Impact of acid additives on the rheological properties of a viscoelastic surfactant and their influence on field application

This paper examines a new class of viscoelastic surfactants (amphoteric) that are used to enhance sweep efficiency during matrix acid treatments. It appears that surfactant molecules align themselves and form rod-shaped micelles once the acid is spent. These micelles might cause the viscosity to significantly increase, and induce viscoelastic properties to the spent acid. The enhancement in these properties depends on the micelle shape and magnitude of entanglement. The effects of acid additives and contaminants [mainly iron (111)] on the rheological properties of these systems were examined over a wide range of parameters. Viscosity measurements were conducted using specially designed viscometers to handle very corrosive fluids. Measurements were made between 25 and 100 degrees C, and at 300 psi at various shear rates from 58 to 1,740 s(-1). Acid additives included corrosion inhibitors, inhibitor aids, an iron control agent, a hydrogen sulfide scavenger, an anti-sludge agent, and a nonionic surfactant. Effects of mutual solvents and methanol on the apparent viscosity were also investigated. It is observed that temperature, pH, shear conditions, and acid additives have a profound influence on the apparent viscosity of the surfactant-acid system. The viscosity and related properties are very different from what were observed with both natural and synthetic polymers. The differences in these properties were characterized and correlated with the type and nature of the additives used. Optimum conditions for better fluid performance in. the field were derived.