Acidisation is a stimulation technique, used to release oil and gas from unyielding rock such as sandstone and limestone. It involves injecting solutions of acids and other chemicals into the ground, either to clean the well, or to create passageways through the rock along which oil or gas can flow.

The industry divides acidisation broadly into three ‘tiers’. In increasing order of intensity, they are:

  • An ‘acid wash’ is a weak acid solution that cleans the well bore at low pressure.
  • ‘Matrix acidising’ uses pressure insufficient to fracture the rock to dissolve short pathways through the rock lying close to the wellbore.
  • Acid fracking’ is done at pressure high enough to fracture the rock, creating longer pathways.

Planning applications for projects involving acidisation tend to be loosely worded. They may mention unspecified ‘stimulation techniques’. Companies may propose an acid wash, but later decide to matrix acidize a well during testing and, in some cases, acidise at pressure during the production stage.

What acids and chemicals are used?

In limestone, the main acid used is hydrochloric, typically in concentrations of up to 15%.

In sandstone, it’s usually hydrofluoric, a highly corrosive acid and a powerful contact poison, usually used at much lower concentrations of up to 3%.

Other chemicals will be used too – such as biocides, polymers to make the liquid gloopy, corrosion inhibitors, detergents, solvents, and other potentially harmful chemicals. However oil and gas companies routinely keep secret the details of the acids and other chemicals they plan to use.

Is it ‘conventional’?

The industry tries to claim that acidisation is ‘conventional’, as it sounds safe and reassuring. But there is no legal definition of conventional in this context.

For geologists, conventional fossil fuel extraction means ‘without stimulation’. Therefore, acidisation, like fracking, is an unconventional extraction technique. Both are stimulation techniques designed to release oil or gas tightly trapped inside the pores of rocks. While fracking is used to crack open shale, acidising is used to dissolve passageways through limestone or sandstone.

Why are we concerned about acidisation?

Wells have been acidised in the Weald in decades gone by, barely regulated or monitored. What is proposed now is on a different scale. There are many reasons to be concerned about acidisation.

Acidising uses much higher concentrations of chemicals than fracking. Matrix acidising and acid fracking fluids could contain up to 17-18% chemicals.

Given the repetitive nature of the process, acidising may use a lot of water.

Acidising has many of the same negative impacts as fracking: traffic, road tankers, air pollution, flares, potential water pollution via spills, leaking wells and faults, processing plants, large volumes of toxic liquid waste, stress on communities.

It threatens industrialisation of the countryside. One company, UK Oil & Gas (UKOG), has promised ‘back-to-back drilling of production wells’ across the Weald.

The vast majority of scientists are now agreed that the burning of fossil fuels is causing potentially catastrophic climate change. The UK government and  others have agreed legally-binding carbon reduction targets. The extraction of oil will make it much harder to meet these. Rather than using unconventional methods to get at these hard-to-extract fossil fuels, we need to switch investment to energy efficiency and renewable technologies.

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