Use of Highly Acid-Soluble Chelating Agents in Well Stimulation Services

Abstract

Abstract 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. Two different types of chelating agents are in use: polycarboxylic acids (including polyaminopolycarboxylic acids) and phosphonates. 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 acids (DTPA) also are in use. Each of these materials has problems: 1) EDTA has low solubility in hydrochloric acid and is not readily biodegradable in standard laboratory tests. 2) NTA is acid soluble and biodegradable, but has a lower stability constant for iron than EDTA (or DTPA) and is considered to be an animal carcinogen. This report describes the search for the "universal" or "ideal" chelating agent for use in oilfield services. The materials evaluated include hydroxy-aminopolycarboxylic acids (HACA) such as hydroxyethylethylenediaminetriacetic acid (HEDTA) and hydroxyethyliminodiacetic acid (HEIDA) as well as other types of chelating agents.