Yes. This is completely true. The acid is designed to react with the rock in order to make it easier to create fractures. The acid is completely used up in that process. Problems would arise only if there were a spillage of the acid at the surface or near the aquifer.
Hydrochloric Acid
It is often said that thousands of gallons of hydrochloric acid are in fracking fluid. This is sometimes the case. If the shale that is to undergo hydraulic fracturing contains a reasonable proportion of calcite or dolomite, it is very likely that hydrochloric acid will be used to help the fracking process. Indeed it is sometimes used without fracking.
Hydrochloric acid (HCl), the acid found in your stomach, reacts with the calcite (CaCO3) in limestone to give Calcium Chloride (CaCl2), water and CO2. The breakdown of calcite to calcium chloride promotes fracture formation and the CO2 adds to the pressure of fluid in the fractures. Although a lot of HCl is put down-hole, because there is so much calcite, all of the acid is turned to water and CO2.
The process is similar if there is dolomite in the rocks, with water and CO2 being produced, but this time with CaCl2 and MgCl2.
Hydrochloric acid is usually used in a 15% by weight solution. In this form, 5000 litres of HCl will react fully with 1.1 tonnes of calcite, which is about 0.4 m3 of that mineral. However, the well is surrounded by millions of cubic metres of calcite containing rock. Hence, although the acid volumes seem large, they are actually quite small compared to the rock's ability to neutralise it, and the acid is completely used up in the process.
Does that mean its safe?
Largely, Yes. All the acid reacts to form water and CO2, which stay in the reservoir or are recouped in the flow-back fluid.
However, there is a small chance that some of the initial acid might be involved in a surface spill. Rigorous procedures should be put in place to avoid this, and UK regulations are already in place. However, if a spill occurs it is unlikely to affect a large area because soil is extremely good at buffering acids.
Other Acids
In fact hydrochloric acid is not the only acid to be used in the acidification during hydraulic fracturing.
Other acids include Acetic acid (CH3COOH, vinegar, 10%), Formic acid (HCOOH), Hydrofluoric acid (HF, 3% usually with HCl) and sulphamic acid (H3NSO3).
The first two are relatively mild and tend not to corrode steels, aluminium or chrome plate at well temperatures and pressures.
By contrast, hydrofluoric acid is extremely dangerous and can eat glass, cause flesh burns that are painless and extreme, and interact with calcium in the blood leading to cardiac arrest. It is one of the most nasty chemicals of which I know. We impregnate rocks with epoxy and then place them in HF for the acid to eat the rock away leaving the epoxy showing where the pores are and how interconnected they are. Hydrofluoric acid reacts very quickly and should not be used in limestones, but only in sandstones. In fact it is hardly ever used in Europe.
Sulphamic acid is probably better for the environment because it can be brought to the well-head in a powder form, which reduces the liklihood of surface spills. However, it is only about a third as effective as hydrochloric acid (volume per volume) and is correspondingly more expensive to use.
In summary, acid (usually hydrochloric) is part of the fracking mix. However, it poses no danger to the environment because it is all used up in the process of making the fractures. The only slight concern is for surface spills, which are rare and fully covered by UK regulations,.
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