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 is injected into offshore 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. 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 separate free oil over time. It is not effective for emulsified or dissolved hydrocarbons.
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.
Gravity separators have no or few moving parts. They are simple to operate and robust in field service. For offshore service, pressurized equipment is used to make the first separation of water, solids and production fluids. Examples of equipment used for primary gravity separation are pressurized cross flow interceptors (CFI), corrugated plate separators (CPS), free water knockouts (FWKO) and multistage three phase production separators. Some offshore facilities rely of floating assets for production, storage and offloading (FPSO). The movement of an anchored ship in a swell can upset gravity based water treatment processes and lead to discharges considerably above the average levels. Because of the movement of these vessels, they rarely carry gravity based separation systems on board.
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.
Vs settling velocity m/h
d particle diameter m
g gravitational force m/s2
density difference between water and oil kg/m3
m viscosity Pa*s
Knowing the velocity, the area required for separation can be calculated from:
A area required for separation m2
Qv volumetric flow rate m3/h
Vs settling (or rise) velocity m/h
Virtually every particle size can be theoretically separated given enough time. However, separation of very small particles would require a rather large unit, and space and weight are primary concerns in offshore platform design. Low liquid hold-up is the strategy. Time consuming operations like settling and biological treatment are generally out of the question. 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 and solid particles must travel. 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.
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 parallel plate primary separation equipment should easily reduce oil content to less than 100 mg/L and remove all solids greater than 40 microns when properly cleaned and maintained.
Primary separation equipment often does not remove enough water from the oil to prepare it for sale or remove enough oil or solids from water to meet disposal requirements. The oil stream is often treated downstream in an electrostatic treater after contact with wash water to reduce bituminous sediment and water (BS&W) prior to transport through pipelines or tankers to shore. Further oil removal from produced water 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.
