How to design your oil and water separator 6 factors you need to consider to achieve maximum performance

By Morten Halleraker and Salvatore Micali

www.sentech.no

Author biography

Morten Halleraker:

Morten Halleraker is the Managing

Director of Sentech AS, located in Oslo,

Norway, where he is responsible for all

the management activities of the

company.

Mr. Halleraker has a technical

background and he started his career in

the oil & gas industry working as

offshore specialist on different North Sea

platforms. He cofounded Sentech AS in

1996, and for over 26 years he has been

heading the company.

Mr. Halleraker is an expert in the field of

industrialization of instrumentation for

process systems applications. He owns

different patents and throughout his

career he has been involved in numerous

onshore and offshore upstream oil & gas

projects, ranging from North and South

America, to Europe and Middle East.

Salvatore Micali:

Salvatore Micali is the Global Sales and

Marketing Director of Sentech AS, where

he is responsible for the sales and

marketing activities of the company and

for the early engagement with clients.

Mr. Micali holds a BS and MS in

Electrical Engineering and he has more

than 16 years of experience from the oil &

gas and energy industries.

During his professional career,

Salvatore worked in 4 countries: Italy,

Norway, Australia and Malaysia and has

developed an extensive experience in the

industrial application of new

technologies ranging from subsea

processing and compression,

instrumentation and automation, power

generation and distribution systems.

www.sentech.no

Index

1- Introduction....................................................................................................................1

2.0- The fundamentals: What is an oil and water separator?............................................2

2.1- How does the oil and water separator work?.............................................................3

2.1.1- Residence time.......................................................................................................3

2.1.2-Gas...........................................................................................................................3

2.2- Main components.......................................................................................................4

3- Why is separation important?........................................................................................5

4.0- 6 factors you need to consider...................................................................................7

4.1- Space availability........................................................................................................7

4.2- Residence Time [Mechanical, Electrostatic, Chemical].............................................8

4.3- Pollution/environmental aspects.................................................................................9

4.4- Oil Quality (Fluid composition).................................................................................10

4.5- Reservoire stability & complexity..............................................................................11

4.6- Productivity (hidden costs).......................................................................................11

5- Conclusion...................................................................................................................13

6- Links and references...................................................................................................14

www.sentech.no 1

Introduction

The need to separate a well stream into

water and oil has existed for as long as

we have been drilling for oil. The water

that comes up from the reservoir simply

has no value in oil production, and

therefore needs to be disposed of.

This separation takes place by using an

oil and gas separator which is installed

either offshore or at processing stations

onshore.

We all know that oil and water does not

mix well. And given enough time to settle,

the fluid stream from an oil well will

separate into oil and water. So why not

simply give it enough time? There are

several reasons for this, where

economic factors and efficiency are the

most important ones. In addition, the

need to separate gas from the liquid

stream must be considered.

In the early days, the challenge was to

separate the well stream into water and

oil. But over time, the oil wells became

deeper with higher gas presence into the

reservoir, and thus the need to separate

gas from the mixture occurred, adding an

extra component to the separation.

That is why, today, the most common

separator within oil and gas production is

the three-phase separator.

In this guide we will point at 6 important

factors you need to consider when

designing an oil and water separator

used in oil production, in order to achieve

maximum performance.

”This separation takes

place by using an oil and

gas separator which is

installed either offshore

or at processing stations

onshore.”

Chapter 1

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The fundamentals: What is an oil and water separator?

A separator is a cylindrical or spherical

vessel used to separate water, oil, gas

and sediment from the fluid stream

produced by one or multiple oil wells.

Separators can be either horizontal or

vertically shaped and are classified into

two-phase and three-phase separators.

The two-phase separator only separates

oil and gas, while the three-phase

separator handles oil, water and gas.

Separators are also categorized based

on operating pressure. There are three

pressure levels: low, medium and high

pressure.

Low-pressure units handle pressures of

10 to 180 psi while

medium-pressure units handle pressure

of 230 to 700 psi. High-pressure units

operate from 975 to 1500 psi.

For the purpose of this paper, we will

focus mainly on the three-phase

separator. Any further descriptions and

illustrations will therefore relate to such

constructions.

Chapter 2.0

”A separator is used to separate water, oil gas and sediment from the fluid stream produced by one or multiple oilwells”

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How does the oil and water separator work?

Water and oil do not mix well and will

separate by themselves over time.

This is defined as gravity separation,

that is the heaviest fluids settle to the

bottom and the lightest fluids rise to the

top. As water has a higher density than

oil, the oil will float on top of the water

inside the separator.

Residence time

The time it takes for the oil and water to

separate is called residence time. In a

separator the residence time is

determined by dividing the liquid volume

inside the vessel by the liquid flow rate.

In typical upstream applications, the

residence time usually varies

between 30 seconds and 3 minutes.

If a foaming crude is present, the

retention time could be increased by

four times its normal values.

Gas

The separation of gas is controlled inside

the separator based on operating

pressure, residence time of the fluid

mixture and the type of flow of the fluid.

Chapter 2.1

Illustration: A window inside a separator, showing the different fluid layers

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Main components

Oil and water separators can be

designed in different ways, depending on

who designs it, the space available, the

components and instrumentation chosen

and if it is vertical or horizontal.

However, there are some main

components that generally are present in

any three-phase separator:

Every separator will have an inlet where

the emulsion flow from the oil well enters

the separator. This is typically located

in the pre-separation zone and secures

separation in the preliminary phase.

To control the direction of the well flow

an inlet diverter or inlet device is needed.

For major phase separation a separation

enhancement device is built in the

primary separation section.

Most separators will also have a type of

weir, where the most common ones will

be a weir plate or overflow weir,

allowing for oil to spill into an oil bucket

and the water to be removed from the

separator through the water outlet, and a

dump valve.

The weir affects the liquid level or

interface level. The interface level

controller in the separator controls when

and how much the water outlet should

open.

The oil level is also controlled by a level

controller.

To separate the gas from the liquids most

separators contain a mist eliminator or

demisting device in addition to a gas

outlet with a back-pressure control valve

which maintains constant vessel

pressure (1).

Illustration: Three-phase separator with overflow weir

”Every separator will have

an inlet where the

emulsion flow from the oil

well enters the separator.”

Chapter 2.2

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Why is separation important?

To answer this question, we must

consider why we need to separate the

fluid stream from the well into three

components in the first place. There are

several reasons for this:

For one, the purpose of drilling for oil is

to produce oil, and not water. The water

is a by-product that will occur in the fluid

stream from a well at some point. The

amount of water mixed with the oil will

vary over time and from oil well to oil well.

Secondly, it is expensive to transport a

three-phase flow over a long distance.

This is due to the complexity of

controlling the three-phase flow into

pipelines or onto shuttle tankers (both

from a HSE and flow assurance

standpoint), which adds to the total cost

of the transportation. From an economic

perspective it is therefore better to utilize

the transportation infrastructures

(pipelines, shuttle tankers, etc.) to a

maximum by only transporting sellable

product like the oil and avoid transporting

the by-products.

Finally, the market for three-phase flow

does not exist. In other words: no one

wants to buy a multi-phase flow.

And those buying the oil barrels from the

production companies,

in most cases refineries, will not accept a

high percentage of water in the mix.

Today, the refineries will make their own

samples testing the oil quality in

addition to the fact that there are

international standards set for how much

water the oil sold can contain. These

standards include (2):

•ISO 9030 - Crude Petroleum -

Determination of Water and Sediment

•ASTM D4007 - Standard Test Method

for Water and Sediment in Crude Oil by

the Centrifuge Method

•API MPMS 10.4 - Determination of

Water and/or Sediment in Crude Oil by

the Centrifuge Method (Field Procedure)

Based on these facts the oil companies

simply must separate the oil and water

at the production site to reduce cost and

ensure buyers for their product.

Therefore, one could claim that there

would be no need or reason to build an

oil production platform if the three-phase

flow could simply be transported to

market and sold as is.

This in turn makes the performance of

the separator a very important factor in oil

production.

Chapter 3

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With a high performance three-phase

separator the amount of oil processed

during a given time period will increase

compared to when the separator has a

lower performance rate.

Therefore, optimizing your three-phase

separator to achieve maximum

performance will have huge cost benefits

as you can increase the oil production

while reducing the oil content into the

separated water, with an overall

improvement of the infrastructure

utilization and a consequent reduction of

the production cost per produced barrel.

”The performance of the separator is a very important factor in oil production.”

Chapter 3

Illustration: A separator render compressor

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6 factors you need to consider

Efficiency, capacity, and cost reduction

are three obvious reasons for ensuring

that the three-phase separator you build

achieves maximum performance.

However, there are 6 important factors

you should consider carefully when

building your three-phase separator.

1. Space availability

2. Residence time

3. Polution/environmental aspects

4. Oil quality

5. Reservoire stability & complexity

6. Productivity

Each of them gives an aspect to

enhanced performance and the benefit

hereof.

1-Space availability

When you design a three-phase oil and

water separator the space available is

important, as this will affect the type, and

size of the separator.

Onshore installations typically are larger

than offshore installations, and the need

to consider the space available onshore

is therefore less relevant when designing

an oil and water separator.

As an example, a three-phase

separator installed at an onshore

installation in Canada could have a

diameter of 42 m, while a three-phase

separator installed in an offshore

installation in Norway would have a

diameter up to 6 m.

Why is this? The obvious reason for this

is the fact that building an oil platform

offshore is more costly than building a

land-based facility. You could say that the

production area of an oil platform is one

of the most expensive ‘real estates’ in the

world.

The oil companies therefore allow

themselves more space onshore than

they do offshore, which again affects the

space available for equipment onboard.

This especially affects the size of the

separators.

Are you building a separator for an

onshore installation you can adjust the

size of the vessel or tank almost as you

like, depending on the residence time

needed for the oil and water to separate.

On the other hand, are you building a

separator for an offshore installation you

cannot adjust the vessel or tank based on

the calculated residence time.

Chapter 4.0-4.1

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Hence the separators are much smaller

offshore with the consequence that the

capacity available for separating the

water from the oil will be limited.

Therefore, the way you equip and

instrument your separator becomes very

important as these elements will

influence the overall efficiency of the

separator.

2-Residence time [mechanical,

electrostatic, chemical]

One way to enhance the separation

process is by decreasing the residence

time needed to separate the water from

the oil.

Again, for an onshore installation the

need to speed things up are less relevant

than for an offshore-based installation.

However, in both cases the most

common way to reduce residence time

or speed up the separation process is to

add chemicals to the fluid mixture

entering the separator. This will reduce

the time-related costs of separation, as

you can increase or accelerate

production.

It is important to note though, that

adding chemicals to the separator does

not only increase efficiency. There are

also financial costs tied to using

chemicals in the separation process.

These costs are higher in offshore

operations, than in separation processes

onshore.

First of all, you must buy the chemicals,

secondly you must transport the chemical

offshore, and finally you must store them

and pump them into the well or the

separator. All these processes are costly.

In addition to these direct costs, there

are also costs related to documenting the

chemicals used both during normal

operation, and in the commissioning and

start-up phases of a project.

Lastly, there is the issue of disposing

of the chemicals when the separation

process is completed. Here there are

environmental issues which need to be

considered. We will cover the

consequences of this more in depth in the

third factor you need to consider when

building your separator - the

environmental aspect of things.

We know for a fact that to speed up the

separation process you need to add

some level of chemical to the separator.

However, it will be in the best interest for

the oil company to limit the use of

chemicals as much as possible due to

cost and environmental aspects.

When you design a separator, you should

therefore consider ways to keep the use

of chemicals to a bare minimum, but at

the same time consider the residence

time and speed of the process.

Chapter 4.2

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3-Pollution/environmental aspects

Considering that the by-products of a

separation process like the produced

water are in general separated,

filtered, and disposed locally (for instance

discharged to the sea), the environmental

aspects are very important when

designing a three- phase separator.

In modern oil and gas production, there is

an increased focus on reducing pollution

and the impact of the production process

on the surrounding environment and

ecosystems. Traditionally different

countries had different philosophies and

rules applied to the environmental

aspects of a producing oil facility,

however there is a global tendency on

applying stricter and stricter requirements

on pollution.

We know for a fact that disposing of

water produced is the largest amount of

aqueous waste arising from production

operations at oil fields. Oil and chemicals

discharged with the produced water may

have local effects on the ecosystem close

to the oil and gas installation. (3)

Therefore, the regulations on the quality/

pureness of water disposed of is also

very strict. Take Norway as an

example. Here the level of oil and chemi-

cals discharged with the produced water

is regulated at national level through

issued permits which companies can

apply for.

The permits are issued by the

Norwegian Environment Agency and

allows the company to discharge

chemicals under the Pollution Control

Act. These discharges are also regulated

internationally through the Convention for

the Protection of the Marine Environment

of the North-East Atlantic. In other

countries/areas we find similar

regulations that needs to be met when

disposing of produced water.

This means that polluting more will cost

more to the oil producing companies and

to avoid this they will need to apply more

technologies able to reduce the polluting

sources.

The design of your separator arguably

plays an important part here. The main

by-product of a three-phase separator

is water. However once separated, this

water will still contain some oil content

together with several chemicals that have

been used during the separation process.

All these polluting elements must be

further cleaned away through additional

expensive treatment processes (installed

downstream of the three-phase

separator), before the water can be

finally disposed of.

Chapter 4.3

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To reduce the quantity of these polluting

elements into the separated water and

reduce pollution and costs, the efficiency

of the three-phase separator becomes

crucial as well as the capability of

dynamically monitoring the various fluid

layers inside of it.

For instance, a water layer too thin,

increase the risk of separating to much

oil with the water; on the other hand, an

emulsion layer too tick, increases the

need of chemical usage (e.g.

demulsifiers) or requires a reduced flow

rate and cut on production.

4-Oil Quality (fluid composition)

One of the main aspects to consider

when designing a three-phase separator,

is the lack of standardization availability.

While the other technical disciplines of an

oil production facility have been through

a high degree of standardization both in

the design, construction and operational

phases (e.g. electrical, instrumentation

and telecommunication, mechanical,

rotating machines, etc.) that simplify the

system definition and the technologies

selection, the process discipline has to

design and build systems able to

manage process fluids that are different

every time.

This is due to the nature of crude oil, and

in general of all hydrocarbons, that is not

a unique homogenous substance without

any differentiation.

In reality, the crude oil has physical and

chemical characteristics that can vary a

lot, for instance in density, consistency,

volatility, viscosity and toxicity.

There are different ways of classifying the

crude oil type. Looking at its API gravity it

can be classified in ultra-light, light,

medium and heavy, while looking at its

sulfur content it can be classified in

sweet, medium-sour and sour.

This variety of characteristics generates

several combinations with the

consequence of having several types of

crude oil by quality produced in the world

today (e.g. light & sweet, light & sour,

medium & sweet or heavy & sour, and

many others).

The quality of the crude oil the

three-phase separator has to process

will heavily affect the performances, size,

material, instrumentation, etc. Therefore,

you need to consider these aspects

carefully when designing your oil and

water separator.

”The quality of the crude

oil the three-phase

separator has to process

will heavily affect the

performance”

Chapter 4.4

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5-Reservoire stability & complexity

The stability of the oil wells is important to

consider when building your

separator. In most cases a separator built

for a process plant must consider the

reservoir performance variations over

many years of production through

natural depletion, increased oil recovery

and enhanced oil recovery activities.

Most process plants today serve more

than one oil well, and the separator will

therefore deal with several well flows.

No oil well is identical, and the

composition of the well flow will therefore

differ from one oil well to another.

In addition, the composition and pressure

of a well flow will typically vary over time.

When building a separator, you are

therefore faced with the challenge of

building a bottle-neck into the processing

system that should perform equally good

under different circumstances over time.

Another aspect you need to consider in

your design is the uncertainties linked

to reservoire performance changes by

disconnecting old wells and by adding

new wells through tiebacks. To achieve

maximum performance your oil and water

separator should therefore include the

flexibility to handle these uncertainties.

6-Productivity (hidden costs)

The last main aspect you need evaluate

when designing a three-phase separator,

is its required productivity. In fact,

associated with the operation of a

separator, there are different hidden

costs that are bigger when the separator

itself is operating at low productivity.

A typical example of a hidden cost

associated to a separator with low

productivity is the loss of production

consequent of a shut down for high liquid

carry over on the gas line. Downstream

of the separator there is typically a wide

range of instruments designed to provide

information on the quality of the

separated products, but also to generate

alarms and in exceptional cases they can

also force a process shut down.

Unplanned shutdowns of the process

system are heavily affecting the

performance, the uptime and ultimately

the profitability of an entire production

plant, so it is very crucial to reduce them

to the bare minimum.

Another hidden cost associated to a

separator not operating properly is

related to the damage and consequent

maintenance cost of a compressor

installed downstream of the gas outlet,

that received gas not dry enough.

Chapter 4.5-4.6

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Typically, the process system is designed

to avoid this kind of event, requiring a

part of the instrumentation also

additional vessels (like scrubbers) to

avoid that foam and liquids can reach the

compressor inlet directly.

However, this kind of damages are still

happening, in particular when the

separator is operating outside of its

optimal operation set-up and when the

process fluid conditions are changing

very fast.

To reduce or to entirely avoid these

hidden costs is closely linked to

monitoring the various fluid layers inside

the separator. For maximum performance

of the three-phase separator you should

therefore consider ways to avoid or

reduce the hidden costs mentioned

above.

Chapter 4.6

”A typical example of a hidden cost associated to a

separator with low productivity is the loss of production

consequent of a shut down for high liquid carry over on

the gas line.”

www.sentech.no 13

Conclusion

To summarize: a three-phase separator

is a crucial part of the oil and gas

production both offshore and onshore.

If the separator is not performing at its

maximum this will have severe financial

consequences as it will reduce the

capacity and increase the costs related to

the oil production.

By considering the 6 factors discussed

above you should be able to build your

separator to ensure enhanced

performance, and the benefits linked to

a three-phase separator performing at a

maximum level.

”If the separator is not

performing at its maximum

this will have severe

financial consequences”

Chapter 5

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Links & References

Sources:

(1) https://www.glossary.oilfield.slb.com/Terms/r/residence_time.aspx

https://blog.kimray.com/3-common-types-of-three-phase-separator-vessel-design/

https://petrowiki.org/Oil_and_gas_separators#Separator_components

(2) https://www.mckinseyenergyinsights.com/resources/refinery-reference-desk/

bsw/#:~:text=BS%26W%20refers%20to%20the%20volume,BS%26W%20are%20

1%25%20or%20lower

(3) https://wedocs.unep.org/bitstream/handle/20.500.11822/8275/-Environmental%20

Management%20in%20Oil%20%26%20Gas%20Exploration%20%26%20Producti-

on-19972123.pdf?sequence=2%26isAllowed=y#:~:text=The%20broad%20environmen-

tal%20issues%20faced,soil%20and%20groundwater%20contami%2D%20nation.

Chapter 6