Primary Oil/Water Separation
Technical Profile
Oil or gas production means nothing more than liberation of hydrocarbons from their natural environment. Produced water, associated with oil and gas production, can have two origins: natural presence or subsurface injections as a means of production enhancement.
Most underground oil or gas reservoirs have a natural water layer called Formation Water that lies underneath the hydrocarbons. However, as wells age and oil becomes difficult to remove, water or steam is injected into the reservoirs to enhance production by helping force oil to the surface. Both formation and injected water eventually make their way to the well bore and are produced at the well head along with desired hydrocarbons. Primary oil/water separation is the critical first step in produced fluid treatment that utilizes the natural tendency of oil and water to separate over time. The objective of primary oil/water separation is to remove water from oil.
The bulk of suspended oil in produced water is free oil. Free oil is effectively removed utilizing gravity separation as the first step of treatment. Gravity separation utilizes specific gravity differences between hydrocarbons and water to make a very effective separation of free oil over time. Gravity separation is not effective for emulsified or dissolved hydrocarbons.
A primary rule of oil/water separation relates to shear force. Any application of shear force will break oil droplets into finer particles, which makes them harder to separate. Well pressure is controlled by a choke and centrifugal pumps are often used for oil movement. Each of these devices imparts shear force that breaks up oil particles. Therefore, primary oil/water separation is often performed upstream of this equipment in enclosed, pressurized equipment.
Due to the presence of gas in the production stream from the wellhead, a three-phase separator is generally employed to separate the stream into a gas phase for recovery or flaring, an oil phase for dehydration and transport and a water phase for treatment and disposal.
In addition to breaking up oil particles, shear force tends to emulsify oil and water. Heat and chemical addition is sometimes effective in breaking these emulsions. Special equipment called heater treaters may be used to further reduce water content before oil is transported to a refinery. This equipment uses tubes heated by fired burners running through a gravity separation vessel to further reduce water content.
Gravity separators have no or few moving parts. They are simple to operate and robust in field service. Examples of equipment used for primary gravity separation are cross flow interceptors (CFI), corrugated plate separators (CPS), tilted parallel plate separators (TiPPS), API separators, free water knockouts (FWKO), heater treaters, and equalization tanks.
Gravity separation of gas, oil, water and suspended solids can be performed simultaneously in one vessel. The size of gravity separation equipment is determined by the diameter of the particle that is desired to be fully separated. Stokes Law relates settling velocity of a particle to the diameter of the particle, gravitational force, the difference in density between the particle and water, and the viscosity of the fluid.
Parallel plates are inclined between 45˚ and 60˚ to the horizontal. Steeper inclinations are generally recommended for suspended solids to let gravity help pull solids down the plate to reduce fouling. A smaller plate inclination angle produces a larger net effective separation surface for the same number of plates. Flow across the plates may be cross flow where oil and solid particles flow perpendicular to treated water or counter flow where oil particles flow opposite to water and solids.
Solids separation is usually not the bottleneck of primary separation units. The specific gravity difference between oil and water is much less than that between sand and water. The majority of solids will be separated without problems. The risk with solids, however, is plugging of the unit if the solids do not slide off the plates efficiently. Plugging will lead to poor performance and loss of capacity.
Properly operating primary separation equipment should easily reduce oil content to less than 300 mg/L and remove all solids greater than 40 microns. Inclined parallel plate equipment should reduce oil to less than 100 mg/L when properly cleaned and maintained.
Primary separation equipment often does not remove enough oil or solids from water to meet disposal requirements. Oil removal is needed to eliminate coating of scale and feeding bacteria in subsurface production or disposal formations. Solids must be removed to the point that they do not plug the pores in the production or disposal formations. Most times, secondary oil/water separation is needed.
Virtually every particle size (oil and solid) can be theoretically separated given enough time. However, separation of very small particles would require a rather large unit. Therefore, primary separators are generally designed to remove particles larger than 40 microns. Stokes Law is also valid for particles with a diameter smaller than the settling diameter as well; therefore, these particles may be partially settled in the ratio of their diameter to the settling diameter equal to (d/ds)2 x 100%.
To improve performance and reduce equipment size, parallel plates are utilized to shorten the distance oil particles must rise and solid particles must sink. These plates promote particle coagulation that helps to increase velocity. Oil and solid particles then follow the plate surface to the surface of the separation vessel where oil is removed or the bottom of the separation vessel where solids are removed.
