Questions Asked in Production & Process Engineer Interview with the Answers:

Q: What is Crude Oil?

A: Crude oils is a complex mixture Chemicals, especially hydrocarbons.

Q: how crude oil is classified into: sour and sweet oil?

A: if the sulfur content in crude oil is less than 0.5%, it is considered sweet, if it is more than 0.5% then its considered sour oil.

Q: How crude oil is classified into light, medium, heavy and extra-heavy oil?

A: it is classified according to its API gravity as follows :

– Light Oil: if its API is more than 31.1o.

– Medium Oil: if its API is between 31.1o and 22.3o.

– Heavy Oil: if its API is between 22.3o and 10o.

– Extra-heavy oil: if its API is less than 10o.

Q: if the GOR of a certain well is 300, what does that mean?

A: it means that this well produces 300 Standard Cubic Feet “SCF” per each barrel of oil.

Q: what does “Flowline” means?

A: it is the line that connects the oil well to the first production facility, mainly GOSP “Gas Oil Separator Plant”.

its diameter is determined according to wellhead closing pressure, the distance between the wellhead and the GOSP, 1st stage separator pressure, and the oil production maximum flowrate.

Q: what does GOSP mean?

A: it means “Gas-oil separation plant (GOSP)”

Q: what does “Manifold” means in GOSP?

A: it is a collection of pipes and valves used to determine the amount of crude oil that enters each separator, sometimes it is used to determine the crude oil to go to the test separator or production separator.

Q: what does flare system consists of?

A: it consists of : flare stack, flame arrestor, and sometimes Knock out drum which eliminates oil droplets from gas stream, preventing them from being burned in the flare.

Q: what are the Auxiliary Systems in any oil production facility?

A: auxiliary systems are: Instrument Air System and Fire Fighting system.

Instrument Air system consists of: Air Compressor, Air Dryer system and instrument air pipelines that reach to control valves.

Fire Fighting System consists of: fire water tank, fire fighting pumps, fire water network and foam network connected to oil tanks.

Q: what is “Choke Valve“?

it is a valve used in oil and gas production wells to control the flowrate and the wellhead pressure, it allows fluid flow through a very small opening, this valve opens according to 64 Basis, i.e., 32/64 means half an inch, 64/64 means 1 inch.

Q: What is “Check Valve” ?

A: this valve guarantee one-way flow, in other words; it prevents back flow, in this way you can protect your equipment that might be affected by back pressure.

Q: what does ESDV means?

A: its an abbreviation of “Emergency Shut Down Valve”, when the facility is shut down ,this valve will be closed or opened according to the required situation; i.e., if it is installed at the discharge of a gas compressor station, it should be open when Shut Down happens to dispose all the gas to the blow down, if it is in the suction of any oil facility, it should be close to prevent any further amount of oil entering the facility. this valve works after getting certain signals like LAHH in a storage tank or separator, or a PAHH which indicates a high value of pressure.

Q: what is gas separator?

A: it is a pressure vessel that deals with well streams then separate them into gas and liquid phases.

Q: how gas separator is classified?

A: Separators are classified in many ways:

1. according to its Shape into: Vertical Separator – Horizontal Separator – Spherical Separator.

2. according to No. of Phases into: 2-phase separator – 3-phase separator.

3. according to operating pressure into:

– high pressure separators “750 – 1500” psi.

– Medium pressure separators “230 – 750” psi.

– Low pressure separators “20 – 230” psi.

4. according to its Function into: Test Separator – Production Separator, test separator is used to test well productivity , while production separator is used as a production facility.

Q: How can you control the separator pressure?

A: separator pressure is maintained by controlling the amount of gas leaving the separator, that is achieved by controlling the gas outlet with a Pressure Control Valve “PCV” ,if not; manual valve is used, when throttling this valve you will raise the separator pressure, and vice versa.

Q: where the separator oil outlet go?

A: the root of separator oil outlet will be as follows:

– if the oil leaving the separator is wet crude, then it will go to the “Wet Crude Treatment Facility”.

– if the oil contains H2S, it will go to the “Oil Sweetening Units” or “H2S Stripping Units”.

– if the oil do not contain water or H2S, then it will be ready to export, or as a refinery feed.

Q: where the separator gas outlet go?

A: it will go to the gas processing facility if it is nearby, if it is far away, then it will go to the gas compressor station, in order to compress it and help it reach this facility.

Q: what is “Gas Dehydration Unit” and what is used for?

A: it is used for removing water vapor from natural gas using one of liquid desiccants such as TEG or DEG, water and H2S reacts to form H2SO4, and that is why Dehydration unit is used.

Q: why mutli-stage separator plat is used?

A: when the GOR is high, there is a need to separate the gas in multiple stages to decrease the pressure gradually and to separate as much as gas possible, the number of stages is determined by a complex calculations, but you have to take into consideration the cost of installing an additional separator which will need also valves and controls.

Q: what is the purpose of the following separator internals?

– Inlet Diverter: making a sudden change in flow speed and direction, helping the initial separation of gas.

– Mist Extractor: to extract the oil droplets from gas leaving the separator.

– Wave Breaker: to prevent wave formation inside the separator, the thing that confuse the level control.

– Vortex Breaker: to prevent vortex formation at the liquid outlet stream, the thing that cause gas pockets.

Q: what is the relation of volumetric flowrate to Retention Time?

A: according to the following equation , it is inverse ratio:

Q: what is Liquid carryover, and how it is happened?

A: it occurs when liquid droplets leaves the separator with oil outlet stream, it has many causes:

1. when the oil entering the separator is much more that its capacity.

2. a problem in the oil outlet valve, this thing will raise the oil level inside the separator, and it may reach the gas outlet.

3. a problem in the gas outlet valve, this thing will cause the separator pressure to be decreased, hence the ΔP between the well and the separator and then the flow to the separator is increased.

Q: what is Gas Blowby, and how its is happened?

A: it occurs when the gas comes out with liquid phase, in this way gas pockets will be formed in the oil pipeline.

it has many causes:

1. when the separator pressure is increased suddenly to high values, this will cause the level to be decreased to a very low level and helps the gas to go with liquid.

2. a problem in the gas outlet valve, or a human mistake when input vessel pressure.

Q: what is Foaming in oil gas separator?

A: foam is s small bubble of gas surrounded by a thin film of oil, it decrease the separation efficiency and confuse the liquid level transmitter. this phenomenon reduce the retention time because foam takes a volume in the separator. we can handle this in 2 ways:

1. installing “Defoaming Plates” inside the separator.

2. adding chemicals called “Foam Depressants” .

Q: Why Water is unwanted in crude oil?

A: We have to reduce the water & salt content because there are many bad effects for the existence of water and salt in crude oil, they are:

1. Salt Sedimentation in heat exchangers and furnaces, the thing that reduces their efficiency, such sediments reduce the pipe diameter, reduce the heat transfer area, thus their diameter will be reduced causing severe damage to these pipes. any damage in these pipes will need repair and maintenance and a great lost in production rates.

2. Forming hydrochloric acid: it is formed because of the decomposition of chloride salts and their reaction with water, this acid is very corrosive, and may corrode pipes and equipment.

3. increasing the energy consumption: water existence needs more power, for example; water needs 8 times the power needed to heat oil.

4. multiple effects: reduce the pipe and equipment capacity used in oil treatment.

Q: How the water is mixed with crude oil?

A: water is spread in oil in 2 ways:

1. Free water: which is spread in the form of big particles of water, they are separated easily when settling.

2. Emulsion water: in which waters is surrounded by a strong film because of emulsification factors, this kind is not separated easily.

Q: what are the components of Wet Crude Treatment facility?

A: any wet crude treatment plant consists of the following:

1. wet crude tank: the main storage of wet crude.

2. main pumps: pumps wet crude to the treatment trains.

3. Demulsifier System: consists of a tank and two pumps.

4. Indirect heater: heats wet crude to reduce its viscosity.

5. Dehydrator.

6. Desalter.

7. Wash Water system: consists of a tank and two pumps.

8. Produced Water system: collects and treats the produced water from the process.

9. Heat Exchangers: they are of two types: water/ water and oil/oil.

10. Auxiliary systems: such as Instrument Air system and Fire – Fighting system.

Q: what is Emulsion?

A: a heterogeneous system that consists of at least one immiscible liquid dispersed in another liquid in the form of droplets.

Q: what are Emulsifiers or emulsifying agents?

A: they are substances that cause the formation of interfacial films around the dispersed oil droplets, preventing the coalescence process.

Q: How to form a stable emulsion between two liquids?

A: there are three conditions to form a stable emulsion between two liquids:

1. should be immiscible liquids.

2. a sufficient power to obtain this dispersion.

3. the existence of an emulsifying agent.

Q: what a demulsifier mean?

A: high molecular weight polymers used to equalize the effect of emulsifying agents and to reduce the surface tension of water droplet.

Q: what are the emulsion treatment steps?

A:

1. breaking the emulsion: means the breaking of the film surrounding the water droplet.

2. the coalescence of water droplets after breaking the emulsion forming big droplets ,easy to settle, this process is made by the aid of electrostatic field.

3. Settling of the big droplets to the lower part of the vessel.

Q: what does Stock’s Law mean?

A: it is the law used to calculate the downward velocity of water droplet in oil ,this is Stock’s law:

reducing oil viscosity is essential in wet crude treatment, this is achieved by heating the wet crude oil by different means such as: heat exchangers, direct or indirect heaters, because viscosity is inversely proportional with the downward velocity of water droplets.

Q: what is wash water? and how it is used?

A: using fresh water is useful to reduce the concentration of the salt solution, this thing will help in reducing the salt content in crude oil, the mixing of crude oil and wash water occurs to get a good mixing between them.

using too much wash water leads to excessive current or a short circuit between the grades, may create paths for current leak , decrease the retention time and may cause water carryover with oil outlet.

using less amount of wash water, will reduce water washing, leading to high salt content in the oil outlet.

Q: how to organize the wash water ratio?

A: wash water ratio is organized by a control valve called “Ratio Valve”, all you have to do is to give a setpoint, this valve receives a signal from crude oil and wash water flowmeters, then closes or opens the wash water valve to control to achieve the required setpoint.

Q: what is the best wash water ratio?

A: the best wash water ratio is (3 – 5)% of the treated oil.

Q: How to calculate associated water?

A: Associated water is calculated as follows:

Associated Water = Produced Water – Wash Water

Q: what is the pump type used for chemical injection, and why?

A: reciprocating pumps is the better option for chemicals injected into ant stream, the main features of this kind of pumps which make it the most preferable type : low flowrate and high pressure, high pressure is required to help the chemical to be injected in a high-pressure stream.

Q: what is the best points of demulsifier injection?

A: the best points are:

– in the main pump suction to help better mixing with wet crude oil.

– before the mixing valve.

– 50 to 60 meters before the dehydrator.

Q: what is the best retention time for wet crude treatment?

A: according to the practical numbers, 10 to 30 minutes is the best retention time for wet crude treatment process, heavy crude needs longer time, to reduce their viscosity first.

Q: How to use electricity in wet crude treatment?

A: there are electric transformers are installed over dehydrators and desalters, these transformers are connected to electrical grids made of carbon steel, this system helps the separation of water by subjecting the emulsion to a high voltage electric field, water droplets becomes charged when water in oil emulsion passes through these grids, so these droplets are attracted and repulsed with each other according to their electrical charge, this will help them collide, form bigger droplet then settle.

electricity weakens the water droplet film, stretching it longitudinally and tangentially because of reducing the surface tension.

Q: how wet crude is mixed with wash water and demulsifier?

A: wet crude, wash water and demulsifier are mixed together in “Mixing Valve” works by maintaining a pressure difference through it, the best setpoint for ΔP through this valve is “10 – 15” psig.

Q: what does “High water content in the desalted crude” means?

A: there are many reasons for this:

– using too much wash water, Remedy: reduce wash water.

– high water content in the inlet wet crude, Remedy: increase demulsifier rate.

Q: what does “High salt content in the desalted crude” means?

A: there are many reasons for this:

– high salt content in the inlet wet crude oil or low wash water ratio, Remedy: increase wash water ratio.

– high inlet wet crude flowrate, Remedy: reduce the flowrate of the wet crude facility.

– insufficient mixing between wet crude and wash water, Remedy: increase ΔP through the mixing valve.

source : https://www.arab-oil-naturalgas.com/