Oil and gas production handbook An introduction to oil ... - ABB Group

As an alternative, when production is mainly gas, and remaining liquid
droplets have to be separated out, the two-phase separator can be a knockout
drum (K.O. drum).
4.2.5 Coalescer
After the third stage separator, the oil can go to a coalescer for final removal
of water. In this unit, water content can be reduced to below 0.1%. The
coalescer is completely filled with liquid: water at the bottom and oil on top.
Internal electrodes form an electric field to break surface bonds between
conductive water and isolating oil in an oil-water emulsion. The coalescer
field plates are generally steel, sometimes covered with dielectric material to
prevent short-circuits. The critical field strength in oil is in the range of 0.2 to
2 kV/cm. Field intensity and frequency as well as the coalescer grid layout
are different for different manufacturers and oil types.
4.2.6 Electrostatic desalter
If the separated oil
contains unacceptable
amounts of salts, they
can be removed in an
electrostatic desalter
(not used in the Njord
example). The salts,
which may be sodium,
calcium or magnesium
chlorides, come from the reservoir water and are also dissolved in the oil.
The desalters will be placed after the first or second stage separator
depending on GOR and water cut. Photo: Burgess Manning Europe PLC
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 4,000 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.
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