A New Amphoteric Surfactant Self-diverting Acid System

In acidizing work, conventional self-diverting acid becomes viscosified at pH between 2 and 4, thus limits the function of converting acids. A new amphoteric surfactant self-diverting acid (VES) was developed based on the viscosifying mechanism viscoelastic surfactant. The viscosity of this acid is increasing during the reaction of acid and rocks, and decreasing when the pH of the reaction system is becoming low. The effects of surfactant, hydrochloric acid, calcium ions on the viscosity of the diverting acid were explored and acid-rock reactions simulated. The equation of kinetics of acid-rock reactions was fitted in accordance with the reaction rate of the acid and rocks. Heat resistance, rheology and gel-breaking ability of VES were studied. In acid-rock reaction, VES had the highest viscosity (144 mPa·s) at hydrochloric acid concentration of 5%, and the acid was diverted. The viscosity of VES was decreasing at temperature above 90℃, indicating that this self-diverting acid is suitable for use in medium- to low-temperature reservoirs. VES has good shear-thinning capacity; more than 90% of the viscosity of VES can be recovered. The reaction rate of VES is 1/2 of the reaction rate of hydrochloric acid, which is beneficial to slow-release diverting. The spent acid, whose viscosity is low (30 mPa·s), is easy to gel-broken and get hydrated when in contact with hydrocarbons and formation water, and the solution after gel-breaking has lower viscosity and surface tension, meaning that it will impose no damage to reservoir formations. VES in field application has obviously enhanced oil production, and shows good prospect in well stimulation.