A comparison of PAHs in produced water discharges and flared gas emissions to the ocean

Produced water represents the single largest source of marine pollution in terms of toxicity and volume from an offshore platform in the production stage of oil and gas operations. In the past few years increasing attention has focused on the presence and possible impact of polycyclic aromatic hydrocarbons (PAH) in produced water. Studies have shown that the concentration of the 16 EPA PAHs can vary from 0.7 to 100s mg/L in produced water. However, particularly on oil platforms, the flaring of produced gas can also be a significant source of PAHs. A study by the Alberta Research Council in 1996 identified many PAHs in the emissions from the flared produced gas (up to 300 mg/m³ for the 16 EPA PAHs). It is likely that much of the PAHs in the flared emissions will end up in the water and so should be assessed against the produced water input of PAHs. However, it is difficult to compare concentrations in the flared emissions and produced water outfall directly as the dispersion in each media, characteristics of the flare stack and outfall, and meteorological conditions are all factors in mass input into the water. In this study the concentration of PAHs in the emissions was converted to an emission rate and input into an air dispersion model. The maximum ground level concentrations were determined with the model and then converted to a seawater concentration using a simple equilibrium model. The produced water outfall was simulated using CORMIX and the resulting concentrations were compared with the air dispersion model results.