Effect of Fe III and Chelating Agents on Performance of New VES-Based Acid Solution in High-Temperature Wells
Authors: Zebarjad Fatemeh Sadat , Nasr-El-Din Hisham A. , Badraoui D. A. 
Published: March 2017

Abstract
Viscoelastic surfactant (VES)-based acid systems are used in acid-diversion applications. However, high- temperature, interaction of the VES and Fe(III) (as a contaminant), addition of alcohol-based additives, and chelating agents all interfere with the apparent viscosity of the VES-based acid and reduce its effectiveness. This paper introduces a new VES-based acid system that can be used for diversion in high-temperature formation matrix acidizing. This VES-based acid system exhibites high thermal stability in the presence of Fe(III) contamination and chelating agents. Also, this work elucidates the reaction mechanisms between VES, Fe(III), and two chelating agents (hydroxyethylethylenediaminetriacetic acid (HEDTA), and Glutamic acid diacetic acid (GLDA)) in spent acids.
To study the rheological properties of the VES-based acid, three different formulations of spent acid (20 wt% hydrochloric acid (HCl), 5 vol% VES) were examined. By comparing the apparent viscosity of the three samples as a function of temperature, the same trend (similar viscosity at same temperatures) was illustrated. Moreover, the effect of chelating agents and Fe(III) on VES viscosity in spent acids was investigated. To understand the VES interaction with Fe(III) in spent conditions, a compatibility test was conducted on the live VES-acid and Fe(III) system. The results showed that the maximum concentration of the Fe(III), which is compatible with live VES-based acid, is 5,000 ppm; however, at higher Fe(III) concentrations, the VES interacted with the Fe (III) and precipitated.
Rheological measurements were conducted on the spent VES based system with different Fe(III) concentrations as a function of temperature (80-400°F) at pH in the range of 4-5. At Fe(III) concentrations lower than 6,000 ppm, the apparent viscosity of the VES-based solutions increased in temperatures below 150°F as the Fe(III) concentration was increased. At higher temperatures (150-400°F), the maximum viscosities reduced with iron concentration, but generally they exhibited excellent thermal stability (150 cp at 400°F). The spent VES-based solution, when combined with 6,000 ppm Fe(III), entirely loses its viscosity.
Experimental results indicated that the first peak of apparent viscosity of the VES-based solution increases at low concentrations (0.010 mol/L) of the chelating agents, HEDTA and GLDA, but for both chelating agents at higher concentrations (0.053 and 0.107 mol/L), the apparent viscosity reduces. Inclusively, the apparent viscosity remained above 140 cp with the highest concentration of chelating agents in the temperature range of 80-400°F. Furthermore, both chelating agents were added (1:1 molar to Fe(III)) to VES-based acid solutions with Fe(III) concentrations of 5,000 and 6,000 ppm. The results demonstrated that the negative impact of the chelating agents on the apparent viscosity does not superimpose on the negative effect of Fe(III). Chelating agents rebuilt the viscosity of the VES-based solution with 6,000 ppm Fe(III). For the 5,000 ppm Fe(III) solution, they reduced the apparent viscosity at lower temperatures (150°F) and increased the maximum apparent viscosity over a temperature range of 150-250°F.