Rheological Properties of a New Class of Viscoelastic Surfactant

Abstract

Summary Surfactant-based acid systems were developed over the last few years for diversion to overcome the severe problems caused by polymer residue and crosslinker precipitate after polymer-based system treatments during matrix and fracture acidizing. Two main types of viscoelastic surfactants have been used: amphoteric and cationic. Surfactant molecules can form rod-like micelles and significantly increase the viscosity in the presence of salts. After acid treatments, the surfactant gel can be broken by mixing with hydrocarbons, external breakers, internal breakers, or by reducing the concentration of salts and or surfactant through dilution with water. This paper introduces a new type of viscoelastic-amphoteric surfactant (amine oxide). It carries a positive charge in live HCl acids. The effects of acid additives and Fe(III) contamination were examined on its rheological properties. Measurements were made at temperatures from 75 to 220°F and 300 psi at various shear rates from 0.01 to 935 sec-1. Acid additives included corrosion inhibitors, a mutual solvent, a nonemulsifying surfactant, iron-control agents, and a hydrogen-sulfide scavenger. The apparent viscosity of surfactant solutions prepared in deionized water, live acid, and spent acid was found to be a function of temperature. Apparent viscosity of live surfactant-based acids was also found to be a function of HCl concentration. Fe(III) contamination caused enhancement of apparent viscosity, then two immiscible liquids, and finally precipitation of surfactant/Fe complex. A demulsifer and mutual solvent decreased the apparent viscosity at all temperatures examined. Multiple iron-control agents were tested and found to reduce the apparent viscosity of this surfactant-based acid. Only up to 1 wt% methanol can be used with this spent-acid system at temperatures below 175°F.