Viscoelastic surfactant-based acids have been extensively used over the last few years in matrix and acid fracturing treatments. Two main types of viscoelastic surfactants have been used; amphoteric and cationic. Once the acid reacts with the carbonate formation and the pH increases above two and the concentrations of calcium and magnesium ions increases, these surfactants form a long rod shaped micelles. As a result, the viscosity of the solution increases significantly. This will result in better acid diversion in matrix acid treatments and deeper acid penetration in acid fracturing treatments.

Viscoelastic surfactant-based acids were used in more than 600 successful treatments. The results, however from some treatments were below expectations. Analysis of the live acid used in one unoptimum treatment indicated that this acid contained nearly 10,000 mg/l total iron. It was clear that there were some interactions between iron and the viscoelastic surfactant used. Therefore, this study was conducted to better understand interactions between iron and various types of viscoelastic surfactants. All tests were conducted on live acid that contained 3 & 20 wt% HCl, 4 to 6 vol% surfactant, and 0.3 vol% corrosion inhibitor. Ferric chloride was added to the live acids at various concentrations from 0 to 10,000 mg/l.

Compatibility tests indicated that there are strong chemical interactions between iron (III) and all types of viscoelastic surfactants examined. A gel like material was noted at a certain iron concentration. SEM analysis indicated that the gel-like material contained iron. Also analysis of the iron in the supernatant using ICP confirmed this result. The critical iron concentration was found to be a function of the surfactant type. Extensive lab testing indicating that there is interactions between iron and the amide and amine groups present in these surfactants. Corrosion inhibitors contain amine and amide groups, and they did affect the interactions between iron and viscoelastic surfactants. These interactions resulted in the formation of the gel-like material that can cause sever formation damage. This paper will discuss the results obtained and give recommendations to avoid the formation of such damaging material.

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