Page 235 - Petroleum and Gas Field Processing

for enhanced recovery operations. This is because the hydrocarbon
droplets cause severs plugging of the formation. In all cases, the produced
water must be treated to lower its hydrocarbon content to acceptable
limits. For the heavy oil field, produced water may be used to generate the
steam needed for oil recovery. In this case, additional chemical treatment
will be needed to reduce the concentration of the salt and other minerals
to make the water quality adequate for steam generation.
The purpose of this chapter is to present the concepts and procedures
used for selecting and sizing the equipment used for removal of oil from
the produced water.

9.2 PRODUCED AND TREATED WATER QUALITY
The quality of treated water (i.e., the maximum allowable oil
concentration and maximum allowable oil droplet size) is determined to
meet water-injection or disposal requirements. From an environmental
point of view, it should be desirable to remove all of the oil from the
produced water or at least allow the technically minimum possible. This,
however, can impose substantial additional operating costs. Therefore,
operators would usually provide the necessary water treatment to achieve
the maximum allowable oil content. To properly design an efficient and
economical treatment system that achieves this objective, knowledge of the
produced water quality (oil concentration and droplet size distribution) is
necessary. This is best determined from laboratory analysis of actual field
samples. Such samples, however, are not normally available, especially
when designing a treatment system for new field development.
Theoretically speaking, it is possible to determine the droplet size
distribution throughout the various components of the production system
and the separation and oil treatment equipment. However, most of the
parameters needed to solve the governing equations, especially those
involving dispersion and coalescence, are normally unknown. As discussed
in previous chapters, the design of separation and oil treatment equipment
determines the maximum oil droplet size remaining in the water. Several
attempts have been made to determine the oil concentration in water for
properly designed separation and treatment equipment. The results showed
that the dispersed oil content ranges from 1000 to 2000 mg oil per liter of
water. Unfortunately, as the water leaves the separation and treatment
equipment, it flows through various restrictions (such as valves and bends)
in the piping system before it reaches the water treatment facility. In its
journey, the oil droplets are subjected to a series of dispersion and
coalescence that makes it difficult to exactly determine the oil droplet size

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