Gas lift
A gas lift injects gas into the well flow. The downhole reservoir pressure to
the wellhead falls off, due to the counter pressure from weight of the oil
column in the tubing. Thus, a 150 MPa reservoir pressure at 1,600 meters
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will fall to zero in the
wellhead if the specific
gravity is 800 kg/m2
(0.8
times water). By injecting
gas into this oil, the
specific gravity is lowered
and the well will start to
flow. Typically, gas is
injected between the
casing and tubing, and a
release valve on a gas lift
mandrel is inserted into
the tubing above the
packer.
The valve will open at a
set pressure to inject lift
gas into the tubing.
Several mandrels with
valves set at different
pressure ranges can be used to improve lifting and startup. Ill: Schlumberger
oilfield glossary
Gas lift can be controlled for a single well to optimize production, and to
reduce slugging effects where the gas droplets collect to form large bubbles
that can upset production. Gas lift can also be optimized over several wells
to use available gas in the most efficient way.
3.6.4 Plunger lift
The plunger lift is normally used on low pressure gas wells with some
condensate, oil or water, or high GOR wells. In this case, the well flow
conditions can be such that liquid starts to collect downhole and eventually
blocks gas so that the well production stops. In this case, a plunger with an
open/close valve can be inserted in the tubing. A plunger catcher at the top
opens the valve and can hold the plunger, while another mechanism
downhole closes the valve.
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The cycle starts with the plunger falling into the well with its valve open.
Condensed gas and oil can pass though the plunger until it reaches bottom.
There the valve is
closed, now with a
volume of oil,
condensate or water
on top. Gas pressure
starts to accumulate
under the plunger and
after a time pushes
the plunger upwards,
with liquid on top,
which eventually flows
out of the wellhead
discharge.
When the plunger
reaches the wellhead
plunger catcher, the
valve opens and
allows gas to flow
freely for some time
while new liquid
collects at the bottom.
After a preset time, the
catcher releases the
plunger and the cycle repeats.
