Application of chromatography and mass spectrometry to the determination of chemical additives in oilfield fluids

Abstract

Corrosion of steel structures is a growing concern within the oil and gas industry. One of the most severe cases of corrosion is that of the internal surface of steel transmission pipelines. Such structures are constantly in contact with crude oil, unrefined natural gas and production fluids such as produced waters and brines. Certain corrosive compounds can be naturally found in such matrices such as carbon dioxide (CO₂), hydrogen sulfide (H₂S) and organic acids.

The most cost effective means of preventing internal pipeline corrosion is the use of chemical corrosion inhibitors (CIs). These will form an impermeable barrier between the pipeline surface and the flowing matrix by adhering to the already corroded pipeline. In most instances, transmission pipelines are located in remote areas where constant monitoring of the rate of corrosion is impossible. This has led to the need of developing an offline detection method for the exact quantification of un-adsorbed CIs in all oilfield production fluids at sub ppm concentrations.

To achieve this, a model CI formulation comprising of four quaternary ammonium species, four imidazoline based species, one aminopropionic acid and two pyridine based corrosion inhibitors was prepared in methanol and used for qualitative and quantitative analysis using high pressure liquid chromatography-mass spectrometry (HPLC-MS), ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS(/MS)) and ultra-high performance supercritical fluid chromatography-mass spectrometry (UHPSFC-MS(/MS)). Use of UHPLC and UHPSFC as the methods of chromatographic separation, coupled with positive ion electrospray ionisation (+ve ion ESI) tandem mass spectrometry have afforded sub parts per million (ppm, <0.5 ppm) detection limits for all corrosion inhibitors in neat solvent.

The UHPSFC and UHPLC methods have decreased the analysis times by a factor of 4 when compared to HPLC. Furthermore, using UHPSFC has eliminated the need of a complex sample preparation step prior to analysis, especially in the case of crude oil. CIs in produced water can also be quantified by UHPLC-MS in the sub ppm levels, since it was shown that the high salt concentrations found in produced waters would not affect the chromatographic behaviour of the CIs.

The combination of the two modern chromatographic methods will allow the rapid determination of residual corrosion inhibitors in the oil flow, so dosing can be adjusted; but also give a real-time calculation of the discharged corrosion inhibitors in produced waters.

It was also shown that, by altering the position of the UHPLC autosampler injector, it was possible to analyse both the hydrocarbon and aqueous phases when found in the same chromatographic vial. Using this approach, an initial effort was made to determine the partition behaviour of the CIs of interest when added to a 1:1 mixture of surrogate oil and simulated brines. It was found that the size of the hydrophobic ‘tail’ will determine the partition behaviour of each CI. A form of synergism with respect to the partition behaviour of CIs having the same ‘tail’ was also observed.

Furthermore, a novel atmospheric pressure ion source called UniSpray was compared to the standard Waters ESI ion source on the UHPLC and UHPSFC instruments. Positive ion ESI and UniSpray mass spectra were recorded for all experiments irrespective of the nature of the sample matrix. The sensitivity of a novel ion source was compared to that of a standard Waters ESI ion source. Using the UniSpray ion source introduces approximately a 6-fold increase in sensitivity compared to ESI if UHPLC is used. A 60-fold increase in sensitivity can be achieved when the UniSpray ion source is coupled with UHPSFC.

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