The purpose of matrix stimulation in carbonate reservoirs is to bypass damaged zones and increase the effective wellbore area. This can be achieved by creating highly conductive flow channels known as wormholes. A further injection of acid will follow a wormhole path where the permeability has increased significantly, leaving substantial intervals untreated. Diverting materials such as surfactant-based acids plays an important role in mitigating this problem. In this study and for the first time, 20-inch long cores were used to conduct the acidizing experiments in two configurations, single coreflood and parallel coreflood. The major findings from performing single coreflood experiments can be summarized as follows: The acid injection rate was found to be a critical parameter in maximizing the efficiency of using surfactant-based acids as a diverting chemical, in addition to creating wormholes. The maximum apparent viscosity, which developed during viscoelastic surfactant acid injection, occurred over a narrow range of acid injection rates. Higher injection rates were not effective in enhancing the acidizing process, and the use of diverting material produced results similar to those of regular acids. The amount of calcium measured in the effluent samples suggests that, if the acid was injected below the optimum rate, it would allow the acid filtrate to extend further ahead of the wormhole; at some point, it would trigger the surfactant and form micelles. When the acid injection rate was lowered further to a value of 1.5 cm3/min, the fluid front developed in more progressive fashion and the calcium concentration was more significant, continuing to increase until wormhole breakthrough On the other hand, the parallel coreflood tests show several periods that can be identified from the shape of the flow rate distribution entering each core. The acid injection rate was confirmed as influencing the efficiency of the surfactant to divert acid. Acid diversion was noted to be most efficient at low rates (3 cm3/min). No significant diversion was noted at high initial permeability ratios, at least for the given core length. The use of surfactant-based acid was also found to be constrained by the scale of the initial permeability ratio. For permeability ratios greater than about 10, diversion was insufficient.

A new type of viscoelastic amphoteric surfactants (amidoamine oxide) has been examined as a diverting agent during acidizing treatment. Rheological properties of viscoelastic surfactants are a function of surfactant concentration, acid additives, pH, temperature and shear rate. A HPHT rheometer was used to test the effect of common acid additives and organic acids/chelating agents on the apparent viscosity of amidoamine oxide-based acids. The compatibility and thermal stability of surfactants with corrosion inhibitor were also investigated. Rotating disk apparatus was used to examine the kinetic studies of surfactant-based acids with limestone. The results show that the apparent viscosity of surfactant solutions prepared in deionized water, live acid, and spent acid was found to be a function of temperature. Apparent viscosity of live surfactant-based acids was also found to be a function of HCl concentration. Most of acid additives could adversely affect the rheological properties of spent acids. Compatibility tests should be done prior the field application. Cryo-TEM studies show the changes of rod-like micelle structures with the addition of additives. The reaction between surfactant-based acid and limestone was found to be mass transfer limited at 170 degrees F.