Page 290 - Hydrocarbon Exploration and Production Second Edition

Surface Facilities 277

gas

free
gas

oil
dispersion
wet crude band

water

gas water oil wet crude
Figure 11.11 Continuous dehydration tank.

For dehydration of very high viscosity crudes, heaters can be used in combination
with dehydration tanks. The temperature to which the crude is heated is a function
of the viscosity required for effective separation.
If oil and water are mixed as an emulsion, dehydration becomes much more
difficult. Emulsions can form as oil-in-water or water-in-oil if mixed production
streams are subjected to severe turbulence, as might occur in front of perforations in
the borehole or in a pump. Emulsions can be encouraged to break (or destabilise)
using chemicals, heat or just gentle agitation. Chemical destabilisation is the most
common method and laboratory tests would normally be conducted to determine
the most suitable combination of chemicals.

11.1.2.6. De-oiling
Skimming tanks have already been described as the simplest form of de-oiling
facility; such tanks can reduce oil concentrations down to less than 200 ppm but are
not suitable for offshore operations. Plate coalescer vessels or hydrocyclones (shown
in Figure 11.13) are generally used offshore and maybe used in tandem with other
water-cleaning devices such as gas flotation units. The choice of de-oiling unit is
influenced by throughput, variability of the feed (in terms of oil content), space and
weight considerations.
Another type of gravity separator used for small amounts of oily water, the oil
interceptor, is widely used both offshore and onshore. These devices work by
encouraging oil particles to coalesce on the surface of plates. Once bigger oil droplets
are formed they tend to float to the surface of the water faster and can
be skimmed off. A corrugated plate interceptor (CPI)isshown in Figure 11.12 and
demonstrates the principle involved. Plate interceptors can typically reduce oil-in-water
content to 50–150ppm.
To reduce oil content to levels which meet disposal standards it is often necessary
to employ rather more sophisticated methods. Two such techniques which can
reduce oil in water to less than 40 ppm are gas flotation and hydrocyclone processes.