Evaluation of Agents for Preventing Precipitation of Ferric Hydroxide From Spent Treating Acid
Authors: Crowe C.W. 
Published: December 2007

Evaluation of Agents for Preventing Precipitation of Ferric Hydroxide Precipitation of Ferric Hydroxide From Spent Treating Acid

Summary
Studies were performed over a temperature range of 100 to 400 degrees F [38 to 204 degrees C] to evaluate the effectiveness of commonly used additives for preventing precipitation of ferric hydroxide from spent treating acid. precipitation of ferric hydroxide from spent treating acid. Citric acid, ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and crythorbic acid were all shown to be effective iron (Fe) stabilizers at temperatures up to at least 400 degrees F [204 degrees C]. In most cases, efficiency increased with elevated temperature. In contrast, acetic acid performed poorly above 125 degrees F [52 degrees C]. When compared on a weight basis. erythorbic acid was the most efficient agent tested, stabilizing nine times as much iron as citric acid. Unlike the other iron stabilizers, which function by complexation. erythorbic acid prevents ferric hydroxide precipitation primarily by reducing ferric FE(III prevents ferric hydroxide precipitation primarily by reducing ferric FE(III ) iron to the ferrous Fe(II) form.

Introduction
Formation damage from ferric hydroxide precipitation is a potential problem during any acidizing treatment. Acid readily dissolves Fe scale in pipeline and equipment and attacks Fe-containing minerals present in the formation. This dissolved Fe remains in solution until the acid spends. As the pH rises above, Fe present in the FE(III) oxidation state precipitates as gelatinous ferric hydroxide. This can cause serious formation damage. Ferric hydroxide precipitation can be prevented by adding Fe stabilizers to the acid. Two types of Fe stabilizers presently are available. The most commonly used type presently are available. The most commonly used type employs a complexing agent or mixtures of complexing agents to solubilize the Fe. Ferric hydroxide precipitation also can be prevented by using reducing agents that precipitation also can be prevented by using reducing agents that convert Fe(III) to the more soluble FE(II) form.

Oxidation State of Dissolved Fe
Fe dissolved during an acidizing treatment can exist in either the Fe(III) or Fe(II) oxidation state. Upon spending of the acid, that Fe present in the FE(III) form will begin to precipitate at a pH of about 2.2. On reaching a pH of 3.2, essentially all of the dissolved FE(III) will have precipitated unless an effective Fe stabilizer is used. In contrast, FE(II) hydroxide will not precipitate below a pH of 7.7. Since spent acid usually reaches a maximum pH of about 5.3, precipitation of FE(II) hydroxide is rarely pH of about 5.3, precipitation of FE(II) hydroxide is rarely a problem. Consequently, one needs only to be concerned about stabilizing the FE(III) dissolved by the acid. Previous studies have shown that most Fe, dissolved by treating acid, is present in the FE(II) oxidation state. 1 3 It has been estimated that, on the average, the Fe(II) to FE(III) ratio of spent acid is about 5:1. This varies greatly, however, depending on well conditions and the type of formation being treated.

Well Conditions Affecting Fe Precipitation
When acidizing sandstones, live acid returns often are observed following the treatment. However, this should not be interpreted to mean that no Fe stabilizer is needed. While much of the acid in the area around the wellbore does remain unspent, a zone of totally spent acid will be formed at the leading edge of the reaction front. This is illustrated in Fig. 1. In this area potentially damaging Fe(III) hydroxide precipitation probably will occur unless an Fe stabilizer is used. In addition, the acid at this leading edge usually contains a greater proportion of dissolved Fe, since it is the first injected and contains most of the Fe resulting from dissolution of scale in the pipe. Because of this high concentration of Fe in the spearhead acid, some operators make it a practice to circulate the first one to two tubing volumes of acid from the well to prevent injection of this Fe-laden fluid into the formation. In any case, a higher concentration of Fe stabilizer probably should be used in the acid initially entering the formation. Other well conditions also can affect the precipitation of Fe(III) hydroxide by shifting the ratio of FE(III) to Fe(II). For example, rust or scale formed by oxygen corrosion will be largely of the FE(III) type and. when dissolved by acid, will present a potentially more serious precipitation problem. As a result, it is advisable to use precipitation problem. As a result, it is advisable to use higher concentrations of iron stabilizer when treating through rusty pipe or when treating water injection wells where oxygen corrosion is suspected. Producing wells usually present less of a problem since most Fe scales formed in the pipe are of the FE(II) type. Another factor tending to minimize ferric hydroxide precipitation is the reduction of FE(III) by metallic Fe. precipitation is the reduction of FE(III) by metallic Fe. A certain amount of the dissolved Fe(III) is reduced as the acid travels down the pipe as a result of the following reaction.
2Fe+++ +Fe-3Fe++....................... (1)
JPT
P. 691