The fluid produced at the wellhead consists usually of gas, oil, free water, and emulsified water (water–oil emulsion). Before oil treatment begins, we must first remove the gas and free water from the well stream. This is essential in order to reduce the size of the oil–treating equipment.
As presented in previous chapters, the gas and most of the free water in the well stream are removed using separators. Gas, which leaves the separator, is known as ‘‘primary gas.’’ Additional gas will be liberated during the oil treatment processes because of the reduction in pressure and the application of heat. Again, this gas, which is known as ‘‘secondary gas,’’ has to be removed. The free water removed in separators is limited normally to water droplets of 500 µm and larger. Therefore, the oil stream leaving the separator would normally contain free water droplets that are 500 µm and smaller in addition to water emulsified in the oil. This oil has yet to go through various treatment processes (dehydration, desalting, and stabilization) before it can be sent to refineries or shipping facilities.
This chapter deals with the dehydration stage of treatment. The objective of this treatment is first to remove free water and then break the oil emulsions to reduce the remaining emulsified water in the oil. Depending on the original water content of the oil as well as its salinity and the process of dehydration used, oil-field treatment can produce oil with a remnant water content of between 0.2 and 1%. The remnant water is normally called the bottom sediments and water (B.S&W). The treatment process and facilities should be carefully selected and designed to meet the contract requirement for B.S&W. Care should be taken not to exceed the target oil dryness. Removal of more remnant water than allowed by contract costs more money without any benefit.
The basic principles for the treating process are as follows
- Breaking the emulsion, which could be achieved by either any, or a combination of the addition of heat, the addition of chemicals, and the application of electrostatic field
- Coalescence of smaller water droplets into larger droplets
- Settling, by gravity, and removal of free water