Interactions of Iron and Viscoelastic Surfactants: A New Formation-Damage Mechanism
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 112465, "Interactions of Iron and Viscoelastic Surfactants: A New Formation-Damage Mechanism," by A.R. Al-Nakhli, SPE, Saudi Aramco; H.A. Nasr-El-Din, SPE, Texas A&M U.; and A.A. Al-Baiyat, King Fahd U. of Petroleum and Minerals, prepared for the 2008 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 13–15 February. The paper has not been peer reviewed.
Viscoelastic-surfactant (VES)-based acids have been used extensively in the last few years in matrix- and acid-fracturing treatments. The results from one treatment were below expectations. Analysis of the live acid used indicated that this acid contained nearly 10 000 mg/L total iron. This study was conducted to provide a better understanding of interactions between iron and VESs.

Introduction
The existence of iron in the stimulation process is almost a given in every step of the treatment. Therefore, the interaction of iron with acid additives should be examined carefully. Acid additives that were introduced recently are VESs. These surfactants (amphoteric, anionic, 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. The full-length paper examines the effect of ferric ion (Fe3+) on amphoteric- and cationic-based VESs.
When stimulating a carbonate, the acid, mostly hydrochloric acid (HCl), will react with calcium carbonate (CaCO3) to produce calcium chloride (CaCl2) and increase the pH value. The increase in pH value will trans-form the special micellars into long rod-shaped ones. This transformation is accompanied by a significant increase in fluid viscosity.
It is very important to measure the effect of Fe3+ on the viscosity of VES solutions because Fe3+ is expected to be significant during well treatment; iron concentration could be as high as 100 000 mg/L. Also, it is important to study the formation of various ferric species with VESs. The presence of iron can affect the behavior and function of VES-based fluids adversely. Therefore, the objective of the present study is to examine potential interactions between Fe3+ and two categories of VES that have been used in the field: amphoteric and cationic VES.
Sources of Fe3+
During acidizing treatments, the acid can be contaminated with iron com-pounds at all stages. Rust in storage tanks and mixing tanks can be dissolved by the acid and produce a mixture of iron(II) and Fe3+. However, dissolved oxygen in the acid will oxidize iron(II) to Fe3+. Large amounts of iron in live acids can result during acid injection as the acid dissolves mill scale in new tubing or corrosion products. Iron content was measured in an acid solution at four different points in the acidizing process. Iron content at the wellhead varied from 200 to 3500 mg/L, and the returning acid showed iron concentrations of 9000 to 100 000 mg/L during cleanup treatments of new wells. Moreover, thin deposits of iron sulfide scale on tubing can result in large quantities of iron(II) in solution during an acid treatment. Also, the formation brine may have iron in solution.