What is gas lifting?
Gas lift is one method of artificial lift. For gas lift, gas is injected continuously or intermittently at the selected location(s), resulting in a reduction in the natural flowing gradient of the reservoir fluid, and thus reducing the hydrostatic 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. This pressure drawdown must not violate restrictions for sand control and water or gas coning.
Gas lift system:
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.
First, a liquid slug must be built in the tubing above the bottom valve. Then the valve opens, displacing the liquid slug upward. The valve remains 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 the intermittent gas lift can be done with a timing device or can depend on the pressure.
Advantages:
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Best artificial lift mechanism, can handle sand and solid materials
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Crooked holes can be lifted easily with gas lift.
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Central gas lifting system can be used to service many wells.
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Installation of gas lift is compatible with subsurface safety valves and other surface equipment.
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Deviated or crooked holes can be lifted easily with gas lift.
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Gas lift is flexible, the gas lift can be used in wide range of volume and depth.
Disadvantages:
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Gas lift is relatively inefficient
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Relatively high backpressure may seriously restrict production in the continuous gas lift
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Good data are required to make a good design.
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High difficulty because of low gravity and friction.