what is Wet Crude?
Raed Al-Obaidi
in the beginning of the production from any reservoir the crude oil contains no water and salt; with time it begins to produce large amounts of water and salt, although the density difference guarantee that water stays bellow crude oil in the reservoir, but sometimes water content will with time.
wet crude facility was founded to reduce the negative effects of water and salt content, especially on the crude oil production and treatment equipment, the crude oil price, and specification.
during wet crude treatment, salt and other impurities are removed from oil, these salts are dissolved in the reservoir water. it is either free water, this thing makes it easy to separate by density difference , or in the form of small droplets dispersed in oil ,the salt content contain chlorides, iron oxides, sand, crystallized salt, and sulfur, these salts are removed during the desalting.
here are some of the negative effects of water and salt contents in crude oil:
1. salt deposition in furnaces and heat exchangers : this will reduce these units efficiency, because salts will stay in the tubes as solid bulks that reduce their diameter, hence; increasing its skin temperature and damaging them, such damages will stop these units for maintenance causing a big lose in production.
2. HCL forming : chlorides decomposition during distillation then its reaction with water that exists in oil will form HCL which causes corrosion for the oil production facilities.
3. water content will increase energy consumption.
4. reducing the oil pipelines and equipment capacity.
How Water is Mixed with Crude Oil?
water exists in 2 forms in crude oil:
- Free Water: it is spreaded in oil as big droplets, easy to separate from crude oil as soon as it has enough time.
- Emulsified Water :water is spreaded in oil as an emulsion, which are very small droplets surrounded with a strong film caused by emulsifier, this type is hard to separate.
for more information read also Emulsion Treating.
Wet Crude Treatment Facility:
any typical wet crude treatment plant consists of the following:
- Crude Oil Tank(s) which is used to store wet crude oil, sometimes used to separate the remained free water as drain.
- Crude Oil Pumps: are used to pump oil to the treatment trains.
- Demulsifier Skid : consists of demulsifier tank and pumps that are used to pump demulsifier to the crude oil pipeline.
- Heater: indirect fired heaters are used in the plant to heat crude oil and reduce its viscosity.
- Dehydrator & Desalter: used to separate water and salts from oil.
- Wash Water system: supplies wash water and consists of wash water tank and pumps.
- Produced Water System: collects the produced water from dehydrator and Desalter and treat it.
- Heat Exchangers: which are of 2 types; oil/oil heat exchanger and water/ water heat exchanger.
- Auxiliary systems: such as air compressor system, and fire-fighting systems.
as we stated in Optimizing Wet Crude Treatment, there are 3 steps to treat any water in oil emulsions:
- breaking the emulsion: break and tear the film surrounding the water droplet, this thing requires heat, electric charge and demulsifier aid.
- Coalescence: combining the water droplets that became free after breaking their films, forming bigger droplets, easy to settle, it is a function of time, increasing time will increase coalescence, and it is supported by water washing and electro-static field.
- Settling of water droplets because of gravity.
in general, these 3 steps are shown in the figure bellow:
Wet Crude Treatment Process Description:
the wet crude is pumped from the storage tank(s) by pump(s) to the dehydration/desalting train(s), which consists mainly from dehydrator, heat exchanger, heater and desalter, wet crude is pre-heated in the oil/oil heat exchanger in which it gains heat from the desalted oil, then it is further heated in the “indirect fired heater” , it reaches a temperature of about 125-150° F.
before entering the dehydrator it passes through the mixing valve in which mixing happens among wet crude, demulsifier and wash water, in the dehydrator or desalter; the wet crude enters the vessel from the lower part of each vessel in a way that guarantee its distribution along the vessel, then it is subjected to an electrostatic field by the means of 2 or 3 electrical grids connected to a transformer, the demulsifier works with electric charges break the emulsion film, then each droplet is charged with a positive or negative charge, this will lead to collision among these droplets, forming bigger droplets, easy to settle down, it becomes a layer of free water known as “produced water”, while the desalted crude oil leaves the vessel from the upper part.
and here are some notes about the wet crude treatment plant:
– if the desalted oil is out of specification, it is redirected to the inlet crude tank to be retreated again.
– fresh water is used as wash water, if there is a lack in wash water, you may re-use the produced water from the desalter and inject it back in the dehydrator inlet. because it has less salinity than the crude oil entering the dehydrator.
what about produced water?
after produced water is collected, it flows through a coalescer to eliminate oil droplets, separate them and take them back to the inlet crude tank, while produced back flows through a filter, then collected in produced water tank; but what happens next?
it depends on the environmental regulations of the country, if they are weak, produced water can be disposed to any land or river, or sometimes are re-injected to a dead well to maintain the reservoir pressure, in this case it need some chemicals to be added into, such as corrosion inhibitor, biocide and oxygen scavenger.
while in other countries, with strict environmental regulations, produced water is further treated to make use of it for irrigation or as a drinking water.
for further information see our Produced Water Treatment Books section.
