(1) (n.) Well Completions, Well Workover and Intervention, Drilling Fluids. A chemical used to bind metal ions to form a ring structure. Chelating agents stabilize or prevent the precipitation of damaging compounds. In the oil field, chelating agents are used in stimulation treatments and for cleaning surface facilities. They are also used to treat or remove scale or weighting agents in reservoir drilling fluids. During acid or scale-removal treatments, various compounds may be dissolved in the treatment fluid. As the acid reacts and the pH increases, reaction products may precipitate as a gelatinous, insoluble mass. Should this occur within the formation matrix, it is almost impossible to remove and permanent permeability damage may occur. Chelating agents prevent precipitation by keeping ions in a soluble form until the treatment fluid can be flowed back from the formation during cleanup. Typical oilfield chelating agents include EDTA (ethylenediamine tetraacetic acid), HEDTA (hydroxyethylenediamine triacetic acid), NTA (nitriolotriacetic acid) and citric acid. See: Acetic acid, chelation, sequestering agent. (2) (n.) Well Workover and Intervention, Well Completions. A chemical added to an acid to stabilize iron. The injected acid dissolves iron from rust, millscale, iron scales or iron-containing minerals in the formation. Iron can exist as ferric iron [Fe+3] or ferrous iron [Fe+2]. If the iron is not controlled, it willprecipitate insoluble products such as ferric hydroxide and, in sour environments, ferrous sulfide [FeS], which will damage the formation. Chelating agents associate with iron [Fe+3 or Fe+2] to form soluble complexes. Citric acid, acetic acid and EDTAare effective chelating agents and can be used at temperatures up to 400oF [204^oC]. Syn: Sequestering Agent. See: Reducing Agent