TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA new type of experimental procedure was used to study iron control chemicals (citric acid, nitrilotriacetic acid and acetic acid) in acidizing treatments at temperatures from 25 to 95 C. This procedure gives plots of iron in solution versus equilibrium pH, while previous work has generally studied iron concentration in completely spent acid solutions. Hydrochloric acid concentrations of 7.5 to 28 wt%, sodium chloride concentrations of 0 to 5 wt%, and iron (III) concentrations from 1,000 to 10,000 ppm were examined with varying concentrations of commonly used iron control chemicals. A systematic study of this kind has not previously been reported in the literature. The following new results were found:1. Iron (III) hydroxide precipitated from spent acid at much lower values of pH than is generally believed in the literature. The general belief is that iron (III) hydroxide precipitation begins at a pH of about 2.2 and is complete at a pH of 3.3. From experimental results, precipitation begins at pH values of about 1 and is nearly complete by pH 2 at 25 C. 2. As the temperature was increased, iron (III) became significantly less soluble in the spent acid. This means that iron (III) hydroxide precipitation is potentially more damaging to the formation than generally believed. 3. Time-dependent iron precipitation can occur in the absence of iron control chemicals. 4. Time-dependent precipitation of ferric/calcium complexes with nitrilotriacetic acid (NTA) and citric acid was found. In partially spent acid, precipitation of previously unreported complexes containing iron/calcium/ chelating agent in a 1/1/1 mole ratio was observed. This behavior is important because of potential formation damage during the acidizing treatment itself, or during shut-in of the well after stimulation. 5. Synergism with citric acid/acetic acid mixtures was found. These two iron control chemicals were able to prevent iron precipitation better in combination than individually.

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