Viscoelastic surfactant (VES) have been successfully applied as acid-diversion fluids. However, high temperature, interaction of VES and Fe(III), addition of alcohol-based additives, and chelating agents all interfere with the apparent viscosity of the VES-based acid and reduce its efficiency. In the present study, the interactions of Fe(III) with a new type of VES-based acid system, which can be applied effectively for diversion at high temperatures, were characterized in a wide range of pH.

The physical behavior of the VES solutions after addition of iron at various pH values were observed visually to determine any change in the viscoelasticity of the solutions. In the present study, because of the similarity between chemical structures, 3-sulfopropyldimethyl-3-methacryl -amidopropylammonium (SMA) was used as a model compound of VES to characterize the nature of the interaction between VES and Fe(III). IR spectroscopy was employed to understand the nature of the SMA interactions with Fe(III) in different pH values. Also, UV-vis spectroscopy was conducted to determine stoichiometry of the interactions as well. Single X-ray crystallography was also utilized to further understand the nature of interaction between SMA and Fe(III).

Bottle test results show the formation of a viscoelastic gel at different pH in the presence of Fe(III)and VES. IR results express that the interaction of SMA and Fe(III) occurs through the amide group in the SMA which is existed in the headgroup of tested VES too. These results confirms previous observations that the interaction of amide part of the VES with Fe(III) results in screening the repulsion forces between surfactant head groups and formation of wormlike micelles that is the primary reason for increase in the viscosity. Results of continuous variation method on SMA and Fe(III) also confirm the 1:1 stoichiometry in their interaction which are in agreement with the results of our previous study on stoichiometry of VES and Fe(III) interaction.

The present paper is the first mechanistic attempt to characterize and understand the nature of a VES-based system interaction with Fe(III) by using a model compound that has the same headgroup as tested VES. The findings of the present study can be utilized to further investigations of the effects of additives on the performance of VES- based systems.

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