TABLE OF CONTENTS The function of all water treating equipment is to cause the oil droplets, which are dispersed in the water continuous phase, to separate and float to the surface of the water so they can then be removed. In gravity separation units, the difference in specific gravity causes the oil to float to the surface of the water. The oil droplets are subjected to continuous dispersion and coalescence during the trip up the well bore through the surface chokes, flow lines, control valves, and process equipment. When energy is put into the system at a high rate, the drops are dispersed to smaller sizes. When the energy input rate is low, small droplets collide and join together in the coalescence process.
The three basic phenomena that are used in the design of common produced water treating equipment are gravity separation, coalescence, and flotation. Dispersion also affects the design but to an unpredictable degree. In the past filtration has been tried, but, due to high maintenance costs, has been found to be unsatisfactory.
Most commonly used water treating equipment depends on the forces of gravity to separate the oil droplets from the water continuous phase. The oil droplets, being lighter than the volume of water they displace, have a buoyant force exerted upon them. This is resisted by a drag force caused by their vertical movement through the water. When the two forces are equal, a constant velocity is reached, which can be computed from Stokes' law as
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