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Exploration & Production: The Oil & Gas Review 2005 - Issue 2 -
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Separation Offshore
The separation process in petroleum production involves a number of equipment units. In many fields the process is composed of a number of separation steps where the pressure is stepwise reduced and gas flashed off. All oil, gas, water and sand separation is based on differences in density and most separators are gravity separators, i.e., they utilise the common acceleration of gravity, in huge pressurised vessels. Later developments have brought compact, reliable high-g equipment to the market, particularly hydrocyclones and centrifuges for oil and water polishing. The fluid flow behaviour of a three-phase separator shows that different physical and chemical phenomena are important in different zones.
Separator sizing must satisfy several criteria for good operation during the lifetime of the producing field. It must:
- provide sufficient time to allow the immiscible gas, oil and water phases to separate by gravity;
- provide sufficient time to allow for the coalescence and breaking of emulsion droplets at the oil–water interface;
- provide sufficient volume in the gas space to accommodate rises in the liquid level that result from surge in the liquid flow rate;
- provide for the removal of solids that settle to the bottom of the separator; and
- allow for variation in the flow rates of gas, oil and water into the separator without adversely affecting separation efficiency.
Horizontal Gravity Separators
Each separator operates at a fixed pressure. The multiphase fluid enters the vessel through the inlet nozzle as a high momentum jet hitting an inlet arrangement such as a momentum breaker device or cyclones. The liquid is diverted into the liquid pool in the lower part of the vessel. The lowdensity gas, together with liquid drops, will flow in the upper part of the vessel. Inside the liquid pool, near the inlet, the multiphase fluid flows as a dispersion with low horizontal velocity. The lowdensity gas and mist rises and the oil, water and emulsion will separate by gravity on the way to the outlet. In the upper part of the vessel, the oil and water particles together with condensed gas will fall down to the liquid interface as the multiphase fluid flows to the outlet. For the modelling and simulation of fluid flow and phase separation behaviour inside a gravity separator, it is helpful to characterise the different flow regimes or zones.
Fluid Systems and Mechanisms of Separation
The separation process is dominated by two factors:
- the emulsification process, which takes place in the choke and other equipment components with high shear; and
- the coalescence and settling effects, where drops grow and settle or cream to its homo-phase.
The Bottle Test (Batch Separation)
A common method of determining relative emulsion stability of water-in-oil or oil-in-water is a simple bottle test (batch separation test). These batch sedimentation and coalescence experiments are an attractive method for the study of separation behaviour of emulsions due to its simple and inexpensive experiments. When the percentage of water in the mixture exceeds approximately 20% by volume, two interfaces become visible as the mixture settles. One is a sedimentation interface between the settling dispersion and the bulk oil phase. The other is a coalescence interface between the dispersion and the bulk water phase. As time goes on, the thickness of the dispersion layer grows smaller and the two interfaces approach each other.
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