Recently, self-diverting acid systems based on viscoelastic surfactants (SDVAs) have been successfully used for carbonate reservoir treatment. Changes in the SDVA viscosity during the interaction with carbonate reservoirs are associated with the transformation of spherical surfactant micelles into worm-shaped micelles as the acid concentration decreases and the fluid salinity increases. The highly viscous fluid acts as a temporary barrier and diverts the acid fluid into the untreated lower permeability zones. After treatment, the SDVA barrier breaks down when it makes contact with newly formed hydrocarbons. The objectives of this study were to examine in detail the viscoelastic surfactant effect on the HCl and calcite reaction and to examine the effects of the surfactant and acid concentrations on the SDVA apparent viscosity. Rheological measurements were conducted using rotational viscometers at 25 degrees C. A proposed semi-empirical rheological model describes the relationships between the apparent viscosity, rate, and HCl concentration. The core flooding tests were conducted in a specialized laboratory unit to simulate the downhole conditions. The testing unit allows for control of the pressure, temperature, and injected volume of acid flowing across the core sample. The results showed that the pressure drop varies as a function of the pumped fluid pore volume. The steady increase in the pressure as the fluid entered the core was a good indication of the actual viscosity increase in the porous rock during acid spending. The experiments were conducted using various flow rates. The core flow tests indicated that the SDVA surfactant delayed the acid breakthrough in carbonate cores. The mathematical model describing the results, obtained from the acidification experiments, is presented.