Gas Lift System Definition / Meaning
A Gas Lift System is an artificial lift method used in oil and gas wells to enhance the natural flow of reservoir fluids to the surface. It works by injecting high-pressure gas into the production tubing or annulus, reducing the hydrostatic head of the fluid column and increasing the gas-liquid ratio, thereby lowering the density of the fluid mixture. This allows the lighter column to be displaced upward by reservoir pressure or by the injected gas itself. Gas lift is one of the most versatile and widely applied artificial lift technologies, particularly suited for wells with high gas-oil ratios, sandy or corrosive environments, or deviated and offshore completions.
Overview of Gas Lift Technology
Gas lift relies on the principle of density reduction. By injecting gas at specific points downhole through gas lift valves, the fluid in the tubing becomes aerated, reducing its weight per unit volume. The injected gas also provides additional energy to lift the fluids. The system typically consists of a surface gas compressor, a gas distribution manifold, a wellhead assembly, and a downhole completion with gas lift valves, mandrels, and packers.
Key Components
- Compressor Station: Provides high-pressure gas (typically 800 to 1500 psi) from a gas supply source (e.g., recycled produced gas, external gas pipeline).
- Gas Injection Manifold: Distributes and regulates gas flow to individual wells.
- Wellhead Equipment: Includes casing and tubing valves, injection ports, and pressure gauges to control and monitor gas injection.
- Gas Lift Valves: One-way valves installed at specific depths in the tubing string. They open when the casing-to-tubing differential pressure exceeds a set design pressure, allowing gas to enter the tubing. Types include injection pressure operated (IPO), production pressure operated (PPO), and fluid operated valves.
- Mandrels: Sub-surface devices that house the gas lift valves. They are threaded into the tubing string at calculated intervals.
- Packer (optional): Isolates the annulus from the production zone. In a cased-hole completion, a packer is often set above the perforations to direct injection gas through the valves into the tubing.
Types of Gas Lift Systems
Gas lift systems are broadly classified into two operating modes: continuous flow and intermittent flow.
| Feature | Continuous Gas Lift | Intermittent Gas Lift |
|---|---|---|
| Injection Method | Constant injection at a steady rate | Cyclic injection – gas is injected in slugs |
| Well Characteristics | High productivity index, moderate to high reservoir pressure | Low productivity index, low reservoir pressure, or high skin damage |
| Gas Volume Required | Lower volume per barrel of fluid lift | Higher volume per barrel, but intermittent |
| Efficiency | Generally higher lifting efficiency | Less efficient, but effective for unloading wells or kick-off |
| Application | Most common for steady production | Used in stripper wells, after workovers, or to handle water slugs |
Design Considerations
Proper design of a gas lift system requires accurate reservoir data, fluid properties, wellbore geometry, and anticipated production rates. Key parameters include:
- Gas injection pressure and volume: Must be sufficient to overcome frictional losses and hydrostatic head at each valve depth.
- Valve spacing and setting depths: Determined by gradient curves and unloading sequence. Typically, valves are placed at intervals of 200 to 500 feet.
- Unloading procedure: Stepwise injection to displace liquid from the tubing annulus, bringing the well to stable production.
- Backpressure considerations: Surface flowline pressure impacts achievable lift depth.
- Corrosion and erosion control: High-velocity gas and liquid mixtures can cause wear; materials selection is critical.
Advantages and Disadvantages
Advantages:
- Handles high gas-oil ratios and sandy or corrosive fluids well.
- Suitable for deviated, horizontal, and offshore wells.
- Relatively low surface maintenance compared to rod pumps; no rotating or reciprocating parts downhole.
- Can be used over a wide range of production rates (50 to 15,000+ BPD).
- Easily adjustable by changing injection gas volume or pressure.
Disadvantages:
- Requires a reliable source of high-pressure gas and surface compression equipment.
- Lower lifting efficiency at very low bottomhole pressures.
- Valve design and unloading sequences can be complex.
- Significant pressure drop across surface chokes can limit flow.
- Not ideal for wells with extremely low productivity or high viscosity crude.
Typical Applications
Gas lift is commonly deployed in offshore platforms where space and power constraints make other lifts impractical. Onshore, it is used in high GOR wells, deep wells, and those with high water cut. It is also frequently employed as a secondary lift after initial natural flow declines, and in well unloading operations after workovers or hydraulic fracturing.
Usage Example
In a typical offshore well producing 5,000 barrels of fluid per day with a 70% water cut, a continuous gas lift system injects 1.2 million standard cubic feet per day (MMSCFD) of gas at 1,200 psi through three injection valves spaced from 2,000 ft to 5,000 ft measured depth. The system maintains stable production and reduces the average fluid density from approximately 8.5 ppg to 4.2 ppg in the tubing.
Related Technologies
Gas lift is part of the broader category of artificial lift systems. Alternatives include electric submersible pumps (ESP), rod pumps, progressing cavity pumps (PCP), plunger lift, and hydraulic pumping. Each method has niche applications, but gas lift remains a top choice for its flexibility and reliability in challenging environments.