Accurate and precise analysis of scale inhibitor residuals is important to managing oilfield squeeze treatments. Phosphonate scale inhibitors are effective for the prevention and control of scale problems in oilfields. The traditional analytical technique for monitoring phosphonate scale inhibitor residuals is inductively coupled plasma optical emission spectroscopy (ICP-EOS). ICP-OES is simple and has been used for monitoring squeeze treatments for decades. However, it can only measure the total phosphorus in the system and is unable to differentiate the different forms of phosphonates in commingled samples.
This paper presents a novel technique using ion chromatography and mass spectrometry (IC-MS and IC-MS/MS) for monitoring and quantifying different phosphonate scale inhibitors with high sensitivity and specificity. Ion chromatography efficiently separates phosphonate ions from other salt ions, and mass spectrometry speciates and quantitates molecular ions or fragment ions of each phosphonate. Previous work in our group (Zhang, et.al., 2014) had shown that IC-MS could be used to differentiate two phosphonates in a squeeze treatment using the characteristic molecular ions of each phosphonate. As the complexity of the squeeze treatment increases with the addition of other phosophates to the local oilfield, the development of an advanced IC-MS/MS method has been required to differentiate up to four phosphonates in a single commingled sample. This innovative technique has a detection limit of <1 ppm for each phosphonate in the mixture. The technique has been validated using both synthetic brine and field brine. Solid phase extraction cleanup work has also been performed to improve the capability of the technique in high-salinity brines. This novel analytical method will provide a powerful tool in squeeze scale management for subsea and deepwater oilfields.