Acid diversion using viscoelastic surfactants: The effects of flow rate and initial permeability contrast

The purpose of matrix stimulation in carbonate reservoirs is to bypass damaged areas 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. This problem can be more significant as the contrast in permeability increases within the target zones. Diverting materials, such as surfactant based-acids, plays an important role in mitigating this problem. Acid injection rate was found to be a critical parameter to maximize the efficiency of using surfactant-based acids as a diverting chemical in addition to creating wormholes. We found that the maximum apparent viscosity, which developed during viscoelastic surfactant acid injection, occurred over a narrow window of acid injection rates. Higher injection rates were not effective in enhancing the acidizing process, and the use of diverting material became similar to that of regular acids. 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. The results were obtained by conducting a large set of acidizing experiments using 20'' long cores. Both single and parallel coreflood experiments were performed in this study. Carbonate cores were used with initial permeability of 4 md to 150 md, and the flow rate was varied from 1.5 cm³/min to 50 cm³/min. The initial ratio of permeabilities between the two cores ranged from 2 to 15. To characterize the wormholes, computerized tomography was used to generate a 3D view of the wormholes in each core.