Acid fracturing is a production stimulation technique in which acids, injected at high pressures, are used to create and sustain flow channels in hydrocarbon formations with the sole purpose of increasing productivity.
Acid fracturing is similar to hydraulic fracturing, the only difference being that with acid fracturing, acids perform the role proppants play in hydraulic fracturing which is keeping the fractures open after the pumps are shut down.
The frac pad used to initiate and extend fractures may be an acid solution or a normal water based fluid. However, the moment fractures of sufficient width and length are created, acids are pumped down into the created fractures to “etch” or irregularly eat up the surface of the fractures.
When operation is completed and the pumps are shut down, pressure exerted on the formation drops and the “fracture faces” will try to close again. But because of the irregularity on the fracture face (etching caused by the acid), the cracks remain open.
The most commonly used acid during acid fracturing is hydrochloric acid in concentrations of 15% by weight. The higher the concentration, the more reactive the acid will be and the deeper the acid can penetrate.
But at the same time, the higher the concentration the more reactive the acid will be on metal surfaces found in pumping equipments and well completions. So achieving a balance in acid concentration is key.
To protect metal surfaces from acid corrosion or hydrogen embrittlement, inhibitive fluids are added to the acid solution. HCl at high concentrations may be difficult to inhibit especially at higher temperatures because the rate of acid reaction with both the formation and metal surface will increase. On occasions of high temperature, formic acid (HCOOH) or acetic acid (CH3COOH) can be used instead.
Acid Selection for Acid Frac Job
Acid fracturing is not recommended for every rock. This is because the rock has to be soluble in the acid in the first place. Carbonate rocks are perfect candidates with acid fracturing treatments.
However, some acids should not be used with carbonate rocks. Hydrofluoric acid (HF) for instance, will precipitate Calcium fluoride (CaF2) which will settle into the etched fractures and plug the flow channels.
Trying to create acid etched fracture faces on sandstone formations with HCl will be a waste of time and resources because sandstone is not soluble in hydrochloric acid. For sandstones, acid wash or an acid job can be done to improve productivity.