Oil and Gas Production Systems
Contents
Why are oil and gas separation equipment required?
Most producing well fluids are a mixture of gas, oil, water, formation materials and corrosion byproducts. The gas, oil and water must be separated from each other to facilitate their handling, measurement and ultimate sale. In many instances the pressures associated with the production of the fluids or gases must be lowered to levels compatible with the surface equipment which will be used to process them. A partial listing of the equipment used to process, store and measure oil and/or gas includes:
- Separators
- Heater treaters
- Electrostatic treaters
- Free-water knockouts
- Gun barrels
- Stock tanks
- LACT units
- Line heaters
Separators
Separators are normally the first piece of equipment used to process produced fluids. Separators can be used to accomplish several different tasks. They can be used to lower the system pressure through stage separation of the produced gas; to separate gas and oil; to separate gas, oil and water; to separate oil and water; and to separate gas and water. Separators can be classified as being either two- or three-phase, vertical or horizontal vessels. A twophase vessel separates gas/ liquid or liquid/liquid mixtures - oil and water. A three-phase vessel separates gas/liquid/liquid mixtures: gas, oil and water. Three-phase separators are only used when free water readily separates from the gas condensate or crude oil.
Gas/liquid separators rely on gravity and centrifugal force to separate the gas from the liquid. The incoming gas is diverted by a baffle and subjected to a circular motion which causes the heavier liquids to separate from the gas stream. The liquids, being heavier than gas, fall to the bottom of the vessel and the lighter gas rises to the top. The gas exits out the top of the vessel and the discharge of the liquid from the bottom of the vessel is controlled by a dump valve assembly, which also maintains the liquid level in the vessel.
Liquid/liquid separators use gravity and the differences in densities of the liquids to separate them. Gas condensate and crude oils are lighter than water, therefore they float on water. The liquid levels are controlled and maintained by dump valve assemblies.
Gas/liquid/liquid separators have three sections in the vessel and they operate by using gravity, centrifugal force and differences in densities to separate the well stream into three distinct phases. The liquid levels, oil and water, are controlled and maintained by dump valve assemblies.
Vertical separators require less surface space (footprint) than do horizontal separators, which makes them more practical for use in offshore operations where space is critical. Vertical separators are manufactured in both two- and three-phase configurations.
Horizontal separators are also manufactured in two- and three-phase configurations. The three-phase vessel has an additional section for water storage and a dump valve to control the water level in the vessel. Horizontal separators can be single or double barrel in design. The double barrel separator provides additional storage and separation capacity.
The well stream flows horizontally in a horizontal separator. The upper section of the vessel contains a series of baffle plates mounted at a 45 degree angle which extends down into the liquid section. As the well stream passes over the baffle plates droplets of liquid form on the plates and fall into the liquid section of the vessel.
Equipment malfunctions can and do occur from time to time and most problems are common to all separators. Some of the problems which can be encountered are:
- Filling of the liquid section with solids. The retention time for oil, water and gas separation is reduced. Oil may travel out of the gas outlet or the water outlet as a result.
- An oil or water dump valve hung in the open position will cause gas to pass with the liquids. A low level alarm should warn of this problem. A dump valve stuck in the closed position will cause liquids to pass with the gas.
- A water dump valve on a three-phase separator hung in the open position will cause oil or oil and gas to pass with the water. If the valve is stuck in the closed position, water will pass with the oil.
- An oil dump valve on a three-phase separator hung in the open position will cause gas to pass with the oil. If the valve is stuck in the closed position, oil will pass with the gas.
Heater Treaters
A heater treater uses heat to separate emulsions and to speed up the separation process. Heater treaters can be thought of as low pressure, threephase separators equipped with fire tubes. They separate gas from the incoming emulsion and separate the emulsion into a water phase and an oil phase. Heater treaters can also function as a freewater knockout, a heat exchanger, a filter, and as a water wash tank.
The heater treater is normally found downstream of a two-phase gas separator on a high pressure system. It can be found downstream of the well, a free-water knockout or gun barrel on a low pressure system. Heater treaters are used where the emulsion cannot be broken using just retention, quiescence and chemical demulsifiers. The addition of heat is required to break the emulsion.
Heat lowers the viscosity of the oil making it easier for the water to settle. It also creates molecular movement which aids in the coalescing of the water droplets. Heater treaters can be vertical or horizontal in design. Horizontal heater treaters have a much larger oil treating section and they are used to treat heavier crudes where addition settling time is required. Both treaters operate using the same principles. The incoming emulsion enters the vessel near its top and any gas associated with the emulsion is vented. Any free water immediately drops to the water section where it is discarded.
The emulsion flows over the fire tubes where it is heated to the desired temperature as it rises through the heated water section. (The heated water serves to wash the emulsion and to aid in the coalescing of the water droplets.) The emulsion and oil continue to rise into the oil treating section of the vessel where the emulsion breaking process continues. Some treaters are equipped with a "hay" or excelsior section to aid in coalescing the water droplets and to act as a filtering media for removal of solids from the crude oil.
The heights of the water and oil columns are controlled by dump valves or by a siphon. The clean oil exits the vessel near the top third of the vessel and the water is drawn off directly under the fire tubes and emulsion distribution pan.
Some of the problems which can be encountered that effects the operation of heater treaters are:
- Overloading of the vessel with well fluids does not allow sufficient settling time for complete water removal.
- Operating temperature too low does not provide for complete water removal.
- Malfunctioning water dump valve allowing oil to carryover with the water if the valve hangs open, and allowing water to carryover with the oil if it remains closed.
- A malfunctioning oil dump valve hung in a closed or semi-closed position will allow oil to carryover with the water.
Electrostatic Treaters
Electrostatic treaters are similar to horizontal heater treaters in design and operation. The main difference between the two units is the electric coalescing section in the electrostatic treater which replaces the hay section in the heater treater. An electrical grid installed in the coalescing section and 440 volt alternating electric current produces an electrical field in the coalescing section as electric current passes through the grid. Power is supplied to the grid through a transformer mounted on the vessel and a control box mounted on a nearby pole.
The operating temperature of the electrostatic treater is normally lower than that of the heater treater. In some instances the emulsion will break without heat being applied to the emulsion. The produced fluids enter the treater and any gas is vented. The liquid passes over the fire tubes and is heated if heat is applied. Free waterfalls to the water section of the vessel and the emulsion rises to the coalescing section.
As the emulsion passes through the electrical field the water droplets in the emulsion become electrically charged and they move about rapidly and collide with one another. As they collide the droplets grow in size forming larger droplets, until they become large enough to settle. The maximum emulsion in the grid section will be 3%. When the emulsion is greater than 3%, the grid shorts out and is non-functional.
Gun Barrels, or Wash Tanks
with an internal or external boot, or flume. Although gun barrels are not employed to the extent now that they once were, many are still used to break emulsions.
In a gun barrel, oil and water are separated in a two-part action:
1 washing, and
2 settling.
The washing is done in the free-water layer, and the settling occurs in the emulsion layer. Because all emulsions are not alike, the amount of free water that should be held in a gun barrel must be established for each lease. For instance, if washing has little or no effect on a certain emulsion, then only a small amount of free water is maintained in the gun barrel’s tank. Conversely, if an emulsion completely breaks down by washing, then a large amount of free water is kept in the tank.
Free-Water Knockouts
Many producing wells produce moderate to large amounts of free water which settle in a relatively short period of time when given the opportunity. Free-water knockouts are used to remove free water from the well stream prior to it entering a treating vessel. Removal of the free water reduces the loading on treating vessels such as heater treaters and electrostatic treaters. Removing the free water also reduces fuel requirements, reduces turbulence, and increases retention time.
Free-water knockouts can be vertical or horizontal in design and they can be either a two- or three-phase unit. The two-phase unit is designed to separate free water from an emulsion. The three-phase unit separates free water from gas and the emulsion.
Well fluids enter near the top of the vessel and any gas present is vented. The water and emulsion fall to the midsection of the vessel. The heavier free water falls to the bottom of the vessel and is discharged. The emulsion and any free oil that may be present flow through a series of baffles which function to reduce turbulence, thus allowing all free water to separate from the emulsion. The emulsion flows to the opposite end of the vessel and is discharged to an oil treating unit.
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