The acid debate – what the Environment Agency says about acidising oil and gas wells and how campaigners respond

171223 Broadford Bridge drone 1 BBAG

UKOG’s Broadford Bridge site in West Sussex. Photo: Broadford Bridge Action Group

The use of acid in oil and gas wells is dividing opinion across the country, particularly in southern England and parts of the East Midlands.

Operating companies have argued it is a routine technique designed to clean wellbores. Campaigners have argued that it could be fracking under the radar with similar risks.

The Environment Agency recently published a set of frequently asked questions and answers (FAQs) about the use of hydrochloric and hydrofluoric acids at oil and gas exploration and production sites.

The FAQs give details of how the Environment Agency will interpret techniques including acid wash, matrix acidisation, acid squeeze and fracture acidising.

DrillOrDrop asked groups researching and campaigning about acidisation in southern England to review the FAQs. This post looks at the key points and the groups’ responses.

Some of the comments are from the Weald Action Group, which has produced its own leaflet (link to pdf here). The group described the Environment Agency’s FAQs as “a start in the right direction” but “too vague to give confidence that this operation can be properly regulated”.

Environment Agency use of acid at oil and gas sites

Extract from the Environment Agency’s FAQs


The Environment Agency’s FAQs describe a range of techniques using acids.

Acid wash The Environment Agency defines this as a technique to clean the well following drilling or after use. It is not considered to be a well stimulation method. The FAQs suggest acid wash usually uses hydrochloric acid (HCl) at a concentration of about 7%.

Matrix acidisation According to the FAQs, this technique involves injecting or squeezing dilute acid into the geological formation at a pressure below that needed to fracture rocks. The acid enlarges natural pores, fractures and fissures, increasing the productivity of the well. The Enviornment Agency said it considers this to be a form of stimulation.

Acid fracturing or fracture acidisation This is defined in the FAQs as pumping dilute acid into the formation at pressures that will fracture as well as dissolve rocks. This is also regarded as a form of stimulation.

Acid squeeze The FAQs say this technique dissolves rock and may open new fractures. It is used for areas close to the well to make it more productive.

Weald Action Group said:

“We are pleased to see some distinction between routine maintenance and stimulation.

“Setting the bar at 7% hydrochloric acid for acid wash is again a good start but how will this be regulated?

“The Environment Agency assumes industry usually uses 7% and that acid wash is not considered a stimulation of the formation. Yet at Markwells Wood, a UKOG site in West Sussex, the company consistently claimed it was going to wash with 15% hydrochloric acid.

“How will the Environment Agency be able to distinguish between a matrix acidisation and an acid frack?

“Matrix acidisation and acid fracking carry similar risks as hydraulic fracturing but there are even more unknowns with acidisation because of the lack of historical permitting and monitoring. “Acidisation also uses a higher percentage of chemicals than hydraulic fracturing and there may be a greater cumulative impact because a well can be repeatedly acidized within its lifetime.”


The FAQs say hydrochloric acid is used to dissolve carbonate rocks, such as limestone (calcium carbonate) and dolomite (calcium magnesium carbonate), or calcite cement.

Hydrofluoric acid is less commonly used and is required to dissolve quartz or silica in rocks, such as sandstone or clay.

The FAQs say other chemicals may be added to protect the integrity of the well and refer to:

  • Inhibitors to prevent acid damaging the steel casing
  • Sequestering agents to prevent the formation or gels or precipitation of naturally-occurring iron compounds.

Weald Action Group responded:

“Acidising tight reserves is complex and typically many kinds of chemicals are used along with the acid, not just two”.

The group asked:

“What about cationic surfactants to inhibit sludge, mutual solvents, friction reducers, acid fluid loss additives, diverting agents, clay stabiliser, calcium sulphate inhibitors, scale inhibitor, ph adjusting agent, clean-up additives and biocides?

“In recent applications at Broadford Bridge, Lidsey and Wressle, there was a list of about 20 chemicals, many of them extremely toxic.”


Plan for the Egdon’s Wressle site in north Lincolnshire where the use of hydrofluoric acid was proposed. Source: Egdon Resources


The FAQs explain that hydrochloric acid reacts with calcium carbonate rock to form calcium chloride salt and water plus carbon dioxide gas. Acid that has reacted with rocks is referred to as “spent acid”. The FAQs say:

“It is expected that the amount of acid used will be calculated to balance with the amount of rock that needs to be treated, so that the acid will have been used up in the chemical reaction. If the fluid coming to the surface is still acidic, it is neutralised with soda ash.

“We require details of the treatment to be included in a waste management plan, which is submitted with the environmental permit application documents.”

Weald Action Group said the description of waste products was “inadequate”:

“There is no consideration of the unpredictable nature of by-products. We’d like to see a requirement for on-site testing of waste water and for logging and monitoring of this.”

“We need to demand that the companies are bonded or pay financial guarantees for transport of waste water considered hazardous, just as with fracking.”

Balcombe,West Sussex, UK Anti Fracking protests..16th September

Tanker leaving Cuadrilla’s oil exploration site at Balcombe in September 2013. Photo: David Burr

No Fracking in Balcombe, which campaigned against Cuadrilla’s oil exploration site in the village, said:

“Companies, for example Cuadrilla in Balcombe, call their waste water euphemistically ‘salty water’, yet it is likely to contain heavy metals brought up from underground, radioactive materials, and quantities of salt up to five times the typical level in sea water.

“It is likely to be difficult and expensive to treat, and once treated it is likely to be dumped on land or (diluted until the contaminants fall below legal limits) into the sea. Returning acidising fluids are by no means always neutralised. Workers have to take great care, especially when using the very highly corrosive hydrofluoric acid. American unions are campaigning to have hydrofluoric acid banned on oil and gas sites.”


The FAQs say the Environment Agency would not permit a proposed activity if it posed an unacceptable risk to the environment.

“We will only issue an environmental permit if we are satisfied the proposed activities meet the requirements of all the relevant legislation”.

Weald Action Group responded:

“What is the relevant legislation around acidisation?

“There are still so many unknowns about acidisation. We are not being adequately protected. Loopholes still exist that make life very easy for industry.”

The FAQs say that as part of its assessment of risk, the Environment Agency looks at the acid and other chemicals, the proposed activity, the geology, the well and the treatment of waste. It applies a standard Source-Pathway-Receptor conceptual model to assess risk, considering how the acid could reach the groundwater.

Weald Action Group said:

“There are no peer reviewed studies available regarding the dissolution and mobilization of naturally-occurring heavy metals and other pollutants from the oil-bearing formation, specifically with regard to acidisation.

“There is no analysis of the chemicals being injected and the fate and effect of well stimulation fluids in the subsurface.

“How then can the EA make proper assessments using the Source-Pathway-Receptor model?”

Brockham well Brockham Protection Site

Angus Energy’s Brockham site, winter 2016-2017. Photo: Brockham Oil Watch

Brockham Oil Watch, which has been following Angus Energy’s operation at Brockham in Surrey, pointed to stricter regulation in the US. The environmental conservation committee of Florida’s senate unanimously approved a bill recently to ban fracking of all kinds, including acid fracking and matrix acidising. In 2013, California’s state legislators required permits for all well stimulation treatments, including the use of acid. The regulations also require the notification of neighbours, public disclosure of all chemicals and groundwater and air quality monitoring.

Conventional or unconventional?

The Environment Agency says the FAQs apply only to the use of acidisation at what it calls conventional oil and gas exploration and production sites.

No Fracking in Balcombe said:

“This document discusses forms of stimulation, including acid fracking, which would clearly be used in unconventional situations. The Environment Agency is either naïve or keen to maintain the political fiction that limestone and sandstone are always conventional sources of hydrocarbons.

“Who do they think they are fooling? The use of acid is proposed at a number of sites in southern England that are described as conventional by operators when speaking to the general public, but unconventional when they address current and prospective shareholders.”

Weald Action Group said until target formations in the Weald and areas such as north Lincolnshire were defined as unconventional, regulation would be “inherently flawed”.


Initial flow testing at Horse Hill in Surrey. Photo: Eddie Mitchell

Brockham Oil Watch has questioned Angus Energy’s description of its operations at Brockham as conventional:

“We have asked the Environment Agency to clarify how they define conventional versus unconventional sites, where Brockham would fall, and how the approach is different for unconventional sites.

“Based on the proposed completion programme of work for BRX4Z that Angus presented to, and received approval for from the Oil and Gas Authority, the proposed perforation interval is a c190 m zone that includes multiple layers of micrite and shale.

“Where shale rock is the source of hydrocarbons, the Planning Practice Guidance on mineral extraction defines them as unconventional.

“This matters because unconventional reservoirs will likely require stimulation to enable commercially viable flow.”

De minimis: what is it and why does it matter?

Environment Agency use of acid at oil and gas sites2.jpg

Extract from the Environment Agency FAQ

The FAQs say a permit for groundwater activity is not needed if the volume of substance used is too small to risk deterioration of the groundwater. It calls this activity de minimis.

The FAQs say an operation would be considered de minimis if:

  • The volumes of acid in an acid wash were very small
  • The acid would come into contact with a very small area of rock in the target formation
  • The acid would react to form an inert salt solution, water and carbon dioxide
  • The activity was not intended to inject acid a significant distance into the formation
  • There would be no significant residual acid fluid remaining in the target formation

The FAQs give as an example:

“The groundwater may contain naturally high concentrations of hydrocarbons and salts, making any impact from the discharge on the groundwater insignificant or trivial. The fluid is introduced under a controlled pressure so as not to create new fractures in the rock and so that it only penetrates the minimum required distance into the formation.”

Brockham Oil Watch said:

“We are concerned that much of the document is focused on the issue of how to define and deal with a de minimis activity, for which, in effect, regulation is not required.

The FAQs make no attempt to quantify what a de minimis activity is and so it is open to interpretation and potential avoidance of regulation by the oil companies.

“We have also asked the Environment Agency what volume of acid per unit of formation (based on its specific geological characteristics) will be the threshold between acid wash and acid stimulation for the two wells that Angus Energy say they will acid wash: BR-X2Y and BR-X4.”

Weald Action Group asks:

“We’d like to have further information on how the Environment Agency reaches the conclusion that a procedure is ‘environmentally trivial’. Will the 7% HCl be regulated?”

The group said some companies were claiming that their operations fell under routine well maintenance and were therefore de minimis when, in fact, they may be intending to stimulate the formation.

“Will the EA enforce these rules? And if the industry is non-compliant how long will this take to enforce?’’


No Fracking in Balcombe said:

“The Environment Agency’s grasp of acidisation is weak and waffly.

“The Environment Agency has learnt about acidisation and developed its stance on the hoof over the last couple of years, much, we would contend, in response to opinion, queries and FOIs from the public.

“On the positive side, it’s good to see the Environment Agency admit that an acid wash typically uses no more than a 7% acid solution, and that matrix acidising and an acid frack are forms of stimulation. It is good that the Environment Agency is beginning to see acidising as a process worthy of its attention.

“But this document selects its vocabulary with care, emphasising ‘cleaning out the well’, comparing acidising an oil or gas well to descaling a domestic kettle, comparing it spuriously to the use of acidisation in the water industry. ‘Only a small volume of dilute acid is required’. The Environment Agency’s vocabulary is de minimis.

“The Environment Agency writes of its commitment to protecting public and environment, and of its regulatory powers and potential sanctions. Guardian angels? After six years dealing with the Environment Agency, we see them as fallen, toothless angels, locked in a government and industry embrace, dwindling in number, and reduced to ticking boxes.”

Weald Action Group concluded:

“It is clear that this is a nascent regulatory regime based on very little knowledge about the impacts of acidisation on our environment and human health.

“We would like to see a moratorium on the practice until there is sufficient research on all aspects of acidisation.

“We deserve strong, meaningful, protective legislation that puts the health of our communities and environment first.”

“The concern is that in UK regulation the use of acid still falls under the wrong category of conventional production. We know that the target geology in the Weald is complex and oil does not flow naturally. We know that it is low permeability and large quantities of chemicals and/or acid fracking is are needed to release the oil. Acid maintenance procedures are passed off as routine and standard and bypass important regulatory scrutiny.

“Data gaps exist in all aspects of acidisation, including acid washing. There are significant uncertainties about the kinds of adverse health outcomes that may be associated with chemicals used in acidising and the effectiveness of some of the mitigation measures in reducing or preventing environmental impacts which could adversely affect public health.

“There is no evidence that this process can be conducted without risk to the environment and human health. We would expect the Environment Agency to have revised guidance and very distinct regulation for all aspects of acidisation. What is the guidance on acidising in unconventional formations?”


Environment Agency FAQ

Weald Action Group leaflet

60 replies »

  1. Thanks for putting this information together – for my simple mind the whole thing is absurd and dangerous in so many ways. The contrast with clean, safe wind and solar tech is stark, and I do believe fracking is only an option because greedy, amoral vultures are investing in it.

    • Thank you Ruth, a valuable investigation for us all, i think we will all consider the implications of the subject of The Environment Agency recently published a set of frequently asked questions and answers (FAQs) about the use of hydrochloric and hydrofluoric acids at oil and gas exploration and production sites.

      The Environment Agency’s interpretation of the techniques such as acid wash, matrix acidisation, acid squeeze and fracture acidising acid fracking perhaps indicates that there are an awful lot of questions regarding its safety and safe use, and apparently few, if any reliable answers?

      On the regulation, verification and oversight of the operational problems of handling such scary chemicals as hydroflouric acids, the resultant toxic dissolved residue left in the ground, and where that eventually travels, and the additional resultant waste bought to the surface, the danger of spillage and accidents, not just on the well head site, but also in transit, and the accountability and supervision of use to be carried out by whom? The operators again? Just another rubber stamp job from The Environment Agency without anything to base its approval or disapproval on and no on site presence?

      We seem to be back once more in the now ever so familiar “never never land” of the industry self regulation and apparent unaccountability for fracking, now transferred lock stock and smoking barrel of acid onto chemical extraction techniques? It has taken long enough for the truth about the true dangers of fracking to emerge, now we have to familiarise ourselves with acid fracking and its attendant set of complications, avoidances and prevarications all over again? it makes us wonder what else is in the pipeline to try and catch us off guard?

      It looks to me like this aspect of acid wash, matrix acidisation, acid squeeze and fracture acidising acid fracking will just serve to further erode and invalidate that whole myth of gold standard regulations in the UK.

      Time to call a halt to all this fracking until all of these secretive accountabilities and uncertainties in all aspects of the industry are made public knowledge to everyone by publishing all the data or the lack of it on the national media as a matter of urgency. Lets open up this whole sorry mess to total and upfront public scrutiny. No more secrets.

      Time for another referendum i think.

      • Dear Mike.
        All energy sources have some impact on our environment
        Nickel mining: the hidden environmental cost of electric cars.
        Batteries using nickel and cobalt, like lithium-ion batteries, have the “highest potential for environmental impacts”. It cited negative consequences like mining, global warming, environmental pollution and human health impacts.

    • Please don’t call wind and solar clean. It is not. The chemicals used and mined in the production of solar panels and turbines are as environmentally damaging as any other energy production. Clean energy is a myth.

  2. Interested in what the EA will make of fenced off circular area, bottom left of BB pad. What is that discolouration of the grass? Also nitrogen wasn’t mentioned and plenty of lorries go in loaded. Too much nitrogen not good for local agriculture or people’s health

    • Paula
      That must be a wind up?
      The nitrogen is in liquid form, which is then evaporated and used as a gas to lift the oil ( or completion fluids and hopefully oil ).
      The good news is that this gas is fixed from the air, and returns to it.
      The danger to agriculture and health is from nitrogen in fertiliser.
      Worth looking up in Wikipedia to the the benefits of fertilisers to the many and the downside to the environment.

  3. This is a good start from the Environment Agency. But why are the Q&As provided only for conventional oil and gas exploration? Are the Agency working on a second set of Q&As to cover unconventional oil and gas, and if not, why not? I think that they should now be given every encouragement to put together a second set of guidance on unconventional. And it should also include guidance on their definition of what constitutes the difference between conventional and unconventional, as the definition given by DCLG in paragraph 91 of the National Planning Practice Guidance is plainly over-simplistic and therefore misleading.

    • Ed K
      I am sure that the EA will provide further QA on unconventional oil and gas in due course.
      In my opinion the difference is a construct which can be used by both industry and protest groups to blur the debate. What you do is a matter of fact across a spectrum of activity, and it is those facts that should drive the risk assessment.

      However, the law always requires a line to be drawn somewhere, it has ever been thus, and once drawn it gets managed.

      Lines may be a speed limit ( safe at 29 unsafe at 31 ), the amount of LPG you store re COMH requirements ( 1litre less, or 1 litre over unsafe or safe? ) through to exposure limits.

      Let’s hope that we can rid ourselves of the term unconventional and conventional in the debate.

  4. What a fuss about nothing. Acidising is a bog standard technique that has a huge history with no issues. The acid reaction is very quick and the residues are harmless.
    Yet again we get people who know nothing passing comments on matters they do not understand.

  5. The technique of acidising has been used for over 120 years in both the drilling of water wells for drinking water and by the oil industry.

    The acid/water solution used for acidising by the drinking water industry in the UK’s limestone reservoirs (mostly chalks), is typically stronger than the oil industry use. It is usually 20% to 30% acid.

    • I am sure we would all be happy for the above acid squeezers and operators to demonstrate on camera the action of hydrochloric and hydrofluoric acid on various rocks and breath the resulting gas and touch the residue and then soak it in water and drink it?

      Or perhaps like John Gummer did, to use your children as a guinea pig demonstration?
      Because you will be asking our children to do the same?

      There you go, proof positive or proof negative?

      We will be interested to see that, from a distance of course and with full ambulance team on call, that should take a couple of hours to get there after our friends the Tories have destroyed the NHS?

      I am sure we will all be interested to see the result?

  6. Well, only a few more weeks before huge quantities of nitrogen rich fertilizer will be spread across UK farms in order to maintain yields through the coming year.
    No wonder, when some seem to be against such standard agriculture, the productivity levels of UK agriculture are actually starting to fall back against some other countries nearby. But then, we can just simply import our food and let the NHS decline rather than support UK agriculture to be efficient and thereby put money into the public services.

    • I think I’ll start slow walking in front of tractors in order to protect my water supply:
      Nitrate concentrations in South Downs public supply boreholes are between 4 and 7 mg l-1 NO3- N (Figure 55). It is likely that this derives solely from agriculture since abstraction tends to be concentrated up ground- water gradient of major urban areas. Nitrate concentra- tions are increasing in about 40% of sources and these are predominantly located in areas of more intensive arable agriculture.

      • Heavens you guys must be so grateful for a diversionary subject to prattle on and on and on and on ad infinitum about?

        Not a single mention of the acid fracking issue from either above?

        Perhaps you will be overjoyed to participate in a demonstration of acid fracking residue drinking for us to show to our grandchildren as one of those funny oops accident videos?

        Funny man drinks the results of acid sqeezing? Just laugh as you see the hilarious results!

        We will all scream with laughter almost loud enough to cover the screams of the victims?

        It’s sure to be all happy smiles and not gory or upsetting in the least little bit?

        Must be a prime candidate for game show television?

        No harm at all is it? Just good clean fun and a creepy clowns game of buckets of acid jokes?

  7. Al-may come as a surprise but farmers add nitrogen (usually NPK) AFTER they have checked their land requires it. If that check shows it does not they do not waste money.
    Perhaps you would do better slow walking the ambulances entering the maternity hospitals as increased population not only requires increased food production but also causes severe pressures upon water extraction, and more use of acid within the water industry, especially in the south east.

    • Wow! Mentioned acid once! Out of context of course, but a mild improvement?
      Given another month or so when it’s safe, we might actually get to mention the relationship of hydrofluoric acid to fracking and the fact that hydrofluoric acid is not used by the water extraction companies?
      I suspect that will be much of a step to take this month though? Lots of more useful diversionary evasions to trawl through for ream after ream after ream yet aren’t there?

      I will await that seminal day with baited breath! Not!

      • It is interesting that hydrofluoric acid is used to make fluorides.

        The use of fluoride in water was first used in nazi concentration camps as a soporific and a chemical suppressant to prevent revolution against the genocide.

        Contrary to popular misinformation, fluorides have no value whatsoever to teeth and in fact fluoridation can actually destroy the enamel on teeth.

        So why is fluoride used in drinking water? Because of the same reason the nazis used it in concentration camps, it suppresses anger and revolution and creates a soporific suggestible compliant population that is then ripe for exploitation and control.

        There are thousands of fluoride compounds, including hydrofluoric acid, many of them are so toxic and caustic, that only total chemical hazard suits with breathing equipment can be used when handling them. The sources of fluorides used in drinking water is also worth investigating, as many such compounds are sourced from China with any specific labelling at all, and considering the thousands of combinations of fluorides, you might find it necessary to ask your water company exactly what compound of fluoride is being put in your water, and at what concentration?

        And then of course we get to acid fracking, and the same questions pertain to that too, what exact compound is being used, where is it sourced and what concentrations are used to achieve the result and who monitors, controls and records that according to what regulations?

        This is a wikipedia link to hydrofluoric acid.

        The message of all this, is never take anything for granted, particularly from those who seek to profit from your ignorance of the issue.

  8. Hydrofluoric acid is not used by the water industry simply because it is not suitable to dissolve limestone, It is used by the oil industry to dissolve sandstone.

    Hydrofluoric acid is very hazardous and regulations are in place for its transportation.

    To avoid the transportation and storage of hydrofluoric acid, a hydrofluoric acid generating formulation is generally used by various industries including the oil industry.

    The formulation consists of Hydrochloric acid and Ammonium bifluoride. The two components are transported and stored separately on site.

    Specialist companies are used to mix, react and control the Hydrofluoric acid generating formulation, the resulting reaction produces ammonium chloride and hydrofluoric acid.

    The hydrofluoric acid solution is immediately pumped down the wellbore and into the formation if used by the oil industry.

    The reaction between the Hydrofluoric acid and Sandstone is similar to the Hydrochloric acid and Limestone reaction one, producing Salt, Water and Carbon dioxide.

    • Now this is interesting John, lets see where this goes shall we?

      “Similar” is not the same thing as saying “identical” is it not? The question is John, would you drink water in which such a reaction had taken place? And would anyone care to do that on camera amongst independent observers and chemical monitoring? Because that is eventually what we are expected to assume isn’t it?

      A Hydrofluoric Acid reaction with any rock strata, there is no way of limiting the reaction to limestone or sandstone only, since there is no pure form of either and certainly not assessable 2000, 3000, 4000m down.

      Limestone and sandstone will have all sorts of additional elements present created in the formation, deposition and ageing, the reaction with other elements over its period of deposition, the degree to which it is exposed or subject to erosion and contamination. fracturing and leakage of other elements into it
      It will also contain the natural gas elements that you are looking for in the first place, so there will be no pure sandstone and the reaction cannot be confined to just the strata you want to have it react to.

      And then also you will have the pipe casing material reaction and the protective chemical compounds, the fracking slip chemicals themselves, and what other chemical is used to assist in the drilling and casings, the residue of other strata carried down with the drilling casings, the cement “seal” mix with the hardening chemicals, there must be a long list of other compounds, chemicals and elements present and all reacting with the hydrofluoric acid in different ways?

      Do you know precisely about every single one chemicals, elements and compounds of those in isolation and also in combination?

      No, of course not.

      Simple physics and common sense isn’t it?

      I am afraid the attitude that the result will only and solely be Salt, Water and Carbon Dioxide simply does not really represent the true practical reaction results does it?

  9. PS, What do you do with the resulting Ammonium Chloride and from where are these chemicals sourced? UK? Abroad? As i said there are thousands of possible fluoride compounds all with different chemical combinations and constituents and the water companies may not even be aware just which one they are using, they are in bags that simply say fluoride?

    I can only assume the chemical constituents of hydrofluoric acid are similarly vague both in precise chemical compound and origin?

    Do you see how interesting it gets when a few simple questions are asked?

    Thanks for replying by the way.

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