Chelating agents are materials that are used to control undesirable reactions of metal ions. In oilfield chemical treatments, chelating agents are frequently added to stimulation acids to prevent precipitation of solids as the acid spends on the formation being treated. These precipitates include iron hydroxide and iron sulfide. In addition, chelating agents are used as components in many scale removal/prevention formulations. This current study is related to development of improved and biodegradable polyaminocaboxylic acid chelating agent formulations.
Chelating formulations based on ethylenediaminetetraacetic acid (EDTA) have been used extensively to control iron precipitation and to remove scale. Formulations based on nitrilotriacetic acid (NTA) and diethylenetriaminepentaacetic acid (DTPA) also are in use. Each of these materials has problems: 1) EDTA and DTPA have low solubility in hydrochloric acid and are not biodegradable. 2) NTA is acid soluble and biodegradable, but has a lower stability constant for iron than EDTA (or DTPA) and may be classified as an animal carcinogen. This paper describes the development and field-testing of formulations containing hydroxyethyliminodiacetate (HEIDA) salts for use in oilfield stimulation fluids.
Sodium HEIDA can be used in all of the formulations requiring EDTA-type chelating agents, including those for iron control during acidizing, scale removal treatments, and well cleanout operations. HEIDA biodegrades much more easily than EDTA or DTPA and has a much smaller environmental footprint (biodegradation, less material and less toxic corrosion inhibitors) than these chemicals. HEIDA is more soluble in HCl than EDTA, and therefore can control more Fe3+ than EDTA. The new formulation can be used to dissolve calcite and gypsum scale. It also can be used in acidic or alkaline well cleanout formulations with surfactants and mutual solvents. Field trials have shown that HEIDA formulations can be 1:1 substitutes for EDTA in oil and gas well treatments.