What Is Acidizing?
A matrix acid job is performed when acid is pumped at low pressure into the oil well and into the texture of the reservoir rocks. The acids dissolve the sediments and mud solids that are inhibiting the permeability of the rock, enlarging the natural pores of the reservoir and stimulating the flow of oil.
Acid fracking involves pumping acid at moderately high pressure – though considerably lower pressures than conventional fracking. The acids physically fracture the reservoir rock and dissolve the sediments that are blocking the flow of oil.
Because HF is so successful at dissolving anything it touches, developers add other substances to the mix to prevent the acid from dissolving the oil well’s steel casings – which are intended to keep the oil and chemicals from leaching into the surrounding rocks and water table. After the acid job is performed, the used acid and oil well sediments are sucked out in a process called backflush.
A low-volume form of acidizing has long been used nationwide, including in California. This process typically occurs in aging oil wells during the final stages of production, as a means of coaxing out the last dregs of oil before the well is abandoned. In contrast, the tactic now being pioneered in California appears to involve much higher volumes of injected acid as a primary technique for new wells.
In past years, both HF and hydrochloric acid (HCl) were used, depending on the geology. However, the sandstones and silicates that are prevalent in the Monterey Shale lend themselves especially to HF use.
In many cases, HF is created at the oilfield by mixing hydrochloric acid with ammonium fluoride and immediately injecting the mix down the well. Creating the HF on site is accepted as safer than offsite production, as it reduces the risk of transport accidents.
In other cases, however, HF is mixed at a remote location, trucked to the oilfield, mixed there and then pumped downhole.
 In addition to the primary acid components, other chemicals used include surfactants, solvents, corrosion inhibitors and oxidizers. The acid solution may also be applied in other forms such as foams, gels or emulsions. The volume injected is typically low, such as 5 gallons per well foot, but there are reports that higher volumes of as much as 250 gallons per foot are being used.