hen 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 cases to make the wells flow.
WHAT IS GAS LIFT?
To understand the principle of gas lift the meaning of the following must be
- 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 increases).
|Basic Components for A Gas Lift System|
PRINCIPLE OF GAS LIFT
The objective is 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 introduced 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 reduces 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 then 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 the 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
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.
|Dual ‑ String Gas Lift Installation|
|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
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 ToolsTo 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|
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.
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.
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.
The mandrel is designed to guide the valve or pulling tool to accurately land the
valve or engage the latch on the valve.
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.
|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.
|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.
|Casing Pressure Operated Gas Lift Valve|
|Chemical Injection from Casing|