Page 138 - Petroleum and Gas Field Processing

provide the best data for design. The next best source of data could be
obtained from nearby fields. If such data are not available, the minimum
water droplet size to be removed from the oil is taken as 500 mm.
Separators design with this criterion have produced oil and emulsion
containing between 5% and 10% water. Such produced oil and emulsion
could be treated easily in the oil dehydration facility, as discussed in
Chapter 5. Experience has also shown that three-phase separators designed
based on the 500-mm water droplet removal produces water with a
suspended oil content that is below 2000 mg/L. This produced water must
be treated before it is disposed of, as described in Chapter 7.
Another important aspect of separator design is the retention time,
which determines the required liquid volumes within the separator. The oil
phase needs to be retained within the separator for a period of time that is
sufficient for the oil to reach equilibrium and liberates the dissolved gas.
The retention time should also be sufficient for appreciable coalescence of
the water droplets suspended in the oil to promote effective settling and
separation. Similarly, the water phase needs to be retained within the
separator for a period of time that is sufficient for coalescence of the
suspended oil droplets. The retention times for oil and water are best
determined from laboratory tests; they usually range from 3 to 30 min,
based on operating conditions and fluid properties. If such laboratory data
are not available, it is a common practice to use a retention time of 10 min
for both oil and water.

4.5 SEPARATOR SIZING EQUATIONS AND RULES

In this section, the equations and rules used for determining the
dimensions of horizontal and vertical three-phase separators are developed
and presented. It should be realized that these equations are generally used
for preliminary sizing of the separators. Other important aspects of the
design should not be ignored. The changes in operating conditions, such as
production rates, gas–liquid ratio, water–oil ratio, fluid properties,
pressure, and temperature, over the life of the field should be incorporated
in the design. For new-field development, there is always some degree of
uncertainty in the available data and information. This should be an
integral part of the facility design. Also, cost, availability, and space
limitation could affect the design and selection of equipment.
The sizing procedure is generally similar to that for two-phase
separator with the exception that the separation of water from oil, and oil
from water are additional constraints for three-phase separators.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.