Oil leaving the separator is not yet oil anyone will buy. It is wet, it is salty, and it is quietly fizzing with dissolved gas. Between that liquid and a sold barrel stand three jobs: dry it, calm it, and count it, to the decimal, in front of witnesses.

Dry it

Much of the water left after bulk separation is dispersed as small droplets, often stabilised by natural surfactants. Treatment combines heat, chemistry, and, where appropriate, an electric field. Heating reduces viscosity; a demulsifier helps weaken the interfacial film; electrostatic forces promote droplet coalescence. Larger droplets then settle more readily. Injection points, mixing, heating method, voltage, and residence time are selected together for the crude and the equipment. A BS&W limit near 0.5% is familiar in some contracts, but the required target is the agreed product specification.

Salt is the same fight in disguise. Most inorganic salt in produced crude is associated with entrained brine, so removing that brine reduces the salt load, with one refinement: when the crude is very salty, fresh dilution water is deliberately mixed in first, rinsing the brine into droplets the grid can catch. A refinery desalter is this trick, industrialised.

Calm it

Even after staged separation, crude holds dissolved light ends, from methane through butane, under pressure. Send it straight to an atmospheric tank and it behaves like a shaken bottle: the fizz escapes in the tank farm as lost product, emissions, and a flammable cloud at ground level. Where the duty calls for it, a stabiliser provides a further controlled separation step: heat at the bottom drives the light ends up and out, until the liquid's vapour pressure is low enough to sit calmly in a tank through the hottest afternoon of the year. The light ends off the top are not waste. They become turbine fuel, LP compressor feed, or the start of an LPG product. "Stable" is not a feeling. It is a specification, measured and certified.

Count it

At the edge of the facility sits the custody transfer unit, known as LACT in the American idiom. It is a short run of pipe with more procedure per metre than anywhere else on site, because when oil crosses it, ownership changes.

Quality and quantity are both established at transfer. Depending on the metering arrangement, an online water-content monitor, representative sampling, and laboratory analysis support the quality checks. Off-spec product may be diverted for retreatment. Flow measurement is corrected to the agreed reference conditions using the required temperature, pressure, and fluid-property information. BS&W includes sediment as well as water, so a water probe alone does not establish the entire specification.

The measurement system must also be verified. Where proving is used, the meter’s response is compared against a recognised reference, and a meter factor is established under the applicable procedure. A calibrated displacement prover is one option; other proving and verification methods exist. Isolation, sampling, traceability, and witnessing requirements follow the transfer agreement and the measurement system. It is engineering with an audit trail, because both parties need confidence in the number.

The waiting is engineered too

Between processing and export, crude may wait in storage, and storage is engineered too. Floating roofs, fixed roofs with suitable vapour-management systems, venting, and containment are selected for the product, duty, and applicable requirements. A floating roof does not mean zero emissions, and inert-gas blanketing is not automatically required for every fixed-roof tank. The quiet corner of the facility still has a defined safety and environmental basis.

Wet crude in; a number two companies both signed, out. Every step exists because a buyer somewhere refused to pay for anything less.

I’m a practising oil & gas facilities engineer. If you have ever stood a proving run at three in the morning, or argued a BS&W result with a pipeline inspector, the YouTube comments are exactly where that story belongs.

Further reading

• SLB: electrostatic treating and coalescence