Interactions of Fe(III) and Viscoelastic-Surfactant-Based Acids

Abstract

Summary Viscoelastic-surfactant (VES) -based acid systems have been used successfully in matrix-acidizing and acid-fracturing treatments. However, the existence of Fe(III) as a contaminant in such systems may lead to many problems because of the interactions between VES and Fe(III). Such interactions can reduce the effectiveness of VES-based acid systems and potentially lead to formation damage. In this study, two commercial VES products were used and the interactions between VES and Fe(III) were studied. Rheological properties of these two VES-based acids were examined with various concentrations of Fe(III). Energy-dispersive X-ray spectroscopy was used to identify precipitates from the reaction products. Inductively coupled plasma was applied to measure iron concentrations, and the two-phase titration method was used to determine the VES concentrations in all liquid phases of the samples. The effect of several chelating agents on the reaction of VES with Fe(III) was also examined. Experimental results indicate that the apparent viscosity of live VES-1-based acids [20 wt% hydrochloric acid (HCl) and 4 vol% VES-1] increased from approximately 2 to 126 cp at a shear rate of 100 s−1 at room temperature when the Fe(III) concentration increased from 0 to 1,400  ppm, and it started to decrease at higher Fe(III) concentrations. This is because of the electrostatic interactions between negatively charged [FeCl4]− groups and positively charged amine groups in VES in live acids. Live VES-2-based acids (20 wt% HCl and 4 vol% VES-2) showed properties similar to those of the VES-1-based acids in apparent viscosity. Both surfactants interacted with Fe(III) and precipitates, which are complexes containing iron and VES. These interactions were noted at Fe(III) concentrations greater than 5,000 ppm. On the other hand, the addition of a chelating agent [1:1 mole ratio to Fe(III)] helped to reduce the apparent viscosity of the sample, which means that the chelating agent reacted with Fe(III) and reduced the interactions between VES and Fe(III). At the same time, when the Fe(III) concentration was 6,000 ppm in VES-2-based acid, the disappearance of precipitates with the addition of chelating agents showed a reduction of the Fe(III) impact on such VES-based acid systems. Moreover, the addition of enough chelating agent [more than 1:1 mole ratio to Fe(III) amount] reduced the amount of precipitates that formed significantly when the Fe(III) concentration was very high. Adding a suitable chelating agent can minimize the impact of Fe(III) on VES-based acids.