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.

Introduction

Chelating agents have been used extensively in oil/gas field treatments. See Figure 1 for structures. Primary uses include stabilization of iron, scale removal and uses as stimulation agents.

Iron Control

EDTA, in particular, is used to stabilize iron in spent HCl. Crowe1 reviewed the use of chelating agents and reducing agents for iron control. According to this author, EDTA will control precipitation of ferric and ferrous hydroxide up to 350°F. Above 250°F, its efficiency actually improves since it then acts as a reducing agent 1. It works better than citric acid/acetic acid2,3 because CaEDTA is believed to be more soluble than calcium citrate. Mixtures of citric acid and levulinic acid4 and citric acid with 5-sulfosalicylic acid5 have been developed to get around the shortcomings of citric acid, a biodegradable material. An alternate complexer is dihydroxymaleic acid6. Taylor7,8 also surveyed the use of iron control chemicals. He concluded that ferric iron would precipitate between pH 1.0–2.0, in spent acid, not between 2.2–3.3 as has been assumed from the solubility product calculations. He also teaches the use of citric/acetic acid mixtures as compared with EDTA or NTA. He claims that EDTA is not soluble (enough) in 15% HCl to control significant amounts of Fe3+.

This assertion is contrary to our practice where >3% tetrasodium EDTA (expressed as the 100% salt) is routinely used in acidizing formulations (though we agree that HCl formulations with >3% sodium EDTA are not stable if left for more than a few hours). Goughler9 advocates cleaning the tubulars prior to acidizing the formation to reduce the iron load introduced into the formation. Chelants also are used with erythorbic acid10 to stabilize iron and prevent precipitation of sulfur in sour acid and with aldehydes11 or ketones12 for the same purpose (the carbonyls react with the sulfide ions). However, Brezinski13 claims that EDTA and NTA are not stable as iron control agents in HCl/carbonate spending tests. He suggests using sulfide control chemicals.

Iron Control

EDTA, in particular, is used to stabilize iron in spent HCl. Crowe1 reviewed the use of chelating agents and reducing agents for iron control. According to this author, EDTA will control precipitation of ferric and ferrous hydroxide up to 350°F. Above 250°F, its efficiency actually improves since it then acts as a reducing agent1. It works better than citric acid/acetic acid2,3 because CaEDTA is believed to be more soluble than calcium citrate. Mixtures of citric acid and levulinic acid4 and citric acid with 5-sulfosalicylic acid5 have been developed to get around the shortcomings of citric acid, a biodegradable material. An alternate complexer is dihydroxymaleic acid6. Taylor7,8 also surveyed the use of iron control chemicals. He concluded that ferric iron would precipitate between pH 1.0–2.0, in spent acid, not between 2.2–3.3 as has been assumed from the solubility product calculations. He also teaches the use of citric/acetic acid mixtures as compared with EDTA or NTA. He claims that EDTA is not soluble (enough) in 15% HCl to control significant amounts of Fe3+.

This assertion is contrary to our practice where >3% tetrasodium EDTA (expressed as the 100% salt) is routinely used in acidizing formulations (though we agree that HCl formulations with >3% sodium EDTA are not stable if left for more than a few hours). Goughler9 advocates cleaning the tubulars prior to acidizing the formation to reduce the iron load introduced into the formation. Chelants also are used with erythorbic acid10 to stabilize iron and prevent precipitation of sulfur in sour acid and with aldehydes11 or ketones12 for the same purpose (the carbonyls react with the sulfide ions). However, Brezinski13 claims that EDTA and NTA are not stable as iron control agents in HCl/carbonate spending tests. He suggests using sulfide control chemicals.

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