Consider the science of oil in water when designing a separation system.
Oil water separation (OWS) is a critical step in the produced water management cycle. As a result, OWS is involved in several steps throughout the produced water lifecycle.
At the wellhead there is initial OWS with separators, heater treaters and free water knockouts that are all part of a wellhead separation system. Further downstream tank batteries collect produced water and provide another OWS step. Finally, produced water goes through a gathering system or is trucked to a disposal well where gun barrel separators provide additional OWS. When managed effectively these systems provide sufficient OWS, but like other water management systems, they have to be monitored and maintained to remain operationally efficient.
Oil water separators are normally standard gravity or enhanced gravity fed units. OWS design can vary but in simple terms, they are vessels which use sufficient retention time to allow oil droplets to rise.
As oil rises it forms a separate layer which can be removed by skimmers, pumps or some other method. To improve retention time, baffles are typically installed. Conventional separators normally have three chambers separated by baffles, an influent chamber, a main separator chamber and the effluent chamber. Baffles also used to reduce turbulent flow to help maintain larger oil droplets. The influent chamber removes free oil (described below) that has already separated on its way to the OWS.
Baffles separate the effluent chamber from the main settling chamber to prevent floating oil and scum from entering the main chamber and allow it to be skimmed. The lower baffle extends to the bottom and directs wastewater to the top of the main chamber to prevent short circuiting.
Oil water separators use retention time to allow oil droplets to rise.
The main separator chamber is intended to settle the flow and prevent turbulence or mixing. It is not unusual for a main chamber to be designed with a slope or V-bottom to allow for accumulation of solids. There is typically an oil skimming device at one end of the main chamber for discharge to an oil collection tank. The effluent chamber is also separated by upper and lower baffles for wastewater discharge.
The point of settling down flow is to improve oil droplet rise. The larger the oil droplet the faster it rises. Oil droplet rise is governed by Stokes’ Law. Stokes’ Law was developed to explain the behavior of small particles as they settle in a liquid medium. Stokes’ Law is as follows:
Fd = 6π η Rv where:
• Fd is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particle.
• η is the dynamic viscosity.
• R is the radius of the spherical object
• v is the flow velocity relative to the object.
Even though Stokes Law was written to describe the settling of small particles, it can be applied to oil droplets and their rise.
There are many factors affecting oil droplet rise including oil droplet size, oil specific gravity, temperature, other particles, flowrate and turbulence, to name a few. Generally, most OWS are designed with a water depth not to exceed 3 feet, a width of 6 to 10 feet, a length-to-width ratio between 3:1 to 5:1, a depth-to-width ratio between 0.3:1 to 0.5:1 and a flow through velocity not to exceed 3 feet per minute or 15 times the rate of the rise of the oil droplet.
Based on Stokes’ Law, a 100-micron oil droplet will rise three feet in 5 minutes as opposed to a 20-micron oil droplet which will take 60 minutes. Stokes’ Law however doesn’t apply to small particles or oil droplets. Oil droplets 10 micron and smaller tend to stay in suspension and their behavior is more adequately described by Brownian Motion which describes random motion of small particles.
Oil droplets are further defined as follows:
• Free oil – Oil droplets of 150 microns and larger
• Dispersed oil – Oil droplets from 20 to 150 microns
• Mechanically emulsified oil – Oil droplets less than 20 microns
• Chemically emulsified oil – Oil droplets less than 20 microns with a chemical bond to other molecules
• Stable emulsion/dissolved oil – Oil in solution
If the only issue was management of free oil, the OWS challenge would be simplified and more directed by Stokes’ Law. Unfortunately, dispersed and emulsified oil must also be addressed. Often, OWS equipment manufacturers specify that centrifugal pumps are not used as they can transform free oil into dispersed oil. An OWS unit can become quite large given all that goes into the design. This gives rise to enhanced gravity separation, which are systems that include plates and coalescers.
Parallel-plate separators operate under the same principles as conventional separators but are smaller. Parallel-plates can increase retention time by 2 to 3 times. They operate by improving the retention time with an array of closely spaced inclined plates. The incline of these plates can be anywhere from 45 to 60 degrees. Oil droplets rise until they contact the plates where they coalesce, gradually rise and grow in droplet size to eventually float to the top.
To promote coalescence of oil droplets, plates can be made of oleophilic (oil attracting) material like polyethylene and fiberglass. The surface of these plates should be smooth to facilitate migration of oil droplets to the top.
Another way to optimize the oil droplet removal is with a corrugated plate design. Corrugated plates provide additional coalescing surface area. Operators should note these plates require regular cleaning to ensure they are effectively working to promote oil droplet coalescence as designed.
A properly designed oil water separator must accurately characterize the type of produced water it is intended to treat. Specifications should include expected hydrocarbon ranges, including free oil, dispersed oil and emulsified oil. An analysis of expected solids should include volatile and settleable solids. Other measurements should include specific gravity of the expected oil and water, viscosity at expected temperature ranges, expected ambient temperatures and fluid pH. Finally, consideration should be given to the expected flow range and other factors like valves and pumps that could disperse oil.
Applying these fundamentals to existing OWS collection points will help optimize performance of existing systems. Where produced water is being supplied directly from a tank battery, a stand-alone system may be needed with consideration given to the variables just described.
Managing all the variables when designing a system in a dynamic environment with constantly changing flows and water qualities can be a challenge. When operating outside of the design of a system, breakthrough events are possible.
For unconventional oil operators, this is not uncommon and part of the reason for separation at the wellhead and then the tank battery and finally at a disposal well. This redundancy anticipates breakthrough events.
Produced water recycling presents a different challenge for operators. When recycling produced water, a best practice is to start the produced recycling program downstream of gun barrel separators. This allows for retention of OWS redundancy.
Unfortunately, redundancy in the separation process isn’t always possible. The challenge comes from water variations that may require significant retention time. Produced water may contain oxidizers for bacteria, iron or sulfides that can break oil emulsions. An oil water strategy may be needed before the water is put into storage. Emulsified oil is difficult to remove in a conventional separation system and as a result, an oil sheen may develop on pits or tanks even with an effective OWS strategy.
To meet this challenge, water storage systems have evolved to better manage oil emulsions. Today, larger aboveground storage tanks (AST) and smaller storage pits are often used for primary separation.
In these systems, water treatment begins with ASTs and pits to allow oxidation to break emulsified oil and coalesce with any free oil and dispersed oil from upstream. Water retention also allows solids to settle. Booms and turbidity curtains take the place of baffles used in separators while sumps collect and remove solids. Some operators use storage vessels as converted oil water separators because their size allows the retention time needed.