Interactions of Iron and Viscoelastic Surfactants: A New Formation-Damage Mechanism

Abstract

Abstract 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 one treatment was below expectations. Analysis of the live acid used 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 conduct to better understand interactions between iron and various types of viscoelastic surfactants. All tests were conducted on live acid that contained 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. Introduction The existence of the iron in the stimulation process is almost given in every step of the treatment. Therefore, the interaction of iron with acid additives should be carefully examined. One of acid additives that were recently introduced is viscoelastic surfactants (VES). These surfactants (amphoteric, anioinic or cationic, or combinations of these types) are used to divert the acid in carbonate formations. Amphoteric and cationic surfactants contain function groups that can interact with iron. This paper examines the effect of Fe (III) on amphoteric and cationic-based viscoelastic surfactants. VES has been used recently as a diverting agent during acid treatments of carbonate formations. When stimulating a carbonate, the acid, mostly HCl, will react with calcium carbonate as follows: CaCO3 + 2 HCl => CaCl2 + CO2 + H2O (1) The produced calcium chloride and the increase in pH value will transform the special micellars into long rod-shaped ones. This transformation is accompanied by significant increase in fluid viscosity. It is very important to measure the effect of Ferric ions (Fe3+) on the viscosity VES solutions as ferric ions are expected to be significant during well treatment; iron concentration could reach up to 100,000 mg/L. Also, it is important to study the formation of various ferric species with VES's. The presence f iron can adversely affect the behavior and function of VES-based fluids. Therefore, the objective of the present study is to examine potential interactions between ferric iron and two categories of ZVES that have been used in the field: amphoteric and cationic VES.