Secondary Deposition of Iron Compounds Following Acidizing Treatments
Authors: Smith C.F. , Crowe C.W. , Nolan T.J. 
Published: December 2007

Iron sequestering agents frequently are misused and overused as acid additives. The attitude "it won't hurt and it might help" is no proper basis for choosing an iron control agent because many agents themselves can cause damage following an acid treatment.

Introduction
Recently there has been increased interest in using chemical additives in acid to prevent secondary precipitation of iron compounds following the acidizing precipitation of iron compounds following the acidizing treatment. Acid readily dissolves iron scale in pipe and also attacks iron-containing minerals in the formation under treatment. This dissolved iron will remain in solution in the acid until the acid is spent. As pH of the spent acid begins to rise, the iron loses its solubility and precipitates. The precipitation of ferric hydroxide or other iron-containing compounds can seriously damage the flow channels recently opened by the acid reaction in the formation.
Simple calculations can be made to show that if the acid dissolved the rust in only 5,000 ft of old pipe, this would be enough to deposit more than 100 lb of damaging precipitant in the formation. While there is no question of the stoichemiometry of this calculation, it is an oversimplification of the conditions under which acid is used, and it does not account for the electrochemical behavior of the multiple valence states of iron under oxygen-free, or anaerobic conditions.
Iron is a definite hazard to successful acidizing in some areas, but this does not mean sequestering agents should be routinely used unless there is positive evidence of their need. Like many chemical positive evidence of their need. Like many chemical additives for acid, iron-control agents can be misused and overused with damaging results. Some agents precipitate if the expected down hole sources of iron are not present. In some cases the iron actually keeps the sequestering agent in solution in spent acid. Thus, the effective use of iron sequesting agents depends upon the chemical conditions existing down hole during acid reaction. Since it is obviously impossible to know exactly what conditions will be encountered during an acid treatment, it is doubly important that care be used in selecting acid additives based on the best information available.
In the case of iron sequestering agents there are several factors that dictate whether or not an agent should be used, and, if so, which one. Several iron-control agents are available at varying costs and each has limitations. There is no single iron-control agent available today that can economically prevent secondary iron precipitation at temperatures from 100 degrees to 250 degrees F without danger of agent precipitation if iron is not found down hole in the precipitation if iron is not found down hole in the expected quantity. This does not mean that iron sequestering agents are ineffective or not needed. It simply indicates some study should go into the decision to use an iron control agent. The thoughtless and excessive use of sequestering agents, like many other acid additives, can ruin an otherwise successful acid treatment.

The Electrochemical Nature of Iron
Underlying the need of an iron-sequestering agent is the chemical behavior of iron down hole when contacted with acid.
JPT
P. 1121