Produced water from gas fields is discharged at sea or, in some cases, re-injected into deeper formations of the sub-soil, once hydrocarbons (HC) have been eliminated.
The target standard for the oil content of produced water discharges is 40 mg alkanes per liter, for the North East Atlantic offshore installations. Gravity separation processes act only on the fraction of HC in suspension and enable the compliance to this standard, but with difficulty.
Prompted by the desire to improve the quality of water discharged from its offshore production installations, Elf Aquitaine instigated research into a technique less sensitive to the quality of emulsions and capable of efficiently reducing dissolved HC too. A process based on extraction was developed and installed on the K6P platform offshore the Netherlands. Application of this process enables condensates to be recovered and does not therefore generate any atmospheric pollution.
Its main application concerns the process water coming from TEG and DEG regenerators which are located on treatment platforms where utilities are available.
PROCESS DES CRI PTION The produced water to be treated is sent to the top of a column where dissolved and undissolved hydrocarbons are stripped by steam generated in a reboiler.
The overhead stream of the column is condensed in an aircondenser and sent to a three-phase separator:
the liquid hydrocarbon phase is recycled to the condensate production through the closed drain system,
the condensed water is recycled to the top of the stripper.
No atmospheric pollution is generated by the unit since no gaseous effluent is produced.
The treated water is recovered at the bottom of the column and directly discharged at sea, after heat exchange with the inlet water stream.
DESCRIPTION OF THE INDUSTRIAL UNIT Site conditions All equipment is skid mounted and installed on the K6P platform, operated by Elf Petroland and which treats the gas from the K6 block (Netherlands). This platform is designed to treat 7 MNm3/d of gas coming from various satellites. The treatment consists is drying the gas in a conventional TEG unit. The DEG used for hydrate inhibition in the field is regenerated on the platform.
The water recovered from the DEG and TEG regeneration units is treated in the steam stripping unit before discharge at sea. This water contains undissolved hydrocarbons and is saturated with aromatics. The unit is designed to discharge at sea treated water with a residual hydrocarbon content below 40 mg/l under normal operating conditions.
Basic data The unit has been designed in accordance with the basic data as indicated below: a Effluent to be treated - total water flowrate: 800 kg/h - total HC content: 3000 mgb, of which