WHAT IS GAS LIFT?
When the formation pressure is not high enough to overcome the hydrostatic head (the total weight) of the fluid in the well tubing, the well will not flow. This situation can exist in a newly-drilled well. It can also exist in an old well where the Bottom Hole Pressure (BHP) has decreased due to production. Gas lift can be used in both these case to make wells flow.
Hydrostatic pressure : the pressure exerted by a column of fluid due to the height of the fluid and the specific gravity of the fluid.
Gradient : the pressure exerted per unit of vertical height of a fluid (gradient increases as specific gravity of the fluid increase).
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Basic Components for A Gas Lift System |
PRINCIPLE OF GAS LIFT
To reduce the hydrostatic head of the fluid in the production tubing so that the formation pressure will be great enough to make the well flow. This is done like this:
- Gas is introduce into the oil in the tubing as deep as possible. This is normally done from the casing tubing annulus into the production tubing.
- As the gas mixes with the oil, it "aerates" the oil (fills the oil with small gas bubbles). This reduce the specific gravity of the oil, which means that the gradient has also been reduced.
- By reducing the gradient, the hydrostatic pressure will be less than the formation pressure at bottom hole. The well will flow.
- As the mixture of oil and gas bubbles moves up the tubing, the tiny bubbles of gas expand. This reduces the hydrostatic pressure further and thw well flows more easily.
OPERATION OF A GAS LIFT SYSTEM
A
source of clean, liquid free gas is needed. This can come from a nearby gas
well
or from a gas/oil separation plant. This gas lift gas has to be compressed
to a high
pressure.
The
gas is introduced into the annulus of the well to be lifted, through a
regulating
device at the surface. The regulating device can be a choke or a
flow controller.
The
amount of gas injected has to be controlled because:
- If there is too little gas, the well will not flow oil.
- If there is too much gas, the gas will come out of the oil in the production tubing and pass to the surface as free gas taking no oil with it.
Produced
oil plus the gas is used to lift the oil flow from the well at the surface.
This
production oil is flowed through a separator to remove the gas from the
oil. The oil
is passed on to production process or to storage.
The
separated lift gas plus separated formation gas produced with the oil is passed
through a scrubber to remove any liquids. It is then recompressed to be used
again for the gas lift cycle or it is passed to the gas processing plant.
This
is a continuous process.
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Dual ‑ String Gas Lift Installation |
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Gas Lift Sequence |
Unloading Sequence in a Well to be Gas Lifted:
Sequence is numbered 1 through 9 as below.
Note: All gas lift valves are tubing
sensitive.
1. Well is ready to unload with casing and tubing full of
fluid. All gas lift valves are open.
2. Gas pressure has U‑tubed fluid from the annulus to
production tubing through the gas lift valves. Gas has not yet entered the
production tubing which is still full of original fluid.
3. Annulus fluid level has been lowered below the first
valve and gas has entered the production tubing through the first valve. From
this moment gas lifting of the tubing contents commences. Formation flui feed‑in
has not yet commenced.
4. Annulus fluid level has been lowered by U‑tubing to
just above the second valve. Gas lift injection into the tubing is continued at
the first valve only.
5. Annulus fluid level has been lowered below the second
valve and gas enters the tubing through the second valve. As flow commences
from the second valve the tubing pressure gradient opposite the first valve
decreases and the first valve closes. At this step in the unloading sequence
the back pressure on the formation has been decreased to the point where
formation fluid starts flooding into the well. The fluid in the tubing is
slowly displaced from the formation. The formation fluid will continue to be
mixed with load fluid as the annulus continues to unload.
6. The annulus fluid level has been lowered to just above
the third valve. The transfer of fluid from the annulus to the tubing is the
only change in the conditions established in (5).
7. Annulus fluid level has been lowered to below the
third valve, and gas enters the production tubing through the third valve. As
flow commences through the third valve the production tubing pressure gradient
opposite the second valve decreases and the second valve closes. Injection of
gas through the third valve lowers the back pressure on the formation further
and additional formation fluid flows into the production tubing.
8. Annulus fluid has been lowered to just above the
fourth valve. The transfer of fluid frorn the annulus to the tubing is the only
change in, the conditions established in (7).
9. The
fourth and deepest valve has been uncovered and gas injection commences at this point. The third
valve closes and flow from the formation stabilises at the maximum rate
possible for the installations.
OTHER USES FOR GAS LIFT
The
principle by which a gas lift system operates can also be used to start a well
flowing.
It will 'kick off' a well.
With
'dead' crude in the well, (crude containing no gas), the hydrostatic pressure
exerted by that crude can be greater than the formation pressure. The well will
not
flow.
Gas
is introduced into the well as deep as possible. This aerates the crude thereby
enabling it to flow.
Once
the well is flowing with 'live' crude in the tubing, the well will continue to
flow
without the assistance of injection gas.
The
gas, natural gas or nitrogen, is introduced deep down into the tubing through a
long, small pipe inserted down the production tubing. This requires special
equipment called a 'coiled tubing unit', and the technique is used extensively,
especially after workover and well stimulation operations.
Kickover Tools
To simplify the wireline work to install gas lift valves in side pocket mandrels, a specially designed kickover tool can be used. This too[ locates the mandrel selectively when two or more mandrels are installed in one well. It also orients in the proper position and offsets the valve (or pulling tool) into position over the pocket for setting or retrieving the valve.![]() |
Kickover Tools for Installing Side Pocket Mandrels |
Schematic 1
The
kickover tool is run below the mandrel. Since the tool is locked in a rigid
position, it is designed to not kick over accidentally.
Schematic 2
The
kickover tool is raised until its key engages the kickover sleeve in the
mandrel.
Continued upward movement rotates the tool until its key enters a
slot. When the
key reaches the top of the slot, the operator is notified by a
weight increase
displayed on the weight indicator. The tool is now properly oriented.
Schematic 3
The
pivot arm is designed to swing and lock in position. This action locates the
valve or pulling tool above the pocket or latch on the gas lift valve.
Schematic 4
The
mandrel is designed to guide the valve or pulling tool to accurately land the
valve or engage the latch on the valve.
Schematic 5
A straight upward pull shears a pin
when the key reaches the top of the slot. This
action allows the trigger to
guide freely out of the slot and through the tubing. When
the pivot arm reaches
the small upper section of the mandrel, it is designed to
snap back and into
its vertical running position. This reduces the drag on the tool
and the valve
as it is removed.
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Side Pocket Mandrel |
GAS LIFT VALVES
These
valves are used down‑hole in the well and stop or start the flow of gas lift
gas or fluid from the annulus into the production tubing.
·
There are generally two basic types in use.
·
Tubing pressure operated.
·
Casing (annulus) pressure operated.
The
pressure exerted against the gas lift valves at depth from the tubing or the
casing opens the valves. The valve opening pressure is pre‑set at the surface
before the valves are run into the well.
These
valve opening pressures are calculated during the design stage of the gas
lift
system. The deeper the valve in the well the higher the operating pressures.
Tubing Pressure Operated
When
hydrostatic pressure near to the valve in the tubing is greater than the
pre‑set
valve opening pressure, the valve will open. This will allow gas/liquid to
flow
through the valve from the annulus to the production tubing.
When
hydrostatic pressure near to the valve in the tubing falls below the
pre‑set
valve opening pressure, the valve will close. Flow from the annulus
to the
tubing will then stop.
Reverse
flow through the valve from production tubing to annulus is prevented
by a
check valve in the valve.
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Tubing Pressure Operated Gas Lift Valve |
Casing Pressure Operated
The function of the valves are
identical to the tubing pressure operated except that
it is the casing or
annulus pressure adjacent to the valve that activates the valve
to open or
close.
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Casing Pressure Operated Gas Lift Valve |
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Chemical Injection from Casing |
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