Laboratory evaluation of the effects of additives and pH on the thermorheological behavior of a viscoelastic zwitterionic surfactant used in acid stimulation

This paper examined a new class of zwitterionic viscoelastic surfactant (VES) used to increase the efficiency of the acid stimulation process. The effects of CaCl₂ salt and methanol additives on the rheological properties of these systems were examined over a wide range of parameters. The behavior of the rheological parameters indicates the presence of a large network of structures in the material, most likely multi-lamellar vesicles. It was observed that temperature, pH, shear conditions, CaCl₂ salt, and methanol additives had a profound influence on the rheological properties of the VES system. The VES system demonstrated shear-thinning and thixotropic properties. The thixotropic behavior diminished as the temperature increased from 50 to 150°C, and it persisted in the presence of salts and methanol. Both the elastic and viscous properties of the VES solution were affected by changes in salt concentration, methanol concentration, pH, and temperature. Elastic and viscous properties under different tested conditions were found to be a complex function with respect to pH where one or two maxima were observed. Methanol concentrations higher than 5.0wt% decreased the material viscosity drastically, which should be taken into account when designing the diverting system. This paper establishes novel rheological data for the studied VES system that could be used as a basis for future investigations and field applications.