Introduction of Gas Lift
Gas Lift System
Gas lift is
one method of artificial lift. For gas lift, gas is injected continuously or
intermittently at selected locations, resulting in a reduction in the natural
flowing gradient of the reservoir fluid, and thus reducing the hydro static
component of the pressure difference from the bottom to the top of the well.
The purpose is to bring the fluids to the top at a desirable wellhead pressure
while keeping the bottom hole pressure at a value that is small enough to
provide good driving force in the reservoir. The pressure drawdown must not violate
restrictions for sand control and water or gas coning.
Two other
considerations must enter the design. First, large amounts of gas injected into
the well will affect the separation facilities at the top. Second, there exists
a limit gas-liquid ratio (GLR) above which the pressure difference in the well
will begin to increase because the reduction in the hydrostatic pressure will
be offset by the increase in the friction pressure.
The positioning
of the gas lift valves and their number is a matter of wellbore hydraulics
optimization. For continuous gas lift, an “operating valve” will be used to
inject the appropriate amount of gas at the desirable tubing pressure at the
injection point. Other valves may be placed below the injection point and may
be put into service during the life of the well as the reservoir pressure
declines or if the water-oil ratio increases.
For intermittent gas lift, both a single
injection point and multiple injection points can be employed. First a liquid
slug must be built in the tubing above the bottom valve. Then, the valve opens,
displacing the liquid slug upwards. The valve remain open until the slug
reaches the top, and then the valve closes. It reopens when a new liquid slug
builds in the bottom of the well.
For multiple
injection points, the bottom valve opens as described for the single injection
point operation, but as the liquid slug moves upward, valves below the slug
open. The valves close after the slug reaches the top. The actuation of the
valves for intermittent gas lift can be done with a timing device or can depend
on the pressure.
Valves can
open and close based on the value of the casing or the tubing pressure. Other
valves may be operated on the basis of the fluid level.
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