Page 52 - Environmental Control in Petroleum Engineering

40 Environmental Control in Petroleum Engineering

The liquid streams exiting the free water knockout are generally in
the form of an emulsion. These emulsions normally require additional
treatment. Emulsions can be broken by adding demulsifiers (chemicals
that cause the water drops to coalesce), by heating the emulsion, by
passing an electrical current through the emulsion (Fang et al., 1991),
or with combinations of these processes. These processes break the
emulsion, allowing the droplets to grow and settle in the gravitational
field. This settling is driven primarily by buoyancy and impeded by
viscous drag, as described by Stokes law. Chemicals used to break
emulsions include surfactants, alcohols, and fatty acids.
The efficiency of the separations equipment in breaking emulsions
depends on the droplet size and density difference between the oil and
water. Small droplets are much more difficult to separate. The droplet
size depends on the interfacial tension between the oil and water and
the shear history of the fluid. If the fluid flows through many shearing
devices at high velocity, e.g., chokes, valves, or pumps, the oil can
be shorn into smaller and smaller droplets. Emulsions are stabilized
by many of the treatment chemicals added to the production stream,
making separations even more difficult.
The hydrocarbon levels in the produced water after exiting dernulsi-
fication equipment may still be too high for unrestricted discharge.
Advanced water treatment methods are available that can further lower
the hydrocarbon levels. These advanced methods are discussed in
Chapter 6.

2.2.2 Produced Water

The largest volume waste stream in the upstream petroleum industry
is produced water. For mature oil fields, the volume of produced water
can be several orders of magnitude greater than the volume of pro-
duced oil. The environmental impact of produced waters arise from
its chemical composition. Produced water contains dissolved solids and
hydrocarbons (dissolved and suspended), and is depleted in oxygen.

Dissolved Solids

Most produced water contains a variety of dissolved solids. The
most common dissolved solid is salt (sodium chloride). Salt concentra-
tions in produced water range between a few parts per thousand to