Oil-in-Water Emulsion

An oil-in-water emulsion (O/W emulsion) is a mixture in which tiny droplets of oil are dispersed throughout a continuous water phase. Water is the dominant liquid, and oil exists as microscopic spherical droplets suspended within it. This is the opposite of an invert emulsion (water-in-oil), in which water droplets are dispersed in a continuous oil phase. Oil-in-water emulsions form wherever oil and water are mixed with sufficient agitation in the presence of natural or added emulsifying agents, including surface-active compounds from crude oil itself. In oilfield operations, O/W emulsions appear in produced water streams, in normal (non-invert) emulsion drilling muds, and as an unwanted product in production equipment that must be broken and separated before oil can be shipped.

Key Takeaways

In an oil-in-water emulsion, the water is the continuous phase and oil is the dispersed phase. The ratio of oil to water determines the emulsion's character: low oil fractions (less than 30 percent oil) produce stable, fluid O/W emulsions; higher oil fractions can invert the emulsion, converting it from O/W to water-in-oil (W/O) as the droplets crowd together and the continuous phase switches.
Natural emulsifiers in crude oil (asphaltenes, resins, naphthenic acids) stabilize O/W emulsions by adsorbing at the oil-water interface and forming a rigid skin around each oil droplet that resists coalescence. The more aromatic and resinous the crude, the more stable the emulsions it tends to form.
The viscosity of an O/W emulsion increases with oil droplet concentration. At low oil content, an O/W emulsion can be nearly as thin as water. As oil content rises, viscosity increases and the emulsion becomes increasingly difficult to transport and separate.
Breaking an O/W emulsion requires overcoming the stability provided by the interfacial film. Common methods include heat (raises droplet mobility), chemical demulsifiers (displace the natural emulsifiers at the interface, allowing droplets to coalesce), electrostatic treatment (aligns droplets to promote coalescence), and gravity or centrifugal separation (allows droplets to rise through the water phase).
In the context of normal emulsion drilling muds (as opposed to invert emulsion or oil-base muds), an oil-in-water emulsion describes a water-base mud that has emulsified diesel or mineral oil added to improve lubrication and fluid loss control, with water as the continuous external phase. These are sometimes called oil-emulsion muds or emulsion muds.

What Is an Oil-in-Water Emulsion?

Shake a bottle of salad dressing vigorously. For a few seconds, the oil and vinegar are dispersed throughout each other as tiny droplets. The mixture looks opaque, almost creamy. Stop shaking and the two phases separate again because there is no emulsifier to keep the droplets from coalescing. If the dressing contains mustard or egg yolk (natural emulsifiers), the mixture stays mixed much longer because these molecules coat each droplet and prevent them from recombining.

Crude oil contains its own natural emulsifiers: asphaltenes (large polycyclic aromatic molecules), resins, naphthenic acids, and waxes. When oil and produced water are mixed under the turbulent conditions of the wellbore, tubing, and surface pipelines, some of these natural emulsifiers migrate to the oil-water interface and coat each oil droplet. The result is an oil-in-water emulsion that can persist for hours, days, or indefinitely depending on the oil's chemical character and the degree of mixing.

The critical distinction between an oil-in-water emulsion and a water-in-oil emulsion is which phase is continuous. In an O/W emulsion, you are surrounded by water with oil droplets floating in it. In a W/O emulsion (invert emulsion), you are surrounded by oil with water droplets inside. A drop of an O/W emulsion placed in water will dilute readily; a drop of a W/O emulsion placed in water will bead up and float because the outer surface is oil.

Fast Facts

Produced water from heavy oil operations in Alberta's Athabasca, Cold Lake, and Peace River areas often arrives at surface facilities as a complex O/W emulsion containing 3 to 15 percent oil by volume as small droplets. These droplets can be as small as 1 to 10 microns in diameter. At that size, settling under gravity takes days or weeks for a single droplet to rise to the oil-water interface. Modern produced water treatment systems use combination approaches: heat exchangers (raising temperature to 70 to 90°C to lower interfacial film strength), chemical demulsifiers (interfacial film displacers), and corrugated plate interceptors (providing a large surface area for small droplets to coalesce on) to break the emulsion within minutes rather than days.

Oil-in-Water Emulsions in Production Operations

The fluid arriving at a production battery from an oil well is rarely a clean, phase-separated stream of oil over water. In most wells, especially those with high water cuts (the fraction of total liquid that is water), the oil and produced water have been mixed and re-mixed by the turbulence of the wellbore, the tubing, and the surface flowlines. The result is some combination of free oil, free water, and emulsified oil droplets.

Most oil production facilities have emulsion treating equipment designed specifically to break O/W emulsions before the oil can be metered and shipped to the pipeline. A typical treating train includes a heater-treater (horizontal vessel where the fluid is heated, held for a residence time, and allowed to settle), a chemical injection point for demulsifier addition, and sometimes an electrostatic treater (where an applied AC or DC electric field aligns and coalesces the oil droplets). The goal is to achieve a clean oil phase (less than 0.5 percent water content or the BS&W specification for pipeline) and a clean water phase (less than 100 to 500 mg/L of oil in produced water, depending on the disposal or reinjection specification).

Heavy crude oils and bitumen produce the most stable emulsions because they contain the highest concentrations of asphaltenes and resins. SAGD operations producing steam-condensate-bitumen mixtures at high temperatures generate emulsions that can be relatively easy to break at temperature (steam heat keeps the temperature high) but re-emulsify quickly if the fluid cools. Cold Lake and Peace River facilities must maintain treating temperatures precisely to avoid re-emulsification in cooling produced water lines.

Oil-in-Water Emulsion Muds

In the context of drilling fluids, an oil-in-water emulsion mud (also called an oil-emulsion mud or just an emulsion mud) is a water-base mud that contains emulsified oil droplets (typically 3 to 8 percent by volume of diesel or mineral oil) added to improve lubrication and reduce fluid loss. Water is still the continuous external phase, so the mud behaves as a water-base system for regulatory and environmental purposes (cuttings can generally be discharged with lower oil-on-cuttings limits than for invert emulsion muds). The emulsified oil provides lubrication at the bit and in the drill collar string, reducing torque and the risk of differential sticking.

An oil-in-water emulsion is also called a normal emulsion or an external-water emulsion. The abbreviation is O/W. Related terms include invert emulsion (a water-in-oil emulsion where water is the dispersed internal phase and oil or synthetic fluid is the continuous external phase; all oil-base muds are invert emulsions; the opposite of O/W), demulsifier (a surface-active chemical that displaces natural emulsifiers from the oil-water interface, allowing oil droplets to coalesce and separate from the water phase; the primary chemical treatment for breaking O/W emulsions in production facilities), emulsifier (a surface-active molecule that adsorbs at the oil-water interface and stabilizes emulsions by reducing interfacial tension and forming a protective film around droplets; natural emulsifiers in crude oil include asphaltenes, resins, and naphthenic acids), basic sediment and water (BS&W, the water and sediment content of a crude oil shipment, measured as a percentage by volume; pipeline quality specifications typically require BS&W below 0.5 percent, meaning the O/W emulsion must be fully broken before custody transfer), and produced water (the water that comes up from the reservoir along with oil and gas; often arrives as an O/W emulsion or contains emulsified oil droplets that must be separated before the water can be reinjected or disposed of).

How an O/W Emulsion Stability Problem Shut Down a Cold Lake Battery for Four Days

An Alberta Cold Lake CSS (cyclic steam stimulation) operator was running a seven-well battery producing bitumen-water mixtures from the Clearwater Formation. The treating system included a horizontal heater-treater operating at 82°C and a demulsifier injection rate of 50 mg/L. Pipeline oil quality specification was 0.5 percent BS&W or better.

In February, an abnormally cold snap dropped ambient temperatures to -32°C. Heat loss from the uninsulated produced water line to the battery was greater than anticipated. By the time the fluid reached the heater-treater inlet, temperature had dropped to 61°C, 21 degrees below the design treating temperature. At 61°C, the interfacial film strength of the Cold Lake bitumen emulsion was too high for the demulsifier dose to break it effectively. BS&W at the treater outlet crept from 0.3 percent to 1.8 percent over 18 hours.

When the treating results crossed the 0.5 percent pipeline specification, the operator had to shut in the wells and divert fluid to a holding tank while the system was troubleshot. The heater-treater was found to be operating correctly; the problem was inlet temperature. Pipe-traced heat tape was activated on the inlet line and the demulsifier dose was temporarily increased to 80 mg/L to compensate. The system returned to on-spec treating over 12 hours. Total downtime: 4 days including stabilization, corresponding to approximately 1,200 barrels of lost bitumen production at CAD 55 per barrel, or CAD 66,000 in lost revenue. Insulating the produced water line cost CAD 18,000. The cold-weather maintenance item had been deferred for three years.