Viscoelastic surfactant (VES)-based acid systems have been used successfully in matrix and acid fracturing treatments. However, the existence of Fe (III) as a contaminant in such systems may lead to many problems, due to interactions between VES and Fe (III). Such interactions could reduce the effectiveness of VES-based acid systems and may lead to formation damage.
In this paper, reaction mechanisms between VES and Fe (III) were studied. Rheological properties of two surfactants were examined with various concentrations of Fe (III). A scanning electron microscope (SEM) was used to identify precipitates from reaction products. Inductively coupled plasma (ICP) was applied to measure iron concentration, and the two-phase titration method was used to determine VES concentration in all phases of the sample. 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-based acids (20 wt% HCl, 4 vol% VES) increased from 14 to 751 cp at a shear rate of 10 s-1 when the Fe (III) concentration was increased from 0 to 2,300 ppm, and started to decrease at higher Fe (III) concentrations. This is because of the electrical bonding between negative charged [FeCl4]- groups and positive charged amine groups in VES in live acids. Both surfactants interacted with Fe (III) and precipitates, which are complexes containing iron and VES, were noted at 5,000 ppm and higher concentrations of Fe (III). On the other hand, adding a chelating agent [1:1 mole ratio to Fe (III)] reduced the apparent viscosity of the sample to no more than 40 cp at the shear rate of 10 s-1 when Fe (III) concentration is lower than 4,000 ppm, which means that the chelating agent reacted with Fe (III) and reduced interactions between VES and Fe (III). Adding a suitable chelating agent can minimize the impact of Fe (III) on VES-based acids.