Page 47 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

4.2.7 Water treatment

On an installation such as this, where the water cut is high, there will be a
huge amount of water produced. In our example, a water cut of 40% gives
water production of about 4000 cubic meters per day (4 million liters) that
must be cleaned before discharge to sea. Often this water contains sand
particles bound to the oil/water emulsion.

The environmental regulations in most countries are quite strict, for example,
in the North-East Atlantic the OSPAR convention limits oil in water
discharged to sea to 40 mg/liter (ppm).

It also places limits on other forms of contaminants. This still means that the
equivalent of up to one barrel of oil per day in contaminants from the above
production is discharged into the sea, but in this form, the microscopic oil
drops are broken down fast by natural bacteria.

Various pieces of equipment are used. This illustration shows a typical water
treatment system. Water from the separators and coalescers first goes to a
sand cyclone, which removes most of the sand. The sand is further washed
before it is discharged.

The water then goes to a hydrocyclone, a centrifugal separator that will
remove oil drops. The hydrocyclone creates a standing vortex where oil
collects in the middle and water is forced to the side.

Finally the water is collected in the water de-gassing drum. Dispersed gas
will slowly rise to the surface and pull remaining oil droplets to the surface by

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