Field-Proven removal of dissolved and dispersed hydrocarbons at commercial scale from offshore produced water by the macro porous polymer-extraction technology

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Abstract
During offshore production of gas, condensate and oil, water is co produced in significant amounts and discharged into the sea. For dispersed oil (“free oil”) emission standards of 40 ppm or higher are applied, depending on local legislation. However for dissolved hydrocarbons such as Benzene, Toluene, Ethyl, Ethylbenzene, Xylene etc. (Aromatics), Polycyclic Aromatic Hydrocarbons (PAH’s), alkylphenols no emission limits are set. Dissolved hydrocarbon concentrations can go up to 100 ppm level in oil produced water and 500 –1000 ppm levels in gas produced water. Both Oil & Gas Industry and Governments in various countries have addressed this issue and set targets but despite considerable effort no agreed reductions have been obtained, mainly due to the lack of suitable and reliable technologies. Within the framework of OSPAR (Oslo Paris Convention for the Protection of the Maritime Environment of the North East Atlantic) alternative techniques (55), have been evaluated by the Dutch Oil & Gas Industry in the course of the nineties. In this study Macro Porous Polymer Extraction (MPPE) came out as a promising option. Subsequent testing and evaluation by the ERT/Orkney Water Technology Center, and offshore field tests of NAM (Shell/Exxon) and Statoil Åsgard A, has led to the first commercial MPPE offshore applications in 2002. Removal efficiencies of 99% have been reported on both aromatic (BTEX) and polycyclicaromatic hydrocarbons (PAH’s). For dispersed aliphatics 50 to 99% removal has been obtained depending on the composition. Next to the separation performance the main issue in offshore application is the effect of the water composition (water environment) on the steady long term separation performance. It shows that the presence of salt, heavy metals, alcohols (glycols) and surfactant containing chemicals (like corrosion, scale inhibitors etc.) do not negatively influence the separation performance, while solids can be adequately dealt with. There are strong indications that offshore produced water with temperatures up to 60o C can be treated with the MPPE technology. For Gas and Gas condensate platforms the MPPE unit size is small enough for installment on existing platforms. For oil platforms this is also considered for streams up to 300 m3 /hr while sizes for 1000 m3 /hr levels are challenging and currently being investigated.

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