Metal chelating materials have been used as iron control agents in acidizing treatments for many years. There are several disadvantages associated with use of the various commercially available products. These disadvantages include low solubility and high toxicity. Furthermore, recent evidence casts considerable doubt on the effectiveness of common chelating agents for iron control in sour environments.
A relatively simple procedure will be presented that allows preparation of pure metal (calcium and iron) chelant complexes. These complexes serve as models for the in-depth evaluation of various chelating agents over a broad range of temperatures and fluid compositions and have been characterized with respect to solubility and thermal stability in spent acid fluids. The experimental results show loss of solubility of calcium complexes as well as thermal decomposition of ferrous complexes at relatively low temperatures. Implications of these results will be discussed.
The problems are exacerbated in sour environments where iron sulfide precipitation occurs at a very low pH. New evidence will be presented that shows iron sulfide precipitation will occur below the pH range where chelating agents are effective. The precipitation of iron sulfide has been found to be dependant upon formation basicity and NOT the overall acidity of the bulk fluid as commonly believed. The result implies that simple equilibrium calculations may not reliably predict the chemistry that will occur downholeand must be used with caution.
The conclusion drawn from these results is that the only effective method of preventing precipitation of iron sulfide during sour well acid treatments is to remove H2S from the fluid with sulfide scavenger products. The elimination of large quantities (6–36 kg/m3, or 50–300lbs./Mgal) of chelating agents will have significant cost savings with regards to fluid cost. The evidence provided in this paper shows that these products are not effective and even worse, may precipitate in the formation and cause severe damage