Crude Oil Treatment

Why Crude Oil Treatment is Required

Each crude oil must be treated in order to make the product competitive with the standards and suitable for sale. Normally, according to law, Crude Oils shall be with following characteristics:

• Water Content: max 0,2 % vol
• Salinity: max 2,8 mg/lt
• HZS max 70 ppm
• Sediments: unmeasurable traces on a 100 cc sample

Based on the required product and based on the reservoir fluid, as shown before, the crude oil contains numerous foreign substances which require removal or reduction at the same value as shown here below:

• Oil/gas/water and sediment separation
• Dehydration and desalination
• Desulfurization
• Stabilization

Separation

The purpose of this initial crude oil treatment is to separate the gas and water from the oil with the maximum possible oil recovery. The separation is mainly performed with sudden changes in the flow direction and speed reductions. These performances, which occur in equipment called separators, allow their separation with different specific weights of the products.

Crude Oil Dehydration and desalination

The water present in the crude oil cannot be always removed from the separators, often a fair amount of water is present in the form of an emulsion. Basically, there are three methods to break this emulsion and thus obtain the dehydration of the crude oil. With these methods, desalination is also obtained, in fact, the salt content in the crude oil is generally a direct consequence of the water content which is salty.

1. Thermal method: Increasing the temperature favours the breakdown of an emulsion.
2. Chemical methods: they consist of the injection of Demulsifier chemical products
3. Electric methods: under the action of an electric field, the drops of water contained in the crude oil coalesce and form larger drops which decant easily (DESALTER).
   
   

   Emulsion

   In our case, an emulsion is defined as a mixture of water and crude oil. The water is in the form of very fine droplets dispersed in the oil.

   An emulsion forms due to violent agitation (which occurs during the ascent of the fluid through the tubing, and then to the wellhead, passing from the Choke Valve and in the process lines). The emulsifying agents in the oil field can be: asfalteni, resine, acidi organici, sali e molte altre sostanze presenti nel fluido. Questi agenti si trovano nell'interfaccia fra l'olio e la gocciolina d'acqua e formando un film attorno a queste ne impediscono la coalescenza.

Desulfurization

For selling the Crude Oil correctly, the value of H2S deve necessariamente contenere un quantitativo di H2S fissato in 70 ppm massimo. The simple separation process is often not sufficient to guarantee this value; this means that a specific treatment is required which is called desulphurisation or softening of the oil must be carried out.

The softening of the oil is obtained by backwashing (stripping) with sweet gas, in fact, the hydrogen sulphide, being a gas, is dragged along by the stream of sweet gas, thus reducing its presence in the oil. With this method, will be obtained an oil with an H2S content of about 20 +/- 3O ppm is obtained.

Stabilization

Oil stabilization means removing the more volatile substances, i.e. the lighter hydrocarbons such as methane, ethane, propane and butane.

The stability of crude oil is measured with a system called R.V.P (Reid Vapour Pressure) which represents the pressure developed by a sample of oil heated to 37.89°C (100°F). Figure C below shows an R.V.P analyzer

Figure C - R.V.P analyzer

Italian standards establish that the oil is considered stabilized if it has an R.V.P. maximum of 1 kg/cm2.

This result can be achieved by placing a series of separators that work at decreasing pressures to release all the volatile products by exploiting the pressure drops. For some types of oil, this method may not be sufficient. In this case, it is necessary to use a stabilization column equipped with bottom heating and reflux. The column works like a distillation column, allowing one to get a product which respects the required parameters.