Oil and grease in produced water is not a chemical substance. It is defined by the method specified to measure it. There are several instrumental methods for measuring oil in produced water. None of them measure all the organic compounds in the water. Comparing what the commercially available methods actually measure will illustrate the problem of interpreting oil in water (OIW) analysis. Organic compounds in produced water exist as dispersed droplets or dissolved compounds. Droplets are crude that was dispersed in the water from production operations and contain the constituents in the produced oil. In addition, dispersed droplets of production or workover chemicals may also be present.
The OSPAR 2005-15 method is designed for produced water and other types of waste water discharged from gas, condensate and oil platforms and allows the determination of the dispersed oil content in concentrations above 0,1 mg/l. The method is a modification of ISO 9377-2 in order to include the determination of certain hydrocarbons with boiling points between 98 and 174°C.
Dispersed oil content by GC-FID
The sum of the concentrations of compounds extractable with n-pentane, not adsorbed on Florisil and which may be chromatographed with retention times between those of n-heptane (C7H16) and n-tetracontane (C40H82) excluding the concentrations of the aromatic hydrocarbons toluene, ethyl benzene and the three isomers of xylene.