Acid job is a term used to describe the stimulation technique performed by pumping reactive acids into a producing formation through its perforations to clear the pathway for fluid flow and improve permeability.
Acid jobs can be used on both injection and production wells to improve permeability. The most common cause of low permeability in near-wellbore regions is formation damage. A producing formation could be damaged when drilling, completing or even during the production phase.
An acid job is different from acid fracturing. With acid jobs, the formation is not fractured; the acid is merely injected at pressures sufficient enough to ensure deep penetrations beyond the damaged zone.
Choosing the Right Acid for the Job
The choice of acid for acid job will depend on how well the formation reacts with the acid. Carbonate formations, like chalk, limestone and dolomite react well with hydrochloric acid (HCl) whereas a mix of hydrochloric acid/hydrofluoric acid (HCl:HF) gives good reaction rates with sandstones.
The acid selected should penetrate the formation dissolving scales, fines, or any minerals that have blocked the pores. HCl and HF acids are very reactive; not just with the formation but also with metal surfaces present in the pumping equipment and downhole completions.
To protect metals surfaces from reacting with the acids used in an acid job, additives like surfactants and inhibitors are common. Inhibitors form a protective layer on metal surfaces and slow down any reaction with the acid solution used for the job.
Precaution during an Acid Job
Care must be taken during acid injection to ensure that the acid job is delivered only to the damaged zones. If the acid is simply forced under pressure into the formation without control, then it is possible for the acid to just follow the path of least resistance, which will be zones of high permeability, leaving the low permeability zones.
These low permeability zones are the damaged zones we wanted to treat in the first place. Meaning, that expected improvement in productivity may not be seen after the acid job. Sometimes, re-perforating the well may be a cheaper and more effective solution to by-pass near wellbore damage.