Evaluating the corrosion index of DTPA at different conditions - key of improving the performance of chelating agents in field treatments

Solutions that rely on chelating agent are widely implemented in upstream oil and gas field processes, due to their unique performance and their ability of capturing specific ions. There is a need to fully understand the impact of chelating agents on operational tools at different conditions. A variety of complications in processing chelating agents are related to their corrosion behavior and iron control capability. In this work, the static corrosion index of chelating agents (DTPA, EDTA, and GLDA) was investigated experimentally over the pH range of 4–12 using different basic media, where the impact of the courter ion (potassium or sodium) is in focus. The impact of preparing the chelating solutions differently on the rate of corrosion was explored. Essentially, the aim was to adjust the pH differently (upwards or downwards, referred to as “Basification” and “Acidification” lines, respectively). The tests were conducted using aging cell isolated by Teflon liner at 500 psi and different temperatures (200 °F, 250 °F and 310 °F). The results show that the dissolution rate of the iron by the chelating agent was influenced considerably by preparation route, that is solutions with the same pH value do not result in the same dissolution rate. For instance, at pH 4.4 and 310 °F test conditions, the corrosion index for the DTPA-K (Basification) was approximately 0.20 lb/ft², meanwhile, the prepared sample in acidification line (DTPA-K) yielded 0.15 lb/ft² at same test conditions. Similar observation was exhibited for the other samples. The reaction rate in the upwards preparation process (Basification line) was higher than the rate yielded in the downwards preparation process (Acidification line). We propose a mechanism that accounts the observations that is rooted in the ability of binding external species such as Na⁺, K⁺ and Cl⁻ to the central system (either the chelating agent itself or the Fe2⁺ chelating agent adduct). The mechanism is supported by a series of ab initio calculations.