Produced water treatment equipments
There are several pieces of equipment available that can be used separately or in series with one another to condition the water. For example, three such pieces of equipment are a skimmer, a coalescer and a precipitator. All three are used to reduce the oil and/or solids content of the water. Equipment is also available which can be used to remove corrosive gases from the water. The types of solids or materials to be removed and their quantity, in respect to the location of the treating facilities, will normally determine the type of equipment to be used to condition the water.
Other equipment that can be used to improve water quality includes:
- Flotation cells
- Pits and skim ponds
- Deaerating towers
- Rapid filters
- Water hold tanks
- Cartridge filters
In addition to the equipment listed above, transfer pumps are used to transfer water from one location in the system to another and water injection pumps are used to provide the pressure to dispose of or inject the water into a subsurface formation.
The skimmer is used to remove oil and large sized suspended solids that have a tendency to settle on their own accord.
Skimmers normally do not contain a filter or coalescing bed to aid in the removal of residual oil or suspended solids. Skimmers may be used alone to condition water, or they may be used in conjunction with a coalescer or precipitator. The skimmer is normally used ahead of a coalescer or precipitator.
Contaminated water enters the skimmer through a flume and spreader plate very similar to the flume in a gun barrel. Gas is vented out the top of the flume and the water falls down the flume and exits around the spreader plate into the skimmer. Free oil floats to the top of the unit and is discharged out the oil outlet line. Solids settle out in the bottom of the skimmer and are removed when a sufficient volume has accumulated. The water is drawn from the tank above the accumulated solids and flows to another vessel for additional conditioning or disposal.
When a skimmer is being used to polish water the skimmer oil box needs to be set to flow constantly and to maintain less than 2% floc and oil being dumped out the oil box. In other words, constantly skim whatever accumulates and let it recycle. Care must also be taken to insure that the water is not over-treated when a skimmer is used to polish the water.
Coalescers are used to remove finely dispersed oil droplets and oil wet solids from produced waters. Coalescers use a filter bed to remove the solids. The filter bed also aids in the coalescing the residual oil into large enough droplets to allow them to rise to the oil section of the vessel.
Contaminated water enters the coalescer near the bottom of the vessel and flows over a baffle plate. Free oil rises to the top of the vessel and is withdrawn. The water passes through the filter bed where the oil wet solids and dispersed oil are trapped and removed. The clean water enters the clear water section and exits the vessel.
Coalescers are outfitted with a backwash system for removing the solids from the filter bed and to lower the incidence of filter plugging and channeling of the water.
Coalescers may be used upstream or downstream of a skimmer in a water treating system, however they function best when placed downstream of the skimmer. Caution must also be exercised to prevent overtreating as the filtering media will plug.
Precipitators are used to treat large volumes of water containing finely dispersed oil droplets and little if any suspended solids. Precipitators utilize a larger filtering-coalescing section for oil removal than the filtering section found in the coalescer. Precipitators are not equipped with backwashing equipment, therefore they cannot handle waters which contain solids.
The water enters the vessel through a downcomer and any incoming gas is vented. The water falls through the downcomer and enters the enlarged section of the vessel where its velocity is reduced allowing free oil to rise to the top of the unit. The water passes through the, filter-coalescing bed where the dispersed oil coalesces and subsequently rises to the skimming section for removal from the vessel. The water flows to the clear water section and exits the vessel.
Precipitators may be used downstream, but not upstream, of a skimmer as a significant amount of solids will plug the coalescing bed in the precipitator. Again, the oil section of the precipitator should be set to constantly skim as much as 99% of the water. Overtreating will plug the filtering-coalescing section and recycled returns can cause interface emulsions that only acids will resolve.
An anionic copolymer injected into the total fluid, upstream of the various water treating vessels, is often advantageous in controlling or minimizing floc-oil interfaces and also it provides additional help in attempting to maintain a oil in water content of less than 48 ppm. The EPA regulations have set the oil in water content limit offshore at 29 mg/l with no visible sheen on the water.
Pits and skim ponds
Concrete lined pits and skim ponds have been used for years to remove suspended materials from produced waters. In fact, the earliest attempts to demulsify crudes involved the use of open pits. Many of the pits or ponds are compartmentized and they utilize a series of baffles and weirs to assist in the separation of solids and- oil from the water. Also the units are of such size in most cases that the retention or settling time may be calculated in the terms of days, not hours.
Surface pits and ponds make for an open system; the fluids are exposed to the atmosphere and they become contaminated with oxygen and any other debris that may be blown into them.
Waters removed from pits or ponds will require treatment with oxygen scavengers to negate the effects of oxygen corrosion and filtration to remove fine particulate materials.