Age 10502

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Process For additional information on this subject, contactFile Reference: AGE10502 R.A. Al-Husseini on 874-2792

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Crude Oil Processing

Engineering Encyclopedia Process


Saudi Aramco DeskTop Standards

CONTENTS PAGES

NEED AND PURPOSE OF THE GOSPS .....................................................................................1

PROCESS FLOW..........................................................................................................................3

SPHEROIDS AND STABILIZERS ..............................................................................................7

GLOSSARY ................................................................................................................................10



Saudi Aramco DeskTop Standards 1

NEED AND PURPOSE OF THE GOSPSSafe and environmentally acceptable handling of the crude is assured by treating the produced crude in theGOSPs and related crude processing facilities. GOSPs were first used in Saudi Arabia to relieve the excesspressure due to associated gas in the produced crude. As the water content of the produced crude increased,facilities for control or elimination of water were added. Later, as the effects of corrosion due to salts in thecrude were recognized, modern desalting equipment was included in the GOSP design. With the constructionof the Master Gas System, gas compression and recovery facilities have been added, saving gas that waspreviously flared.

Raw crude oil may have the following materials present:

• Water – Emulsions (up to 3% of crude)– Free water (above 3%, to 30%)

• Salt – 50,000 to 250,000 mg/liter formation water

• Gas – Dissolved gas (up to 600 scf/bbl crude). Results in production pressures in excess of 3000psi in some fields

• H2S – Up to 1000 ppm

The sour, wet crude must be treated to make it safe and environmentally acceptable for storage, processing, andexport. The concerns, due to various contaminants, are shown in the following table.

Contaminant Safety Environmental Corrosion

Water X

Salt X

Gas X X

H2S X X X

Specifications of crude for export include:

• Water content – 0.3 vol %, maximum

• Salt content – 10 pounds (as NaCl) per 1000 barrels

• Vapor pressure – 11 psig TVP (4-5 psi RVP)

• H2S – 70 ppm




Gas is evolved from the produced crude when the pressure is reduced to the crude bubble point, thetemperature and pressure at which part of a liquid begins to convert to a gas. For example, if a liquid is held atconstant pressure, but its temperature is increased, a point is reached at which bubbles of gas begin to form inthe liquid. This is the bubble point. Similarly, if a liquid is held at constant temperature but the pressure isreduced, the point at which gas begins to form is the bubble point.

The volume of gas associated with a typical crude is shown graphically in Figure 1.

FIGURE 1. Vaporization and Expansion




Process FlowA modern GOSP includes the process equipment required for pressure reduction, gas removal and recovery,water and salt removal, and water cleanup and disposal, as well as a number of associated items.

The GOSP has evolved as engineers sought to control additional crude quality concerns close to the point ofproduction (water, salt, recovery of gas). The design also reflects the quality, in terms of pressure, water andsalt content of the crude, and accessibility to gas gathering systems. Thus, it is not practical to establish oneGOSP design as a standard for all production fields in the Eastern Province.

A typical GOSP flow diagram is shown in Figure 2.

FIGURE 2. Typical GOSP Plan




The dominant features of all GOSPs are the long, cylindrical oil-gas separators used for stepwise pressurereduction. A modern oil-gas separator or trap, as shown in Figure 3, contains elements designed to ensuresmooth flow and elimination of oil mist carryover into the gas stream.

FIGURE 3. Oil/Gas Horizontal Separator

The horizontal separator has the maximum liquid surface area for releasing gas from a large volume of crudeoil.

Key components of a modern trap, as shown in Figure 3, include:

A. Impact baffle – reduces the velocity of the incoming crude

B. Defoaming element – breaks up foam produced by gas escaping from the liquidcrude

C. Mist extractor – collects fine droplets of crude oil mist that is carried from the oil by the gas so that the gas leaving the trap will be as “dry” as possible

D. Wave breaker – keeps the liquid surface as smooth as possible for gas releaseand for control of the liquid level in the vessel




Modern desalting operations use wash water and emulsion-breaking chemical injection to remove salts carriedby the formation water produced with the crude oil. A modern two-stage desalting operation is depicted in theflow diagram shown in Figure 4. Details of a desalter/dehydrator are shown in Figure 5.

FIGURE 4. Electrical Desalting – Two Stage




FIGURE 5. Desalter/Dehydrator Details




SPHEROIDS AND STABILIZERSAdditional processing is required to release the final portions of associated gas as well as the H2S present in thecrude. A typical spheroid, used to reduce the crude pressure to very near atmospheric, is shown in Figure 6.

FIGURE 6. Typical Spheroid

The liquid level in the spheroid is held at approximately 30 per cent to allow mist in the vapor to settle out andreturn to the liquid.




To release the H2S associated with the crude, mild heating followed by vapor stripping is employed, as shownin Figure 7.

FIGURE 7. Crude Stabilization

While the spheroids are usually located at oil processing centers such as Abqaiq and Ras Tanura, rather than atthe GOSP locations, the crude stabilization operations are always centrally located. Figure 8 shows the overallprocessing employed at Abqaiq, including spheroids, crude stabilization, and gas recovery. Followingstabilization, the crude oil is safe for storage in fixed or floating roof tanks and for transportation by oceantanker.




FIGURE 8. Abqaiq Plants Processing

After stabilization, the crude oil should meet all specifications for shipment.

• RVP 4 psia, maximum (11 TVP, maximum)

• H2S 70 ppm, maximum

• Salt 10 pounds per 1000 barrels, maximum

• Water 0.3 vol %, maximum

Figure 8 shows the overall crude processing at Abqaiq, including spheroids and stabilizers. The stabilizedcrude is ready for export or refining. The gas collected from the spheroids and stabilizers is compressed, cooledto condense materials heavier than ethane, and then distilled to separate methane and ethane from heaviermaterials, which are sent to Ras Tanura for further fractionation.




GLOSSARYassociated gas Gas released from the crude oil when the pressure on the crude is

lowered below the crude bubble point. Associated gas comes fromthe crude as opposed to gas produced from a gas well.

bubble point The temperature and pressure at which part of a liquid (crude oil)begins to be converted to a vapor (released gas).

desalting The process of using chemicals, heat, wash water, and anelectromagnetic field to remove salts from the crude oil.

emulsion A mixture in which one liquid (water) is uniformly andhomogeneously distributed in another liquid (crude oil).

flow diagram A graphical representation of a process or system showing majorflows, equipment, and control schemes for that system. Small flowsand instrumentation details are not included.

formation water Water contained in the rock formation from which crude isproduced, and which is produced concurrently with the crude.

gas recovery The processes and systems used to recover gas released from thecrude for a useful purpose rather than flaring the gas.

GOSP Gas-oil Separation Plant.

material balance The concept, applied to a process or system, of accounting for allinput materials in the output streams ( “what goes in must comeout”).

pressure reduction The process of systematically lowering the pressure held onproduced crude to allow gas to be released and make the crude safefor handling at low pressure.




produced crude A term applied to crude oil as it comes from the wellhead, prior toany treatment or processing.

spheroid A spherical tank, operated at near atmospheric pressure, for thepurpose of making the final step in lowering the crude pressure.

stabilization The process consisting of heating followed by vapor stripping of thecrude and intended for control of the vapor pressure and H2S contentof the crude.

trap A long, cylindrical, horizontal vessel used for gas-oil separation inall GOSPs.

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