Free oil — the large, buoyant droplets (>150 micron) that separate readily by gravity in the first stage of produced-water treatment.
Oil-in-Water Chemistry — in depth
Free oil is the easy fraction: droplets larger than about 150 microns that rise quickly under gravity. API and plate separators remove the bulk of it in the primary stage, dropping oil-in-water from thousands to low hundreds of mg/L before finer dispersed and dissolved fractions are tackled.
What matters in practice
Large droplets rise fast by gravity.
API/CPI capture the bulk free oil.
The readily-separable size range.
Removed first in the train.
| Property | Value | Note |
|---|---|---|
| Droplet | >150 µm | Large |
| Removal | Gravity | API/CPI |
| Inlet | >1000 mg/L | Bulk |
| Outlet | 100–150 mg/L | To flotation |
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Fundamentals, design drivers and practical guidance
Free oil — the large, buoyant droplets (>150 micron) that separate readily by gravity in the first stage of produced-water treatment.
Reynolds & Bauhm designs produced-water trains stage by stage against the governing discharge or reinjection spec — selecting separators, hydrocyclones, flotation and polishing media, and the demulsification chemistry that makes the physical stages perform.
Produced water is the largest waste stream in oil and gas, and its treatment is a staged de-oiling problem: free oil, dispersed oil and emulsified oil are removed in sequence because no single device handles all three economically. The target — whether for overboard discharge, reinjection or reuse — is set by regulation such as the OSPAR 30 mg/l dispersed-oil limit offshore or the US EPA effluent limitation guidelines.
The treatment train follows droplet size. Bulk gravity and plate separators (API/CPI) take out free oil and coarse droplets; hydrocyclones use centrifugal force to remove finer dispersed oil compactly, ideal for the weight and footprint constraints offshore; induced or dissolved gas flotation attaches micro-bubbles to smaller droplets to float them out; and polishing media such as walnut-shell filters or adsorption capture the residual to meet tight discharge limits.
What our engineers assess on every scope of this type
| Parameter | Typical basis | Why it matters |
|---|---|---|
| Dispersed oil | Hydrocyclones | Compact, high-G separation |
| Fine droplets | Gas flotation (IGF/DGF) | Floats out small droplets |
| Residual | Walnut-shell / adsorption | Polishes to discharge limit |
| Emulsions | Demulsifier / heat / electro | Breaks stabilised oil |
| Limit | OSPAR 30 mg/l / EPA ELG | Sets the train target |
| Free oil | API/CPI separators | Removes coarse droplets first |
Common questions on produced-water treatment
Stabilised oil-in-water emulsions resist gravity separation and must be broken first — with demulsifier chemistry, pH adjustment, or thermal/electrostatic treatment — so the downstream physical stages can then remove the freed oil.
It depends on the route: offshore discharge typically targets the OSPAR 30 mg/l dispersed-oil standard, US operations follow EPA effluent limitation guidelines, and reinjection is governed instead by solids and particle-size limits to protect the formation.
Reinjection shifts the priority from oil concentration to suspended-solids and particle-size control, because plugging solids damage formation permeability. Free Oil is then tuned around filtration and solids removal rather than de-oiling alone.
Because free, dispersed and emulsified oil behave differently and no single device removes all three economically. Free Oil sits within a sequence sized by droplet size, from bulk separation through to fine polishing.
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