Abstract
The use of aminopolycarboxylic acids (APCAs) is increasing rapidly in several industries because of their unique properties of chelation and their effectiveness in high-temperature conditions. One of the major design considerations before their application is their thermal stability and their corrosivity to tubulars, especially the ones used in the oil and gas industry. Their disposal is also an active topic of discussion. The coordination bond formed between the chelator and metal ions is strong and thus can have long-lasting effects on the environment in terms of the metal's bioavailability. Therefore, its biodegradation and photodegradation must be considered. There is a lack of a single source of these major decision criteria for the selection of suitable APCAs and this paper provides an outlet for researchers and industry professionals to further their understanding of APCAs. Several types of APCAs including EDTA, DTPA, HEDTA, GLDA, NTA, MGDA, CDTA, HEIDA, EDDS, and ASDA were reviewed for their corrosion mechanisms and corrosion rates to the most common tubulars used in the oil and gas industry. In some cases, these chelating agents were implemented as corrosion inhibitors as well. The degradation of APCA was divided into three major categories: thermal-, bio-, and photo-degradation. The influence of temperature, microorganisms, and light play an important role during and post-treatment. To fully understand these degradation mechanisms, literature from several industries including medical, mining, toxicology, hydrometallurgy, materials, environmental sciences, mineral sciences, and electrochemical sciences was examined and elucidated. This paper provides a unique perspective of design considerations with the application of the frequently used APCAs. This review connects literature from several industries and can provide an important step-change in the overall understanding of APCAs from the initial design phase to their final disposal and treatment.
Highlights
Chelating agents have a wide range of applications in the oil and gas industry that involve extended exposure to harsh conditions
The main conclusions of this review paper can be summarized in the following points: (1) Acidic solutions of chelators are more corrosive compared to basic solutions of chelating agents and the corrosion rate of both cases increases as temperature increases
(2) Low carbon steel corrosion by ligands occurs through a 2step process: chelator enhanced dissolution of the iron oxide layer followed by a redox reaction between the base metal and the chelating agent
Summary
Chelating agents are multidentate organic molecules that can form two or more coordination bonds with a central metal ion. APCAs have been shown to harm certain types of bacteria by destroying cell membranes and harm plants by increasing toxic heavy metal uptake.[90] APCAs may cause water eutrophication due to the presence of nitrogen atoms in their structure that results in undesired algae blooms.[90,91] In such cases, these effects can be minimized if the chelating agent readily degrades when introduced to the environment. MGDA and HEIDA were extremely stable at temperatures up to 177 C (350 F), experiencing no degradation over 6 hours.[21] ASDA was shown to degrade with less than 10% of the APCA remaining when degraded at 149 C (300 F) for the same duration This would require the application of an APCA with adequate thermal stability for hightemperature wells since the degradation products will no longer be able to perform the function of the original ligand, such as iron control, scale removal, or matrix acidizing. Ting et al.[163] used a chelating agent-based mud acid system to stimulate a reservoir of 49 C (120 F) and were able to increase production to approximately 200%
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