Chapter 05 GS

Electric Submersible Pumps Mohamed Dewidar 2013

Chapter 5

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ESP Gas Technology

Table of Content

Section Content Page

1 Introduction 2

2 Basic types of intake sections 3

2.1 Intake 3

2.2 Static gas separator 4

2.3 Dynamic gas separator 5

2.3.1 Rotary gas separator 5

2.3.2 Vortex gas separator 7

2.3.3 AGH 8


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ESP Gas Technology

5.1. Introduction

When an oil Well is initially completed, the downhole

pressure may be sufficient to force the Well fluid up the Well

tubing string to the surface. The downhole pressure in some

Wells decreases, and some form of artificial lift is required

to get the Well fluid to the surface. One form of artificial

lift is suspending an electric submersible pump (ESP) downhole

in the tubing string. The ESP will provide the extra lift

necessary for the Well fluid to reach the surface. In gassy

wells, or wells which produce gas along with oil, there is a

tendency for the gas to enter the pump along with the well

fluid. Gas in the pump decreases the volume of oil transported

to the surface, which decreases the overall efficiency of the

pump and reduces oil production.

In order to prevent the gas from interfering with the pumping

of the oil, various downhole separators have been developed to

remove gas from the Well fluid prior to the introduction of

the well fluid into the pump. A typical gas separator is

attached to the lower end of the pump assembly, Which in turn

is suspended on production tubing. Normal gas separators

separate most of the gas and discharge the separated gas into

the annulus outside the tubing string where the gas flows up

the Well to the surface. The separator discharges the liquid

into the tubing to be pumped to the surface.

When free gas is present in the first stage impeller (or first

few stages), it takes up useable space and restricts the

volumetric efficiency of the pump. The result is a decline in

expected production. In fact, if the impeller eye fills

completely with gas, the pump will "lock" or stop producing at

all, see fig (5.1).

The amount of gas a pump can handle without gas locking

depends on stage designs and sizes. Smaller pumps with radial

stages have been known to handle 10 to 15 vol% free gas, and

larger pumps with mixed-flow staging can tolerate 20 to 25

vol%.

There are two basic types of intake Sections:

Intakes

Gas Separators

Static

Dynamic


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Fig (5.1)

5.2. Basic types of intake Sections

5.2.1. Intake

A standard intake does not separate gas. Some gas

separation might occur with a standard intake, but it will

only be natural separation due to some of the gas not turning

and going into the intake when the rest of the fluid does.

Intakes can be either standard or ARZ. The ARZ intake uses

Zirconium bearings and sleeves to better protect against

abrasive wear and lateral vibration.

Fig (5.2) pump intake


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5.2.2. Static (Reverse flow) gas separator

A gas separator is still an intake, but with some

special features designed to keep free gas from entering the

pump.

Original gas separator designs were based on increasing gas

separation by forcing the fluid flow to reverse in the

wellbore. This is where the name of this type of gas

separator, REVERSE FLOW, comes from.

Since this type of gas separator does no real "work" on the

fluid, it is also called a "static" gas separator.

This technique relies on causing the well fluid to flow

downward before reaching the pump intake to cause separation

of gas. Gas bubbles within the well fluid flow tend continue

flowing upward as a result of the buoyant force of the gas

bubbles. The downward flowing liquid in the Well fluid creates

an opposing drag force that acts against the upward moving

bubbles. If the upward buoyant force is greater than the

downward drag force, the bubbles will break free of the

downward flowing Well fluid and continue moving upward.

Buoyancy is a function of the volume of the bubble, and the

drag force is a function of the area of the bubble. As the

diameter of the bubble increases, the buoyant force will

become larger than the drag force, enabling the bubble to more

easily separate from the liquid and flow upward. Consequently,

if the bubbles can coalesce into larger bubbles, rather than

dispersing into smaller bubbles, the separating efficiency

would be greater.

Accordingly, as well fluid enters the gas separator it is

forced to change direction due to buoyancy force. Some of the

gas bubbles continue to rise instead of turn or rise inside of

the gas separator, exit the housing and continue to rise, see

fig (5.3) and (5.4).


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Fig (5.3)

Fig (5.4) static gas separator

5.2.3. Dynamic gas separator

5.2.3.1. Rotary Gas Separator (RGS)

Dynamic gas separators actually impart energy to the

fluid in order to get the gas to separate from the liquid.

The original gas separator was called a KGS (Kinetic Gas

Separator). This design uses an inducer to increase the

pressure of the fluid and a centrifuge to separate the gas and

liquid.

This design could likewise be called a centrifugal gas

separator.

The RGS (rotary gas separator), is a descendant of the original

KGS design which has been improved with Zirconium bearings,

hardened materials, and a few hydraulic enhancements.

The rotary gas separator design works in a similar fashion to a

centrifuge. The centrifuge "paddles" spinning at 3500 rpm cause

the heavier fluids to be forced to the outside, through the


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crossover and up into the pump, while the lighter fluid (vapor)

stays toward the center, and exits through the crossover and

discharge ports back into the well.

Fluid travels up the Well and enters separator through

openings at its lower end. The fluid is separated by an

internal rotating member with blades attached to shaft

(inducer) at its lower end to aid in lifting the fluid to the

rotating separating member. The rotating separator member

causes denser fluid to move toward the outer Wall of separator

due to centrifugal force.

The fluid mixture then travels to the upper end of separator

and passes through a flow divider or cross-over member.

A radial support bearing is often required to support the span

of such a long central shaft.

Divider comprises a circular ring and a conical upper end.

Divider is oriented to be parallel to and coaxial with central

shaft. One or more gas exit ports communicate an opening in

the sidewall of separator and the interior of flow divider. As

the fluid nears flow divider, the outer (more dense) fluid

remains in the annulus surrounding flow divider and is

diverted radially inward and upward to a liquid exit port. The

inner (less dense) fluid enters flow divider and is channeled

radially outward and upward to gas exit ports. Liquid exit

port leads to pump, but gas exit ports open into annulus (fig

5.5).


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Fig (5.5) rotary gas separator

5.2.3.2. Vortex Gas Separator (VGS)

In certain applications in the past, we have

experienced some reliability problems and associated failures

in the rotary gas separators.

Problem:

Extended length of shaft in an area with a rotating mass of

very high inertia which may be radially unbalanced either in

manufacture or in operation because it is filled with a non-

homogeneous fluid of variable specific gravity or may be n0m-

uniform eroded by abrasives with time or combination of the

three.

Accordingly, reducing the mass of the rotating element and

decrease the radial bearing spacing become the solution of

instability of the dynamic gas separator, in addition to

adding Axial Impeller to induces a fluid vortex, this is the

latest dynamic gas separation devices (Vortex Gas Separator).

Fig (5.6)

The Vortex* gas separator is a dynamic gas separation device

that utilizes a natural vortex action created by a specially

designed inlet configuration, axial flow inducer, multiple

vortex generators, multiple flow-through bearings, and a

discharge crossover to provide highly effective gas

separation.

The Vortex gas separator offers an extended range and greater

efficiency over a broader range of flow conditions than

previous 400 and 540 series rotary gas separators.


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Additionally, the Vortex gas separator features the patented,

compliant mount abrasion-resistant zirconia radial bearing

technology coupled with a dramatically improved overall

bearing system to provide far superior reliability over

previous designs.

To further extend life expectancy, the rotor assembly has been

designed to impart very little energy to solid particles

produced through the separator. The improved hydraulics in the

Vortex gas separator allow for more effective gas separation

at higher fluid flow rates than previously possible (fig 5.7).

Fig (5.7) Vortex Gas Separator

5.2.3.3. Advanced Gas-Handler (AGH)

The AGH advanced gas-handling device was designed to

improve the overall lift efficiency of a submersible lift

installation by maintaining a higher gas-to-liquid ratio in

the tubing string. The systems higher GVF reduces the

hydraulic horsepower required to lift fluid to surface. The


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AGH system uses a unique centrifugal stage design to alter the

pressure distribution of the impeller, creating a homogenized

mixture with reduced gas bubble size. This conditioned fluid

behaves as a single-phase fluid before entering the pump.

So, the objectives of AGH are:

Homogenize the mixture.

Reduce bubble size.

Put gas back into solution.

Help gas to move to main stream.

The AGH can be used with a standard intake or with a gas

separator. The choice will depend on how much free gas will be

present at the intake for producing condition and on whether

there is a packer preventing gas production up the annulus,

see fig (5.8).

Fig (5.8)

The AGH results in stable operation with reduced restarts due

to under load shutdown (i.e. gas lock). This improves

production and enhances reliability.

See fig (5.9) of amperage chart.


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Fig (5.9)

Chapter 05 GS

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