Separation

What arrives

A well stream is a mixture of oil, gas, water, and often sand, at whatever pressure and temperature the well delivers. The facility's job is to split it into three streams that each meet a specification, and to do so reliably as the composition changes over field life.

How separation works

Separation relies on density difference and residence time. Gas disengages from liquid; water settles below oil. Everything in a separator's design exists to make that happen in the smallest practical vessel:

• Inlet device — kills the momentum of the incoming stream and performs the initial bulk separation. A damaged inlet device degrades everything downstream.
• Gravity settling section — the bulk of the vessel volume, where residence time does the work.
• Coalescing internals — plate packs that bring small droplets together so they settle.
• Mist extractor — mesh or vane pack at the gas outlet removing entrained droplets.
• Weirs and buckets — establishing and holding the oil-water interface.
• Vortex breakers at liquid outlets.

Configurations

• Two-phase — gas and liquid. Three-phase — gas, oil and water.
• Horizontal — the usual choice, with a large interface area, good for high liquid volumes and for three-phase duty.
• Vertical — smaller footprint, better with sand, and better able to handle surging liquid, which is why they are used as scrubbers and knockout drums.
• Stage separation — a train of separators at successively lower pressures. Flashing the gas off in stages rather than all at once increases the amount of liquid recovered at stock tank conditions, which is a direct revenue gain and the reason multi-stage separation is standard.

Control

Each separator has pressure control on the gas outlet, level control on the oil outlet, and interface level control on the water outlet. Level control is deliberately tuned loosely, because the vessel exists to absorb variation — tight level control passes every surge straight downstream and defeats the purpose of the vessel.

Protection includes high and low level alarms and trips, high and low pressure trips, and pressure relief. Level instrumentation deserves particular attention: it is density dependent if hydrostatic, it is the most common source of upsets, and the control and trip measurements should be independent of one another.

What degrades separation

Throughput above design; a change in the gas, oil and water ratio; foaming; stable emulsions; sand accumulation reducing effective volume; damaged internals; and slugging arriving from the flowlines. The symptoms are carryover of liquid into the gas outlet, carry-under of gas into the liquid outlet, water in the oil export and oil in the produced water.