Well-stream processing, a review.

clankflaxMechanics

Feb 22, 2014 (3 years and 7 months ago)

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Well
-
stream processing, a review.

Introduction

The modern world relies on petroleum product
s

for everything from plastics to energy. In
order to get to a useful product, however, the crude petroleum from the ground has to go
through a host of processes. A very important part of the vast processing train is the treatment
of the crude taken up from

the ground so that it can be safely transported and used in a
refinery. This involves removing impurities such as water and sediments, and the removal of
gas. This review aims to give a comprehensive overview of the processes
leading from well
effluent to

export oil
.


Summary

Crude oil is taken from reservoirs underground and arrive
s

at the surface as well effluent.
The
well effluent contains

a variety of elements in addition to the oil itself. These elements include
gas, water, salt
and sediments
. I
n orde
r for the oil to be acceptable for both transport and
refining
, these must be reduced to appropriate levels
. The fluid from the wellhead is first
treated to remove gas and free water, either in two phase or three
-
phase separators. After this
initial separa
tion, the crude still
contains salt

water in em
ulsions, and sediments. In order to
reduce the content of these, the crude undergoes treatment usi
ng an emulsion breaking
process. Such an emulsion breaking process could be the use of an

electrostatic coalesc
er.
This makes the water clump together, making it easier to separate. Once the oil has a content
acceptable for transport and processing, it is known as export oil.



Table of Contents

Introduction

................................
................................
................................
................................

1

Summary

................................
................................
................................
................................
....

1

What is oil?

................................
................................
................................
................................
.

3

What does the well effluent contain and why must it be removed?

................................
.......

3

Refinery feedstock

................................
................................
................................
..................

4

Overview of steps taken

................................
................................
................................
.............

5

Phase
separation

................................
................................
................................
.........................

6

Two
-
phase separators

................................
................................
................................
.............

6

Horizontal two
-
phase separator

................................
................................
..............................

7

Vertical two
-
phase separator

................................
................................
................................
..

8

Three
-
phase separators

................................
................................
................................
...............

9

Emulsion

treatment/desalting

................................
................................
................................
...

10

Sweetening

................................
................................
...............

Error! Bookmark not defined.

Environmental

................................
................................
................................
..........................

10

Other processe
s

................................
................................
................................
........................

10

References

................................
................................
................................
................................

11




What is
oil?

Oil is a complex mixture of hydrocarbons of different shapes and sizes
, from alkanes to
aromatics

and cycloalkanes
.
Hydrocarbons with a carbon number above four

and up around
thirty three

are generally considered oil, while the lighter hydroc
arbons ar
e seen as natural
gas
[
1
]
. In the reservoir
,

this mix of hydrocarbons, along with water and dissolved salt exists
mostly as a single
-
phase liquid under immense pressure. Once on the s
urface and the crude

encounters surface tempe
rature and pressure,
it
separates into different phases

[
2
]
.

What does the well effluent contain and why must it be removed?

When the crude

oil

arrives from
the wellhead
as effluent it contains

a variet
y of elements in
addition to the crude oil itself.

There will be a certain amount of natural gas dissolved in the crude, and this gas will be
liberated as free gas when the pressure drops as the crude arrives from the pressured reservoir

[
1
]
.
Par
t of the gas phase will contain

some larger hydrocarbons know
n as condensate.
Removal

gas is done to stabilize the crude to allow safe storage and handling. Having a
hydrocarbon gas phase would make handling and transport difficult, as the gas in free form

takes up a large volume and would escape easily, presenting a problem with regards to both
environment and general health and safety. Sudden separation of oil and gas during pipeline
transport could create pockets of gas, making
flow uneven and unpredicta
ble.

This
phenomenon is known as slugging.



Water

comes to

exists in the effluent both as free water and as emulsified water. The water
itself may be from reservoir water
or from injected or leaked seawater. The water will contain
a varying amount of diss
olved salts, and it may range from almost fresh water, to completely
saturated brine, at around 30 wt%. If the reservoir water is completely saturated, there may
even be salt

crystals in the crude itself.

Seawater generally has a salt content of

3,5
-
4 wt%
salt.

The

water phase forms because water is largely immiscible in oil, as it is a polar
compound, while

crude is largely hydrophobic.

Removal of the water is mainly done as a way
to remove dissolved salt
s
, as they present a corrosion hazard and may also c
reate deposits on
surfaces such as heat exchangers

or in transport pipelines
.

The definition of free water is
generally water that separates out by itself over a certain time. This could be within a range of
2
-
20 minutes, though 5 minutes is suggested
[
1, Ch 4
]
. The water that is in a more stable state
is the emulsified water. The water
-
in
-
oil emulsions may themselves contain oil
-
in
-
water
emulsion, complicating matters somewhat. The emulsions may be stab
ilized by components
in the crude such as organic acids, sediments and asphaltenes.

The amount of emulsified
water may vary from 1 to 60 vol %. More general values for light oil are 5
-
20 vol% and for
heavy oil 10
-
35 %






Refinery feedstock

For refining p
urposes, the important

characteristics

regulated by the upstream processing

of
the oil are the vapour pressure of the oil and the basic sediments and water. The vapour
pressure of the export crude represents the pressure of remaining gas in the liquid and
is
important because liquid pumps may easily fall prey to cavitation. Furthermore, a two
-
phase
system presents problems regarding

measurement and control systems.
Basic sediments and
water accounts for water, inorganic salts and general sediments.
General
sediments include

asphaltenes and waxes
,

as well as colloids such as clay.
For use in a refinery, basic sediments
and water should be below 0,5 %. In straight salt content, the content should be below 100
mg
salt
/l
oil

with a water content below 0,2 %

accor
ding to T
otal

[
2
]
. From lecture notes in
TKP4150, salt content should be between 50
-
200 ppm (with ppm roughly equal to mg/l) with
a water content below 0,5 wt
%
[
3
]
.

The sediments and water contents are important because
they may deposit on for example heating surfaces or present a corrosion hazard. Having a too
l
arge content of water also means that part of the product essentially contains dead weight as
best, and a massive amount of material causing havoc on the processing equipment at worst.

It
was previously given that even light oil may contain up to 20 % emul
sified water and
sediments, illustrating the importance of well effluent treatment.

If the water was run through
the entire refining process, it would massively increase energy use.

The refinery does carry
out its own desalting, but this is tuned with the
specifications given in mind.

Figure
1

shows
what a refinery might look like
.



Figure
1
: What a refinery might look like

[
4
]



Overview of steps taken

The processing train from well effluent to export crude
consis
ts mainly of phase separation
and emulsion breaking steps.
Figure
2

shows a process overview
.


Figure
2
: Process overview from wellhead

effluent

to export oil
[
2
]
.

From the wellhe
ad, the well effluent is led to either a single stage or a two
-
stage process using
phase separators. The aim of this first separation is to separate out the free phases

to obtain
the oil phase by itself
. As pressure decreases from the wellhead, a gas phase consisting of
light hydrocarbons and some water forms. This gas is separated and treated further. If a single
step is used, a three phase separator allows the removal of both gas and water phases at on
ce.
The two stage process will use several two
-
phase separators, where the gas is removed first,
followed by the free water. After this processing, the crude will still contain water as a water
-
in
-
oil emulsion. This emulsion consists of small droplets with

dissolved sediments and
inorganic salts dispersed in a continuous crude oil phase.

This oil phase is led to an emulsion
breaking step to

remove this water.

Some of the sediments will precipitate in different places
of the processing train, and this may be

removed using jetting nozzles.


It is possible to further desalt the oil by adding fresh water to the oil, in order to dilute the salt
water, before again removing a portion of the water. While total water content will not be
reduced, the problematic salt

content will.

This is only done if the salt content cannot be
reduced below specifications through merely reducing the amount of water to its
specifications.

If there is too much sulphur in the oil, a further step may involve sweetening
the crude

using fo
r example a gas stripper
.

This is mainly done to ease corrosion and handling
concerns
[
1, Ch 5
]
, though sweetening may occur as a result of final stabilization of the oil,
where some residual volatiles are removed
.

Once

the crude meets specifications, it is ready
for transport and fu
rther processing in a refinery.

Phase separation

The three main phases are separated either using one or two steps using three
-
phase and two
-
phase separator respectively.

Two
-
phase separator
s

A two
-
phase separator is a pressure vessel that mechanically separates the gases in the
mixture from the liquids, and it is one of the most important units in the processing and
treating of oil and gas. The two
-
phase separator is often the first processin
g unit in a facility,
thus it is very important to have proper separator design to avoid a “bottleneck” which would
affect the production capacity of the facility.

Every two
-
phase separator has the same four major sections, no matter which size or shape
t
he separator is. The sections are the inlet diverter, liquid collection section, gravity settling
section and mist extractor section.

The inlet diverter absorbs the momentum from the high velocity inlet flow by abruptly
changing the direction, which causes

the initial separation of gas and liquid. The inlet diverter
is often referred to as the primary separation section.

The liquid collection section is at the bottom of the pressure vessel, and it is designed so that
it allows for gas bubbles in the liquid
to rise to the gas
-
liquid interface. The amount of
separation depends on the difference in density of the fluids/gases and retention time of the
mixture, which depends on the volume of the liquid collection section and the inlet flow rate.

The gravity set
tling section is designed so that any liquid droplets larger than 100
-
140 µm in
the gas phase will have time to fall to the gas
-
liquid interface, and any smaller droplets will
stay with the gas. This is because larger droplets can overload the mist extract
or.

The mist extractor section, also known as the coalescing section, is a section with a large
surface area, and paths with many directional changes that the gas has to take. Since the liquid
droplets have a higher mass than the gas, they cannot change di
rections as quickly. This
causes them to collide with the surface, which coalesces them to larges droplets which drop
down to the liquid

[
5
]

[
6
]
.


Horizontal two
-
phase separator

A horizontal two
-
phase separator (
Figure
3
) has a liquid collection section with a retention
time that allows the gas to reach equilibrium with the liquid, and a large enough surge volume
to handle sporadic

slugs of liquid. The liquid leaves the separator from the liquid dump valve
at the bottom, where the rate is controlled by the level control valve to keep the gas
-
liquid
interface at the desired level. The gas leaves at the gas outlet at the top, where th
e rate is
controlled by the pressure control valve. The liquid level is controlled to achieve the largest
surface area on the gas
-
liquid interface, and the pressure is controlled to get a vapour pressure
conductive to separation. The horizontal separator h
as a higher gas/liquid flow rate at a given
size than a vertical separator, and it is well suited for handling mixtures with high gas/oil ratio
and foaming crudes

[
5
]

[
6
]
.


Figure
3
: Horizontal separator schematic
[
5
]



Vertical two
-
phase separator

A vertical two
-
phase separator (
Figure
4
) works very much the same way as a horizontal
separator. The vertical design helps with liquid droplet separation in the gravity settling
section, since the probabilit
y of a large drop collecting small drops on the way down is higher,
and the tank surface area is larger. A vertical separator is normally used for streams with low
-
to
-
medium gas/oil ratios, and for mixtures with high sand and other sediment contents. To
re
move the sand, they’re fitted with a false cone bottom

[
5
]

[
6
]
.


Figure
4
: Vertical separator schematic
[
5
]

There are other separator designs that can be used, where each have different advantages and
limitations, and the design is selected to obtain the desired result for the lowest cost duri
ng the
life cycle. Other designs include spherical separators, centrifugal separators, venturi
separators, double
-
barrel horizontal separators, filter separators, scrubbers and slug catchers

[
5
]

[
6
]
.


Three
-
phase sepa
rators

An oil and water mixture that has been left to settle for 3
-
30 minutes will form a liquid
-
liquid
phase, where the water phase is known as free water. It is often best to remove this water
before emulsified water is removed from the oil. A separator
which separates gas, water and
oil into three separate fractions is known as a three
-
phase separator. If there is very little gas
in the mixture, a separator known as a free
-
water knockout will separate water into one
fraction and oil/gas into another frac
tion, it does not attempt to separate gas and oil in any
way. Both three
-
phase separators and free
-
water knockout follow the same design principles.
Figure
5

shows the design of a horizontal three
-
phase separator.


Figure
5
: Horizontal three
-
phase separator with interface level control and weir
[
5
]

The three
-
phase separator is similar to the two
-
phase separator, with some significant
differences. The inlet diverter is extended so that the gas doesn’t bubble through the liquid,
but the fluid
mixture enters the liquid collections section below the water
-
oil interface. The
water droplets in the oil continuous phase coalesce when rising through the water phase, and
this process is called water washing. This makes sure that fluid doesn’t fall on t
op of the gas
-
liquid or water
-
oil interface, which would cause mixing. In a three
-
phase separator the vessel
needs to be have a higher retention time than in a two
-
phase separator, because it takes longer
time to separate water and oil due to the lower dif
ference in specific gravity.

Another difference is the weir that the oil phase flows over, and the two separate level
controllers and level control valves for each out
-
stream. The water level controller senses the
level of the water
-
oil interface, and remo
ves enough water through the water dump valve to
keep the interface at the design height. The weir height decides the level of the oil phase, and
downstream of the weir the oil level controller operates the oil dump valve.

The gas flow is directed through
a mist extractor, and constant pressure in the vessel is kept
due to the pressure control valve. The height of the gas
-
oil interface can vary from 50
-
75% of
the vessel diameter where 50% is standard. This depends on the importance of the liquid
-
gas
separat
ion
[
5
]

[
6
]
.

Emulsion treatment

A certain amount
of water exists in emulsion. This must also be removed. The presence of salt
water presents both a corrosion hazard and a sedimentation hazard.

Environmental

Cleaning

waste water from oil/water separation is important because there are limits to oil
content in effluent water, around 30 mg/l for most countries. High salt content oilfield brines
might kill freshwater fish and vegetation, disposal of water into freshwater

is prohibited
unless the salinity is very low.

Water treatment before disposal is necessary, secondary treatment might be required in severe
cases. Offshore produced water can often be pumped back to the sea after treatment, onshore
produced water is norm
ally injected back into the formation or pumped to a disposal well.

Water treatment equipment causes the dispersed oil droplets to separate and float to the water
surface. Gravity separation, coalescence and flotation are the three basic phenomena used in
the water treating equipment design.

Gravity separation is due to oil droplets having a lower density than water, and thus has a
buoyant force exerted upon them. This force is resisted by the movement of the oil droplet
through the water. When those forces

are in balance, a constant rising velocity for the oil is
reached, which can be calculated from Stokes’ Law as follows:




















Where V
r

is the rising velocity of the oil droplet (m/s),
Δ
SG is the difference in specific
gravity between the oil and water, d
O

is the oil droplet diameter (µm) and µ
w

is the water
viscosity (cp). It can be seen that the rising velocity increases when the difference in specific
gravity is higher, the droplet dia
meter is larger and the viscosity is lower.

Specific gravity is important to size the equipment.
[
7
]

Other processes

The processes in this processing train results in several product streams. The export crude
stream generally goes to a refinery, where it goes through additional processing such as
fractionation. The separated gas will con
tain low carbon natural gas, but also some heavier
compounds and some water. These are condensed out of the gas and the liquid hydrocarbon
product is known as condensate. The gas may be used for power production on
-
site, flared, or
exported if present in c
onsiderable amounts. Water from the process will contain some oil and
other chemical, and this may need to be treated. The water from the process may be injected
into the reservoir to assist with production.

References

1.

Manning, F.S.
, R.E. Thompson, and R.E. Thompson,
Oilfield Processing of Petroleum, Vol. 2:
Crude Oil
. 1995: PennWell Books.

2.

ENSPM Formation Industrie IFPTraining.
PRO01198

CRUDEOILTREATMENT
-
Separation
. 2007
[cited 2012 10.02]; Available from:
http://www.scribd.com/doc/22321276/1
-
Crude
-
Oil
-
Treatment
-
Separation
.

3.

Moljord, K.,
Oil Refining lectures, Structure of a modern refinery
, 2013, NTNU.

4.

Siegmund, W.
Anacor
tes Refinery
. 2008; Available from:
http://en.wikipedia.org/wiki/File:Anacortes_Refinery_31911.JPG
.

5.

Stewart, M. and K. Arnold,
Surface Production Operations
3rd ed. 2008, Bur
lington: Gulf
Professional Publishing.

6.

Stewart, M. and K. Arnold,
Gas
-
Liquid And Liquid
-
Liquid Separators
. 2008, Burlington: Gulf
Professional Publishing.

7.

Stewart, M. and K. Arnold,
Produced Water Treatment Field Manual
. 2011, Burlington: Gulf
Profes
sional Publishing.