**3.4 Acid-based fracturing fluids**

The common types of acid-based fracturing fluids are described in Section 2.2.7 and 2.3.4. These are hydrochloric acid (HCL), hydrofluoric acid (HF), and organic forms of acids such as methanoic (formic) acid (HCO2H or HCOOH) and acetic acid (CH3COOH). During acid fracturing, fracture conductivity is increased by etching channels along fracture walls. This method of fracturing is, therefore, effective in reservoir rocks that are soluble in acid. Carbonate formation rocks (sedimentary) such as limestone and dolostone are soluble in acid; hence, they are the most common beneficiaries of acid fracturing. Nevertheless, innovative applications of acid fracturing on rocks with low acid solubility (e.g., sandstone) are becoming more recognised [83, 97, 124]. HCL is the most popularly used acid fracturing fluid, especially for carbonate rocks, but the solubility of some reservoir rocks (e.g., sandstone) to this acid is low. Thus, the application of HCL in low-soluble formation rock is limited. Although weaker in strength to HCL, HF are more reactive to formation rocks rich in aluminosilicates and quartz—such as sandstone—and, hence, better candidates. In practice, acid blends (mud acids) are preferred and frequently used (e.g., [2, 83, 97, 104]). Examples are HCL-HF, NaOH-HCL, fluoboric-formic acid (H3OBF4-HCOOH) and hydrofluoric-phosphoric acid (HF-H3PO4).
