Information Act requests

Health and Safety Executive response on notifications

Oil & Gas Authority notifications on notification about acid wash and matrix acidisation

Oil & Gas Authority notifications on Horse Hill

Oil & Gas Authority on number of hydraulically fractured wells

Environment Agency notification about injection operations

Further information was provided to Brockham Oil Watch in follow-up correspondence.

25 replies »

    • Paul

      Ada is one of the more polite users of the FOI process in relation to oil and gas questions. Some hector the departments they write to, or just cannot be bothered to find out what is already in the public domain.

      My favourite type though is when the requestor creates a fiction and then asks why the regulator or public body has not dealt with it. The responder struggles to politely provide some form of answer.

      Maybe there should be a charge for asking. Per year, One free, £30 for the next two and £100 from then on. After all, it is taxpayers money that is being spent.

      • hewes62 – excellent idea. Perhaps a 38deg petition is called for to start charging for FOIs as you have noted. Just joking as those that sign 38deg petitions are probably the same FOI abusers.

        Another DOD fake news article / irrelevant etc. Perhaps there is not much going on although I expect I-Gas will be spudding soon?

  1. Let’s hope that Brockham oil watch campaign for the same conditions and regulations to be applied to the water industry when treating drinking water wells.

    • John – And the food industry and many other industries. The photo shows it is only 15% HCl – these people need to stop worrying about very little. Clearly not enough to do down there….

  2. Right antis and Corbyn ppl and white poppy wearers get yourselves a copy of ‘Chemistry for Dummies’ this Christmas!

  3. So, with so little information in the public domain, how did Stephen Hicks of Southampton University, come to the conclusions that;
    “Based on our knowledge of activities at the nearby drilling sites of Horse Hill and Brockham, as well as our knowledge of human-induced seismicity around the globe from the scientific literature, there is no clear reason why these earthquakes should have been directly caused industry. Injection volumes and pressures appear to be too low, whilst the distances and timescales involved do not indicate a clear, physical link. Whilst earthquakes have been shown to be induced over long distance and timescales due to fluid injection in Oklahoma, the injection rates and volumes are much higher than what we know to have occurred in the Weald Basin.
    Overall, there is no substantial evidence at present which indicates that these earthquakes have been directly induced by nearby industrial activities.”
    In his report on into the 14 earthquakes(?) in the ara of the Horse Hill drilling site and between Brockham and Balcombe:
    Where they re-injecting waste water at the time of these seismic events occurring?
    Also, the link to the UK Onshore Geophysical Library did not work for me, can anyone else actually access the site?

    • Patrick
      Stephen Hicks explains his conclusion in the extract you have pasted.
      Low volumes of re injection fluid it seems are a key issue.
      Large volumes would no doubt require lots and lots of tankers to roll up to the injection site.

  4. Yet another case of encountering scary sounding science and not understanding one jot of it and banking on the general public not to know enough about it so that it scares them as well. 15% Hydrochloric acid is used to clear carbonate minerals in pore spaces in contact with the wellbore to allow hydrocarbons to reach the well. Its only marginally stronger than Acetic acid in table vinegar and marginally stronger than the hydrochloric acid in Harpic toilet cleaner.

    Once the acid is in contact with the carbonates, it is chemically neutralised and turned into water, calcium chloride and CO2. It is pumped as a discrete package downhole, left to react for however long is necessary (minutes usually) in the chosen part of the well, and then pumped out of the well for disposal.

    The acid is ‘spent’, its neutralised to harmless salts, the well is cleaned. They dont use any more than they need because theres point paying for something you dont use as there are only a limited amount of carbonates in the wall and fractures of the well. Anything that comes out that is still acidic is mixed with a sack of lime and neutralised in a skip or tank.

    This is one of the most pointless of all the anti’s scare stories and really betrays their fear of, and absolute ignorance of basic school science.

  5. Weekend long read? Took me about 4 mins.

    Fake headline.

    Total non-story.

    Desperate times for the ever-shrinking anti crusade.

    RIP DoD.

  6. Well Patrick, as HH was not actually doing anything at all at the time of the tremor cluster, and only restarted testing some time after, your comments demonstrate perfectly how dots that are random are tried to be pulled together for reasons that have absolutely nothing to do with science.

    How about joining up theses dots?

    Since HH has restarted testing and has removed around 50 tankers of oil there have been no more tremors! Obviously that proves the tremors were caused by the forces of the oil presence and once it starts to be removed-problem solved. Pure speculation, just as the antis utilise, but equally “plausible” in the Alice in Wonderland world where some prepare their cups of tea with the help of acid treatment of water extraction, but try and prevent it applying for their supplies of diesel.

  7. Good to see sensible comments being posted here by people who actually know what is happening. Using food additive E 507 in varying concentrations is soooooo well established its difficult to understand any complaints being made. But there they go!
    ‘Protesters have concerns’. Well I and anyone else who knows anything about drilling dont.

    Pity that science says there is absolutely no cause for concern. The industry is massively regulated anyway, and the EA will ban any activity that risks pollution, as they did at Markwells Wood.

  8. There is much more in the acidising fluid than 15% HCl or acetic acid. This study for example identified 200 specific chemicals used in acidisation, including multiple hazardous chemicals. They are known carcinogens, mutagens, reproductive toxins, developmental toxins, endocrine disruptors, or high acute toxicity chemicals. If all of this is used to make wells productive in the US, what makes you think that it would be possible to exclude all these chemicals in UK? How about this one for example: 1H-imidazole-1-ethanol,4,5- dihydro-2-nortall-oil alkyl (CAS 61791-39-7) – sounds good doesn’t it? There is too little information to establish it is non hazardous, and so the EA assume it is hazardous. This is what is being pumped into the Horse Hill well as part of PROTEKT 14-10. In addition, the claim that all acid is neutralised is highly questionable too – the study referenced above says that the waste returns can be highly acidic, in the range of pH 0–3. What studies tests have been done in UK? Would you like to be a guinea pig in this experiment? Here is a link to study abstract:

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