Acid flooding can be successful in formations that are dissolvable in the particular acid mixture, thus opening the pores. Hydrochloric acid is common, in a concentration of 6% to 30%, sometimes also with hydrofluoric acid and surfactants added. The acidic environment has still another effect on surfactants. It converts the sulfonates into sulfonic acid, which has a lower interfacial tension with oil. Therefore a higher oil forcing-out efficiency than from neutral aqueous solution of sulfonates is obtained. Cyclic injection can be applied, and sulfuric acid has been described for acid treatment. Injecting additional aqueous lignosulfonate increases the efficiency of a sulfuric acid treatment.
Hydrochloric acid in combination with chlorine dioxide can be used as a treatment fluid in water-injection wells that get impaired by the deposition of solid residues. The treatment seems to be more effective than the conventional acidizing system when the plugging material contains iron sulfide and bacterial agents because of the strongly oxidative power of chlorine dioxide. Mixtures of chlorine dioxide, lactic acid, and other organic acids also have been described.
Iron control chemicals are used during acid stimulation to prevent the precipitation of iron-containing compounds. The precipitation of these compounds in the critical near-wellbore area can decrease well productivity or injectivity. A time dependence of iron III hydroxide precipitation was observed. Acetic acid can prevent the precipitation of iron III at high acetic acid concentrations at low temperatures.
If the injected acid itself contains iron III, a precipitation of the asphaltic products can occur when it comes in contact with certain crude oils. This leads to practically irreversible damage of the zone treated. The amount of precipitate generally increase with the strength and concentration of the acid. Certain organic sulfur compounds, such as ammonium thioglycolate, mercaptoethanol, cysteamine, thioglycerol, cysteine,and thiolactic acid, can reduce the iron III.