A reaction by-product. In sandstone acidizing, the reaction between hydrofluoric acids (HF) or spent HF acids with formation minerals can precipitate nondamaging products, such as silica, borosilicates or fluoborates. However, other insoluble or difficult to remove by-products can create formation damage.

Ferric iron (Fe⁺³) and ferrous iron (Fe⁺²) are potential sources for precipitates. Ferric iron present in some formation minerals, including chlorite and glauconite clays, and in tubing rust (iron oxide) can precipitate as ferric hydroxide (Fe(OH)₃), which is a gelatinous, highly insoluble mass that can plug pore channels and reduce permeability. The precipitation of ferric hydroxide or ferrous hydroxide (Fe(OH)₂) depends on the pH of the spent acid. The former needs a pH higher than 2. 2, while the latter requires a pH higher than 7. 7. Since the maximum pH for a spent acid is approximately 5. 3, the precipitation of ferric hydroxide is more common. Iron-sequestering or iron-reducing agents can be used in acid to maintain the ferric iron in solution.

Calcium fluoride (CaF₂) precipitates when HF contacts calcite or any other calcium source, and alkali-fluosilicates or iron sulfide form crystal-like by-products that can bridge pore throats. Additionally, some sequestering agents, corrosion inhibitors or friction reducers can also form residues that may plug formation pores.

The formation of precipitates can be avoided or reduced by using a preflush, which dissolves calcareous material, iron rust or iron scales, and displaces formation brines (K, Na, Ca ions) away from the wellbore, thereby reducing the formation of CaF₂, ferric hydroxide and alkali-fluosilicates.