Quantifying total iron in brine is critical in order to mitigate its unfavorable effects. An API-sponsored work group has developed a robust field method for determining the levels of iron contamination in all oilfield completion brine, including zinc-based brine. This colorimetric, semi-quantitative method is based on chemistry involving acidification, peroxide oxidation, and thiocyanate complex formation. The iron content is quantified by comparing the intensity of the resulting colored complexes to standards. Using newly-developed, commercially available vacu-ampoule test kits (Figure 1), this assay is quick and particularly user-friendly, and it is easily implemented in the field. It is anticipated that this method will be incorporated into the API Recommended Practice 13J, Testing of Heavy Brine.
Accumulation of iron salts in a brine completion fluid can lead to significant formation damage and greatly affect the productivity of a well. In addition, iron can cause cross-linking and gelling of polymers and increase the stabilization of crude/brine emulsions. Quantifying total iron in brine is critical in order to mitigate its effects.
Iron contamination in oilfield brine typically is a result of corrosion processes of iron-containing metallic components and equipment. This can occur in both aerobic and anaerobic environments, either electrochemically or microbiologically-induced. In the corrosion process, metallic iron is first converted to Fe+2 [the ferrous cationic species] with the loss of two electrons. Fe+2 can be converted to Fe3 [the ferric cationic species] with the loss of an additional electron. The electron acceptor depends on the environment and the configuration of the system. Generally, Fe+2 salts are water soluble, and Fe+3 salts are water insoluble.
In 1994, Subcommittee 13 Task Group 6 convened a Work Group to develop a field-friendly assay for iron quantification in heavy brine that would be effective across the full range of halide and organic brine. Inductively Coupled Plasma (ICP) and Atomic Absorption (AA) techniques with matrix matching work well, but these methods require expensive equipment, high levels of chemical expertise, large and controlled physical settings, and extensive laboratory manipulation. Neither of these techniques is readily amenable to field application.
Prior to the development of the new technique, the Work Group evaluated a number of alternative methods. Many of these are familiar to the oilfield, water, and wastewater industries, but do not work well in brine containing even minor levels of zinc bromide. This includes the standard self-filling ampoules (1,10-phenanthroline chemistry) iron assay commonly used for low-density brine and wastewater analyses. Other techniques considered, including reactive colorimetric strips, were shown to have poor accuracy below 75 ppm iron, to have a high degree of sensitivity to moisture and temperature, or to require development with strong acids. A proposed spot test showed poor reproducibility in round robin exercises.
The limitations detailed above were addressed using a spectrophotometric technique. The sample was acidified, oxidized with peroxide, and complexed with thiocyanate. A quantitative result was determined using a hand-held single-wavelength spectrophotometer with blanks and a calibration curve. Although the technique addressed the zinc issue, the extensive sample, standard, and reagent preparation made this technique difficult for field applications.