Some wells have pure gas production which can be taken directly to gas treatment and/or compression. More often, the well gives a combination of gas, oil and water and various contaminants which must be separated and processed. The production separators come in many forms and designs, with the classical variant being the gravity separator.


In gravity separation the well flow is fed into a horizontal vessel. The retention period is typically 5 minutes, allowing the gas to bubble out, water to settle at the bottom and oil to be taken out in the middle. The pressure is often reduced in several stages (high pressure separator, low pressure separator etc.) to allow controlled separation of volatile components. A sudden pressure reduction might allow flash vaporization leading to instabilities and safety hazards.

Test Separators and Well test

Test separators are used to separate the well flow from one or more wells for analysis and detailed flow measurement. In this way, the behavior of each well under different pressure flow conditions can be determined. This normally takes place when the well is taken into production and later at regular intervals, typically 1-2 months and will measure the total and component flow rates under different production conditions. Also undesirable behavior such as slugging or sand can be determined. The separated components are also analyzed in the laboratory to determine hydrocarbon composition of the Gas oil and Condensate.

The test separator can also be used to produce fuel gas for power generation when the main process is not running. In place of a test separator one could also use a three phase flow meter to save weight.

Production separators

The main separators are gravity type. On the right you see the main components around the first stage separator. As mentioned the production choke reduces will pressure to

the HP manifold and First stage separator to about 3-5 MPa (30-50 times atmospheric pressure). Inlet temperature is often in the range of 100-150 degrees C. On the example platform, the well stream is colder due to Subsea wells and risers.

The pressure is often reduced in several stages; here three stages are used, to allow controlled separation of volatile components. The purpose is to achieve maximum liquid recovery and stabilized oil and gas, and separate water. A large pressure reduction in a single separator will cause flash vaporization leading to instabilities and safety hazards.

The retention period is typically 5 minutes, allowing the gas to bubble out, water to settle at the bottom and oil to be taken out in the middle. In this platform the water cut (percentage water in the well flow) is almost 40% which quite high. In the first stage separator, the water content is typically reduced to less than 5%.

At the crude entrance there is a baffle slug catcher that will reduce the effect of slugs (Large gas bubbles or liquid plugs). However some turbulence is desirable as this will release gas bubbles faster than a laminar flow.

At the end there are barriers up to a certain level to keep back the separated oil and water. The main control loops are the oil level control loop (EV0101 20 above) controlling the oil flow out of the separator on the right, and the gas pressure loop at the top.(FV0105 20 above) These loops are operated by the Control System. An important function is also to prevent gas blow-by which happens when low level causes gas to exit via the oil output causing high pressure downstream. There are generally many more instruments and control devices mounted on the separator. These will be discussed later.

The liquid outlets from the separator will be equipped with vortex breakers to reduce disturbance on the liquid table inside. This is basically a flange trap to break any vortex formation and ensure that only separated liquid is tapped off and not mixed with oil or water drawn in though these vortices. Similarly the gas outlets are equipped with demisters, essentially filters that will remove liquid droplets in the gas.

Emergency Valves (EV) are sectioning valves that will separate the process components and blow-down valves that will allow excess hydrocarbons to be burned off in the flare. These valves are operated if critical operating conditions are detected or on manual command, by a dedicated Emergency Shutdown System. This might involve partial shutdown and shutdown sequences since the flare might not be able to handle a full blow-down of all process sections simultaneously.

A 45.000 bpd design production with gas and 40% water cut this gives about 10 cubic meters from the wellheads per minute. There also needs to be enough capacity to handle normal slugging from wells and risers. This means the separator has to be about 100 cubic meters, e.g. a cylinder 3 m in diameter and 14 meters long. At the rated operating pressure this means a very heavy piece of equipment, typically around 50 tons for this size. This limits the practical number of stages. Other types of separators such as vertical separators, cyclones (centrifugal separation) can be use to save weight, space or improve separation.There also has to be a certain minimum pressure difference between each stage to allow satisfactory performance in the pressure and level control loops.

Chemical additives are discussed later.

Second stage separator

The second stage separator is quite similar to the first stage HP separator. In addition to output from the first stage, it will also receive production from wells connected to the Low Pressure manifold. The pressure is now around 1 MPa (10 atmospheres) and temperature below 100 degrees C. The water content will be reduced to below 2%.

An oil heater could be located between the first and second stage separator to reheat the oil/water/gas mixture. This will make it easier to separate out water when initial water cut is high and temperature is low. The heat exchanger is normally a tube/shell type where oil passes though tubes in a cooling medium placed inside an outer shell.

Third stage separator

The final separator here is a two phase separator, also called a flash-drum. The pressure is now reduced to about atmospheric pressure (100 kPa) so that the last heavy gas components will boil out. In some processes where the initial temperature is low, it might be necessary to heat the liquid (in a heat exchanger) again before the flash drum to achieve good separation of the heavy components. There are level and pressure control loops. As an alternative, when the production is mainly gas, and remaining liquid droplets have to be separated out, the two phase separator can be a Knock-Out Drum (K.O. Drum).


After the third stage separator, the oil can go to a coalescer for final removal of water. In this unit the water content can be reduced to below 0.1%. The coalescer is completely filled with liquid: water at the bottom and oil on top. Inside electrodes form an electric field to break surface bonds between conductive water and isolating oil in an oil water emulsion. The coalescer field plates are generally steel, sometimes covered with dielectric material to prevent short circuits. The critical field strength in oil is in the range 0.2 to 2 kV/cm. Field intensity and frequency as well as the coalescer grid layout is different for different manufacturers and oil types.

Electrostatic Desalter

If the separated oil contains unacceptable amounts of salts, it can be removed in an electrostatic desalter (Not used in the Njord example) The salts, which may be Sodium, Calcium or Magnesium chlorides comes from the reservoir water and is also dissolved in the oil. The desalters will be placed after the first or second stage separator depending on Gas Oil Ratio (GOR) and Water cut.

Water treatment

On an installation such as this, when the water cut is high, there will be a huge amount of produced water. In our example, a water cut of 40% gives a water production of about 4000 cubic meters per day (4 million liters) that must be cleaned before discharge to sea. Often this water contains sand particles bound to the oil/water emulsion.

The environmental regulations in most countries are quite strict, as an example, in the North-East Atlantic the OSPAR convention limits oil in water discharged to sea to 40 mg/liter (ppm). It also places limits other forms of contaminants. This still means up to one barrel of oil per day for the above production, but in this form, the microscopic oil drops are broken down fast by natural bacteria.

Various equipment is used; the illustration shows a typical water treatment system. Water from the separators and coalescers first goes to a sand cyclone, which removes most of the sand. The sand is further washed before it is discharged.

The water then goes to a hydrocyclone, a centrifugal separator that will remove oil drops. The hydrocyclone creates a standing vortex where oil collects in the middle and water is forced to the side.

Finally the water is collected in the water de-gassing drum. Dispersed gas will slowly rise to the surface and pull remaining oil droplets to the surface by flotation. The surface oil film is drained, and the produced water can be discharged to sea. Recovered oil in the water treatment system is typically recycled to the third stage separator.


Separation of Oil and Gas

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