Treatment of wastewater from natural gas wells by a filtration process

Abstract

The FSO, Floating Storage and Offloading, is a vessel designed to store 660,000 barrels of condensate in the Gulf of Thailand. Currently, water associated with the condensate shipped to the FSO is being segregated from the condensate and is being stored as free water in dedicated tanks on-board the FSO. This amount of water can not be directly discharged overboard due to the environmental concern. This situation causes major problems, e.g. customers complain due to high water carry-over with condensate, high arsenic, mercury and hydrocarbon accumulation, difficulty in tank balancing, lost space for condensate storage, etc. In order to determine the feasibility of cleaning the ship-retained produced water, Unocal Thailand conducted pilot scale testing of a combined ultra-filtration (UF) and RO membrane system. The ultra-filtration membrane from Osmonics-Desal was pre-treated to render it hydrophilic. As a result, the UF membrane resists fouling by dispersed hydrocarbons in the water being filtered. The spiral-wound UF membrane was prone to plugging by solids in the feed water. However, a 1" desanding hydrocyclone was successful in removing most of the solids from the feed water and a 5-micron cartridge was used as a polishing filter. The UF filter removed over 98% of the dispersed hydrocarbons in its feed stream, along with essentially all of the mercury. Typical UF permeate contained 1 to 4 PPB of mercury. Water-soluble arsenic was not removed by the UF filter. The RO membrane was only partially successful, reducing arsenic by between 90 and 95% from incoming values. Although this rejection rate might be considered good, the RO permeate did not meet the 250 PPB as Unocal Thailand self-imposed limit for arsenic in discharge water. The performance life of the UF filter was limited mainly by the deposition of iron-based solids within the spiral-wound membrane. At the conclusion of testing, it was found that the UF membranes had retained over 98% of their original permeability. Some initial swelling of the RO membrane occurred due to the presence of dissolved aromatics in the produced water. The swelling reduced RO membrane permeability to about 50% of its initial value