Most oil wells produce saltwater along with gas bubbling out of the oil. They are separated in a long cylindrical steel tank called a separator. The separator can be either vertical (up to 12 ft or 3.7 m high) or horizontal (up to 16 ft or 4.9 m in length). Vertical separators take up less surface space, but horizontal separators have longer retention times, the time that the produced fluid is in the separator. The separator is either a two-phase separator that separates gas from liquid (fig. 1) or a three-phase separator that separates gas, oil, and water.
Fig. 1. Vertical two-phase separator. (Modified from Baker, 1983.)
If gravity readily separates the produced oil and water, the water is called free water. In contrast, an emulsion has droplets of one liquid that are completely suspended in another liquid. A water-in-oil or reverse emulsion that has droplets of water suspended in oil is the most common emulsion produced from a well. Less common is an oil-in-water emulsion that has droplets of oil suspended in water. The tightness of an emulsion is the degree to which the droplets are held in suspension and resists separation. An emulsion can be tight and resist separation or loose and readily separate.
On each separator there is an inlet for fluids from the flowline and separate outlets at different elevations for each of the separated fluids. Every separator has a diffuser section that makes an initial separation of the gas and liquid from the inlet. The gas rises to the gas-scrubbing section at the top of the separator, where most of the remaining liquid is removed from the gas before it goes out the gas outlet. The liquid falls to the bottom, where the liquid-residence section removes most of the remaining gas from the liquid before it goes out the liquid outlet.
All separators use gravity to separate gas, oil, and water. In the diffuser section of a vertical separator, the fluids from the inlet are spun around the shell of the vessel to let centrifugal force help the initial separation. In a horizontal separator, the fluids from the inlet strike a flat, metal plate or angle irons to slow and divert the flow direction to help the initial separation.
In the gas-scrubbing section, mist extractors are often used to coalesce and remove liquid droplets before the gas flows out the gas outlet. A wire mesh pad type of mist extractor uses finely woven mats of stainless steel wire packed in a cylinder. Vanes that are parallel metal plates with collection pockets for the liquid can also be used. The liquid-residence section is often a relatively empty chamber that lets gravity make the separation. Baffles, flat plates over which the liquid flows as a thin film, can be utilized. A weir is a dam in the lower, liquid-residence section of the separator that impounds liquid behind it. It aids in separation by allowing only the lightest liquid, such as oil floating on water, to flow over the weir.
Separators have liquid level controls, which are floats on the oil-gas and water-oil surfaces that control the level of those fluids by opening and closing valves. Controls on a separator include liquid level, high- and low-pressure, high- and low-temperature, safety relief valve, and safety head or rupture disc that breaks at a set pressure. A backpressure valve on the gas outlet maintains gas pressure in the separator.
A free-water knockout (FWKO) is a three-phase, horizontal, or vertical separator used to separate gas, oil, and free water by gravity (fig. 2). Water is drawn out the bottom, oil from the middle, and gas from the top.
Fig. 2. Horizontal free-water knockout (FWKO). (Modified from Baker, 1983.)
A double-barrel or double-tube horizontal separator has two horizontal vessels mounted vertically (fig. 3). The produced fluid enters into the upper vessel where it flows over baffles to make an initial gas-liquid separation. The liquid then flows to the lower vessel to complete the oil-water separation. Oil-free gas flows out the upper barrel and gas-free oil flows out the bottom. A double-barrel horizontal separator can process a higher volume of produced fluids than a single horizontal separator.
Fig. 3. Double-barrel separator
To separate or break an emulsion, the emulsion has to be heated. It is treated in a heater treater, a vertical or horizontal separator that has a fire tube in it (fig. 4) where natural gas is burned. The fire tube can be either in contact with the emulsion (direct-fired) or in contact with a water bath that transfers the heat to the emulsion (indirect-fired). Gravity then separates the heated emulsion.
Fig. 4. Heater treater. (Modified from Baker, 1983.)
Electrode plates and electricity can also be used in an electrostatic precipitator to separate emulsions. If the emulsion is not very stable (loose), a large settling tank called a gun barrel or wash tank is used for gravity separation. A demulsifier is a chemical that can be injected into a treating vessel to help separate emulsions.
Separators are rated by operating pressures that are between 20 and 1,500 psi (1.4 and 105 kg/cm2 ). To maximize the retention of highly volatile components of oil, stage separation with several separators operating at decreasing pressures is used. The produced fluids flow first into a high-pressure separator and then through progressively lower pressure separators. The stock tanks are considered one last stage in the separation. Three-stage separation (fig. 5a) uses a high- and a low-pressure separator along with the stock tanks. Four-stage separation (fig. 5b) uses high-, medium-, and low-pressure separators and the stock tanks. Retention time varies from one minute for light oils to five or six minutes for heavy oils in three-phase separators.
Fig. 5. Stage separation: (a) three-stage and (b) four-stage