An improved viscoelastic surfactant (VES) based self- diverting acid system has been developed for matrix treatment of carbonate formations. The self-diverting acid contains a novel viscoelastic surfactant that undergoes an increase in viscosity as the fluid penetrates carbonate formations. The decrease in acid concentration as the acid reacts with the carbonate rocks promotes the transformation of spherical micellar structure into a worm-like structure that imparts high viscosity to the fluid. The highly viscous fluid acts as a temporary barrier to reduce further fluid loss into the wormholes and allows complete stimulation of all treating zones. After acid treatment, the viscous fluid is broken by either formation hydrocarbons or pre-flush fluids. The VES based self-diverting acid contains no solids, so there is no bridging when it is pumped through tubing. Rheological studies showed that the acid rapidly developed viscosity as the acid was spent with CaCO3, and the spent fluid was stable up to 300oF. Diversion tests using multiple-core flood equipment showed effective wormholing in both high permeability and low permeability cores. The effectiveness of the new system in diversion and stimulation was confirmed in more than 70 field applications, both through increased well production and/or injection and production logs.

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