Evaluation of analytical methods for determination of kinetic hydrate inhibitor (KHI) in produced waters

Hydrate inhibitors are one of many field chemicals used in the oil and gas operation to ensure uninterrupted flow of fluids in the pipelines. Thermodynamic inhibitors such as methanol and monoethylene glycol (MEG) are commonly used as field proven products. The use of kinetic hydrate inhibitors (KHIs) is becoming popular due to reduced capital expenditure and operational costs. There is limited information on analytical methodologies for determining KHIs. This paper evaluates the analytical methods for the determination and characterization of a hyperbranched poly (ester amide)-type KHI in the produced water (PW) from oil and gas operations. Methyl orange ion pair extraction and iodine-complexation methods were evaluated as two alternative colorimetric methods for quantitative analysis of the residual KHI in PW generated in one of the Qatari gas fields in the Gulf region. Colorimetric measurements showed that KHI-iodine complexation has two orders of magnitude higher response (response factor 0.02±0.001) than the methyl orange ion-pair extraction method (response factor 0.0005±0.0001). Interferences to the colorimetric methods due to brine matrix, the corrosion inhibitors (CIs), organic components, and sulfide can be minimized for the iodine-complexation method within ±10% relative error with proper dilution of the sample. Size exclusion chromatography (SEC) equipped with refractive index detector (RID) and charged-aerosol detector (CAD) was evaluated as an alternative to the colorimetric methods. The ability of SEC to characterize the shift in molecular weight distribution (MWD) of a selected KHI product that was subject to separation by heated centrifugation was demonstrated. The SEC-RID showed linear response over the range of 0-10 mg/mL KHI while the SEC-CAD was more sensitive with low dynamic range (<1 mg/mL). At present, the SEC can determine KHI in PW semi-quantitatively within 10-20% accuracy with both detectors. The SEC composition of the selected KHI (1.5 wt% in brine matrix) solution showed predominantly mono-modal size distribution in the chloroform extract with an average MW of 2156 Da. The SEC revealed high-molecular-weight fraction (average MW 2993 Da) of KHI was left in the bottom portion of the heated and centrifuged solution leaving the light-molecular-weight size fraction (average MW 1262 Da) in the supernatant. These investigations provide some basis to the KHI analyses in PW.