Produced Water Treatment
In production equipment, produced water often refers to the aqueous phase effluent or dirty water coming off the bottom of a FWKO vessel, separator, heater-treater or electrostatic coalescer. The term produced water is often used to describe the connate water that exists in the oil or gas reservoir and can be evident. It may also describe free-water or emulsified water together with the produced oil.
Since produced water often contains oil-coated solids or emulsified oil, the water must be treated to reduce the hydrocarbon content to an acceptable level. The operational process or equipment that cleans the effluent water is referred to as the produced water treatment system. The effluent specification for produced water often depends on where the water is to be discharged and is based on local regulations.
For example, produced water from oil and water separators is treated so that the clean water may be re-injected into the reservoir or discharged overboard while the separated oil can be returned to the other downstream processes. Depending on the process used for cleaning up the produced water, it may need to be sent to a de-gasser vessel before being re-injected or discharged.
Produced water treatment systems are often supplied as complete turnkey, skid-mounted or modularized packages and typically consist of one or more of the following components:
- Desanding hydrocyclone
- Deoiling hydrocyclone
- Oily water separator
- Corrugated plate interceptor (CPI)
- Corrugated plate coalescer (CPCTM pack)
- Degasser or gas flotation vessel
- Dissolved gas floatation (DGF)
- Induced gas floatation (IGF)
- Nutshell filter
As with any packaged system, skid-mounted or modular packages would also include piping, valves, instrumentation, utilities, and control facilities.
Typical Produced Water Treatment Flow Diagram
Desanding Cyclones and Hydrocyclones
As production fluids arrive from the wellhead or, further downstream, from production equipment, they often contain unwanted sand and sediment. These undesirable components can accumulate in low velocity tanks, plug small diameter pipes, foul heat exchangers, and cause severe erosion. Thus it is preferable to remove these problematic solids as far upstream as reasonably possible in desanding cyclones or hydrocyclones.
Desanding hydrocyclones are used to remove the solids content of water before re-injection or subsequent water treatment equipment. Production fluids enter the cyclone body tangentially and sand is removed by centrifugal force, pushing the solids to the cyclone wall. The heavier solids on the outer wall are removed from the cyclone tube at the bottom and accumulate in the lower chamber. The chamber can be emptied manually or via an automated blowdown system. The tapered cyclone tube allows the liquid to continuously flow back up and out of the hydrocyclone separator.
Deoiling hydrocyclones are designed and commonly used to clean produced water containing 2,000 mg/l of oil down to a produced water specification 40m g/l or less. Deoiling hydrocyclones are similarly constructed and operate in much the same way using differential pressure and a tangential inlet to create centrifugal separation inside the cyclone body. Hydrocyclones are also very reliable as they do not contain any moving parts. Liners are removable and the vessel can be taken out of service to replace the liners as needed. The mechanics of hydrocyclones are based on the density difference between the aqueous and hydrocarbon phases. For greater detail, see this article by M. Bennett and R. Williams.
In many water treatment applications, a degasser vessel is used as a final polishing stage. The primary function of the degasser is to release dissolved gas prior to discharge of the produced water. Its secondary function is flotation and skimming of any remaining oil. As the liquid passes through the vessel, the different phases separate due to the difference in liquid densities. Inside the degasser, both the oil and gas will rise to the top surface and separate from the water phase. The gas droplets stick to the oil droplets and, due to the flotation effect, increase the speed of the oil droplets rising in the vessel, thereby improving the separation efficiency. Oil accumulates inside the centralized oil bucket and is sent to a slop tank or wet oil system.
Degasser vessels have other operating benefits. The vessel degasses the water prior to discharge and removes oil in the event an oil slug coming from the separator or hydrocyclones. Further enhancement of the flotation effect can be achieved by inducing gas and creating a fine dispersion of gas bubbles.
The key to efficient operation of the gas flotation vessel is to control the bubble size and induce the correct amount of swirl. Gas Flotation Vessels require a certain quantity of gas in the feed to act as flotation gas. This can be provided either as a direct fuel gas feed, or gas can be recycled around the vessel using an educator loop.
A water recycle stream is taken from downstream of the vessel using the Flotation/Recycle Pumps to the gas educator. The recycle stream passes through the gas educator, which uses this motive water to suck gas from the flotation vessel and create a gas/water mixture at the educator discharge. This mixture is then injected into the flotation vessel to facilitate flotation.
Nutshell Filters are vertical vessels designed to remove solids and oil from water. Typically these filters are down-flow filters packed with nutshell/garnet media filters, supported on a coarse garnet bed.
The fluid enters the filter vessel and impinges on the inlet deflector, sending an even flow onto the media bed. The water will flow down through the bed at a low flux rate. The filtration duty is provided by a combination of granular walnut shell and fine garnet. This is specifically designed to remove the solids impurities in the water and the majority of the remaining oil.
Raw produced water passes first through the upper layer nutshell media where the majority of the dispersed oil is absorbed by the media. Nutshell media is well-known as the optimum media for oil removal. Coarse solid particles are also trapped in this layer. The water then passes through a lower layer fine garnet media for polishing. This layer has a significantly finer grain size than the upper layer of nutshells, and fine garnet has long been established as the optimum media for fine particulate removal, being capable of high removal efficiencies at 3 microns and above.
...fine garnet has long been established as the optimum media for fine particulate removal, being capable of high removal efficiencies at 3 microns and above.Click to tweet
Residual fine oil droplets are also entrapped in the fine garnet layer. Finally treated water passes through a coarse garnet support layer, which has no function in filtration terms but is a vital part of the overall design. This layer provides support for the fine garnet and ensures that no blockage of the collection laterals occurs. The coarse garnet also assists in the water and gas distribution during backwashing. Backwash on the Nutshell Media Filters is triggered either on a fixed interval timer or on a high bed differential pressure.
Depending on the client’s requirement, VME may also include sand jetting in the vessels. Sand jetting is used for flushing of solids that have accumulated in a vessel by a series of nozzles and drains. The sand jet nozzles are series of nozzles located on a pipe that runs along the bottom of the vessel. During the sand jet sequencing, the drain valves are opened and the nozzles are activated. Clean water is used to flush the solids particles that have accumulated in the vessel, to the drain ports. Sand jet sequencing is done periodically on the vessels, typically on a daily or weekly basis. The vessel size and solids loading will determine the frequency.