Acidising and Fracking

***During a meeting with local residents and the main protector at the Brockham wellsite on 24th January 2017, the CEO of Angus Energy agreed that a commitment to not use acidisation would be put into a legal document and this had still not been forthcoming…***



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Acidising, Acid Fracking and Industrialisation of the Weald Basin

The geological layer being targeted currently at Brockham is tied up in the Kimmeridge Limestone layers, underlying the ‘Portland reservoir’ which has been exploited for oil briefly in 1988 and from 2002 to 2015. 

The Kimmeridge Limestone resource is classified as tight oil – a form of unconventional reservoir. This oil does not flow naturally and the geology requires a range of chemicals, water and pressure to dissolve the rock and force it out. The most common form of well stimulation for carbonate geology is matrix acidising or acid fracturing (Schlumberger). It also requires drilling of multiple horizontal wells in close proximity. 


A 2016 EY report commissioned by UKOG – oil firm involved in exploration at several  other sites located in the Weald Basin – reads:

“Kimmeridge Limestone Oil likely requires ‘stimulation’ to flow to the surface at commercial rates. The primary stimulation method for wells in limestone rock formations is acidising.”

“The development of an onshore Kimmeridge Limestone Oil industry will require efficient regulation, and back-to-back drilling of production wells.”

The ‘Gatwick Gusher’ at Horse Hill

Angus Energy have now confirmed that the geology at Brockham to resemble closely the nearby Horse Hill well site near Gatwick, nicknamed the Gatwick gusher, after very successful flow test results were announced by UKOG in the first quarter of 2016. Angus Energy oversaw the drilling of Horse-Hill-1 well in 2014, and is the founder of Horse Hill Developments Ltd – the current Operator of the Horse Hill site.


Acidising, or acidisation has been used for a while in the industry, the main use being to remove debris, rust or scale from the well. This is known as an ‘acid wash’. Acid wash must not be confused with acid stimulation methods: matrix acidising and acid fracking. These are much more similar to  the well known and very controversial hydraulic fracturing (also known as fracking). 

Matrix acidising involves greater pressure with the purpose of dissolving rock within a small distance of the well (generally 1-5ft or 30-150cm although ‘wormholes’ of 20ft/6m may open up) thereby increasing overall oil flow. If results from this are favourable, the yet more intensive technique of fracture acidising or acid fracking may be used. This involves greater pressure and a higher concentration of acids and other chemicals, with fractures opening up typically to lengths of 50-100ft/15-30m or further.

The two most common acids used in acidising are Hydrochloric (HCl) and Hydrofluoric (HF) acids, depending on the type of rock and the kind of acidising process. A number of other toxic substances are added to the mix. The concentration of chemicals used in acidising fluids is higher than in fracking fluids, and the cumulative effect is larger because each well will likely be acidised multiple times.

Imported from N. America 

Ever higher acid concentrations are experimented with by oil firms in matrix acidising and acid fracking and the large scale use of these technologies seems to be imported from the US, where acidising has proven more effective than fracking in producing oil from the Monterey shale in California, for example.

Regulation – Do we have a loophole here?

In the UK, little distinction has been made in regulation between acid washing and either matrix acidising or acid fracturing, and a regulatory loophole seems to have developed. Inclusion of proper details and disclosures is not required in applications. Acidising as a well stimulation technique to enhance production (as opposed to restoring the natural porosity of the formation in the case of acid wash) is not mentioned in any UK regulations and the industry is allowed to self-monitor. 

The Environment Agency’s latest onshore oil and gas sector guidance (Aug 2016) makes references to well stimulation methods including matrix stimulations. However, acidising is not mentioned anywhere and there is no specific definition in terms of substances and volumes used. This creates room for confusion between acid wash and acidising or acid fracturing, and opens up a loophole which could be exploited by oil firms. 

In addition, EA’s data collection and the ability to monitor the use of these methods seem vague (for example the EA doesn’t monitor how many onshore oil wells have used hydrofluoric acid – one of the most dangerous chemicals on earth). 

– Regulation in US

Meanwhile in California, which can be seen as an analogue but some years ahead in terms of use of acidising and legal framework development, a bill regulating acidising and fracking was signed into law in late 2013. It puts acidising in the same risk category as fracking. Several California municipalities have passed measures advocating for more stringent regulations or a moratorium on fracking, acidising and other extreme oil drilling methods.

In Florida, the issue of acidising has also been controversial. As of February 2016, nearly 80 counties and cities have passed ordinances to ban or oppose fracking and matrix acidising, and a bill is currently going through the state legislature proposing a ban on advanced well stimulation treatments including hydraulic fracking, acid fracking, and matrix acidising.

Health and Environment Impact, Risk to Groundwater

There is very little scientific information about the effects of acidising, which is in itself an argument against extensive use of this method. A recent University of California, Los Angeles (UCLA) study that looked at this process in California, states that chemicals used in acidising potentially directly threaten groundwater and surface water quality and it urges further evaluation by authorities. According to the report:

“There are close to 200 specific chemicals used in acidisation, with at least 28 of them being F-graded hazardous chemicals”. Some are used frequently in the range of 100–1000 kg per treatment, such as hydrofluoric acid, xylene, diethylene glycol, and ethyl benzene. Unlike hydraulic fracturing the chemical concentrations in acidising are high, ranging from 6% to 18%, and the waste returns can be highly acidic, in the range of pH 0–3.”

Other Wells in the Pipeline for Acidising*

  • Balcombe, West Sussex
  • Markwells Wood
  • Broadford Bridge, West Sussex
  • Horse Hill, Surrey
  • Bury Hill Wood, near Leith Hill, Surrey
  • Palmer’s Wood, Godstone, Surrey
  • King’s Farm, Tilburstow Hill Road, South Godstone, Bletchingley, Surrey
  • Wressle, North Lincolnshire

*Some are yet to be drilled and at this stage suspected to target tight oil reservoirs.

See a recent post on acidising by Kathryn McWhirter on Drill or Drop here (it links to an in-depth report on this technique as well as new technologies including gels and fishbones).