The matrix acidizing method has been widely employed in the petroleum industry to increase the output of oil and gas from reservoirs that have low permeability and are experiencing formation degradation as a result of drilling, stimulation, and completion activity (Mohsin et al., 2019). It is a method of dissolving carbonate reservoirs (calcite or dolomite) by injection of HCl, which reacts with minerals in the rock formation to increase permeability and production close to the wellbore (Buijse and Domelen, 1998). The new ‘wormhole’ channels are created by the introduction of the treatment fluid at a downhole injection pressure just below the fracturing pressure to bypass the damaged zone near the wellbore (Amro, 2002; Kiani et al., 2021). The wormholes are formed from the reaction of HCl with the CaCO₃ and CaMg(CO₃)₂ minerals in the matrix, which avoids the possibility of formation damage (Chang et al., 2007).

In carbonate formations, the purpose of matrix acidizing is primarily to restore and enhance the formation’s productivity; but, it also reduces near-wellbore damage and establishes novel hydrocarbon flow channels (wormholes) (Ibrahim et al., 2020). By obstructing the path that the acid normally takes, these agents redirect it to the less permeable, untreated zones as shown in Figure 1 (Ghommem et al., 2015). The use of HCl alone for acidization, especially in higher temperature reservoirs, has suffered some signal failures. A large volume of acid is required because the rapid reaction of the acid with the carbonate minerals of the formation consumes acid at a far faster rate (Chavez, 2007). Due to the shorter residence time of the acid in the formation, especially at high temperatures, the wormholes created are less efficient hydrocarbon channels (Chacon and Pournik, 2022). Therefore, gelled acid, foamed acid, micro-emulsified acids, and other chemical agents have been formulated in an effort to circumvent the drawbacks of the fast acid-rock reaction by lowering H⁺ ion dissociation, reducing H+ ion mass transfer, and changing the wettability of the rock surface (Zhu et al., 2022).

Due to the fact that carbonate reservoirs are heterogeneous with varied permeability zones and a high temperature environment, the selection criteria for choosing one of the three proposed matrix acidizing methods must be careful observed. Various researchers have reported that foam-based acids were superior to traditional acidizing methods. They provided effective diversion, reduced the acid reaction rate and water sensitivity, increased energy for discharge of acid into the formation, and caused less harm to the formation. Foaming lowered the effective acid diffusion coefficients into the formation, thus regulating convection and diffusion rate. Foamed acid remains in contact longer with the less permeable zones of the formation, and the process prevents the carbonate-acid reaction from happening too rapidly in the highly permeable zones. At low injection rates, the poor diffusivity of micro-emulsified acid produces narrow but deeper effective wormholes.

The gelled acid relies on viscosity differences in the fluid to reduce the mass transfer of H⁺ and the acid reaction rate. Despite its good volumetric efficiency and acid diversion tendency, the field application of gelled acid is limited by the buildup of residuals that impair the flow of oil back through the wormholes to the wellbore. The polymers used as viscosifiers can remain as residual chemicals in the formation and lead to damage; therefore, VES formulations are preferred.