The separation of oil and water phases is one of the most common and least understood processes in a production facility. As fluids flow into the bottom of the well bore, up the tubing, and through surface chokes and equipment, the oil and water are thoroughly mixed. The liquid must be eventually routed to a vessel where it is separated into a continuous oil phase containing dispersed droplets of water (sometimes referred to as an emulsion) and a continuous water phase containing dispersed droplets of oil. These are then routed to oil treating and water treating systems, respectively.
The vessels which perform this separation are usually called 'three-phase separators' where there is a significant amout of gas to be separated from the liquid in the same vessel or 'free water knockouts' where there is little or no gas to be separated. In some areas of the world the term free water knockout refers to a vessel where there is very little gas to be separated and the gas which is separated is recombined with and flows out the oil outlet. Other names which are sometimes used to describe equipment performing this initial separation of the liquid phases are wash tanks, settling tanks, gun barrels, etc.
The purpose of this paper is to describe the process going on within these vessels and to discuss the validity of different techniques for choosing a vessel size.
Figure 1 shows a typical horizontal three-phase separator. Fluid enters the vessel and hits an inlet diverter where the majority of the gas is separated. The liquid falls to below an oil/water interface where the liquid is 'water washed' before rising through the interface. The oil and water droplets entrained in it then flow horizontally to the oil weir and a level controller regulates the rate at which it leaves the vessel. The water continuous phase and other oil droplets entrained in it flow horizontally to the water outlet, The rate at which it is discharged is regulated by an interface controller. As the oi1 and water continuous phases flow the length of the vessel gravity forces cause the water droplets to settle perpendicular to the bulk flow in the oil continuous phase and the oil droplets to rise perpendicular to the bulk flow in the water continuous phase.
Similarly, liquid droplets in the gas fall perpendicular to the bulk flow of the gas phase. Figure 2 shows a horizontal free water knockout where very little gas is expected and the gas is recombined with the oil. As can be seen, the oil/water separation mechanics are identical to Figure 1.
