Page 236 - Petroleum and Gas Field Processing

distribution in the water to be treated. Experience showed that a
conservative assumption for design purposes would be to represent the
droplet size distribution by a straight-line relationship between the droplet
size and cumulative oil concentration, with the maximum oil droplet
diameter being between 250 and 500 mm.

9.3 PRODUCED WATER TREATMENT SYSTEM

In general, produced water always has to be treated before it is disposed of
or injected into the reservoir. The purpose of the treatment is to remove
enough oil from the water such that the remaining amount of oil in the
water and the oil droplet size are appropriate for the disposal or injection
of the water. For example, for water disposal into underground formations
and water injection into the producing reservoir, the pore size of the
formation determines the allowable oil droplet size in the treated water.
The maximum droplet size of the remaining oil in the water should be less
than the minimum pore size of the formation to avoid plugging of the
formation by the oil droplets. For water disposal into the sea, as is
normally practiced in offshore operations, the amount and droplet size of
the oil in the water is governed by environmental constraints.
Depending on the amount and droplet size of the oil in the produced
water, the required quality of the treated water, and the operating
conditions, water treatment may be achieved through a single or two
stages of treatment. The single, or first, stage of treatment is normally
known as the primary treatment stage; the second stage of treatment is
known as the secondary treatment stage.
The equipment used for water treatment serves the function of
allowing the oil droplets to float to the surface of the water, where they
are skimmed and removed. For primary treatment, this may be achieved
by using skim tanks for atmospheric treatment or skim vessels
for treatment under pressure. Plate coalescers such as the parallel plate
interceptor and corrugated plate interceptor are used to promote
coalescence of the oil droplets to increase their size and thus speeds
their floatation to the surface. Another device, known as the SP pack,
is also used to promote coalescence of the oil droplets. For secondary
treatment, plate interceptors, SP packs, and flotation units are
normally used.
For offshore operations, water disposal must be through a disposal
pile, skim pile, or SP pile. The deck drains normally contains free oil and
must be treated before disposal. This can be done either in similar primary
treatment equipment or directly through the various disposal piles.

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