Gas Lift Definition / Meaning
Gas Lift is an artificial lift method used in oil and gas production to increase the flow of reservoir fluids (oil, water, and gas) from a wellbore to the surface. It involves injecting high-pressure gas—typically natural gas or nitrogen—into the production tubing at a predetermined depth. This injected gas reduces the density of the fluid column in the tubing, lowering the bottomhole flowing pressure and allowing reservoir pressure to push more fluids to the surface. Gas lift is one of the most common and versatile artificial lift techniques, particularly suited for wells with high gas-to-liquid ratios, deviated or offshore wells, and those producing sand or corrosive fluids.
How Gas Lift Works
The fundamental principle of gas lift is based on reducing the hydrostatic head of the fluid column. By injecting gas into the tubing, the average density of the fluid mixture decreases, which reduces the pressure required to lift the fluids to the surface. The injected gas also helps to aerate the liquid, creating a multiphase flow that is easier to lift. The process is controlled by gas lift valves installed in side-pocket mandrels along the tubing string. These valves regulate the injection depth and rate, allowing for optimization of the lift process.
Key Components
- Gas Source: High-pressure gas from a compressor station, a nearby high-pressure gas well, or a gas processing plant. The gas must be clean and dry to prevent valve erosion or hydrate formation.
- Gas Injection Line: A pipeline that transports the high-pressure gas from the source to the wellhead.
- Wellhead and Casing: The gas is typically injected into the casing-tubing annulus. The wellhead must have appropriate connections and valves to handle high-pressure gas injection.
- Gas Lift Mandrels and Valves: These are installed at specific depths in the tubing string. Mandrels are side-pocket devices that house the gas lift valves. Valves are pressure-sensitive devices that open and close to control gas injection. Common types include injection pressure operated (IPO) valves and production pressure operated (PPO) valves.
- Orifice Valves: Used for continuous gas lift, these have a fixed opening and provide a constant injection rate.
- Unloading Valves: Used during the initial startup of a gas lift well to remove the liquid column from the tubing in stages, allowing the well to begin flowing.
Types of Gas Lift
| Type | Description | Typical Application |
|---|---|---|
| Continuous Gas Lift | Gas is injected continuously at a constant rate. The injected gas mixes with the produced fluids, reducing density and maintaining steady flow. This is the most common type. | Wells with relatively stable reservoir pressure and moderate to high productivity. Ideal for wells that can sustain continuous production. |
| Intermittent Gas Lift | Gas is injected in cycles or slugs. A volume of liquid is allowed to accumulate in the tubing, then a high-pressure gas slug is injected to push the liquid slug to the surface. This is more efficient for low-productivity wells. | Wells with low reservoir pressure or low productivity, where continuous injection would be inefficient or uneconomical. |
Advantages and Disadvantages
Advantages
- Versatility: Suitable for a wide range of well conditions, including deviated, horizontal, and offshore wells.
- Handling Solids and Corrosive Fluids: Since there are no moving parts downhole (unlike pumps), gas lift can handle sand, scale, and corrosive fluids better than many other artificial lift methods.
- High Gas-to-Liquid Ratio Wells: Works well in wells that produce significant amounts of natural gas, as the produced gas can be used as the injection gas.
- Easy to Adjust: Injection rates and depths can be changed from the surface without pulling the tubing.
- Relatively Low Maintenance: Downhole equipment is simple and robust, reducing workover frequency.
Disadvantages
- Requires High-Pressure Gas Source: A reliable and sufficient supply of high-pressure gas is necessary, which may require compressors and associated infrastructure.
- Limited by Reservoir Pressure: In very low-pressure reservoirs, gas lift may not be effective because the reservoir cannot provide enough energy to lift the fluids even with gas injection.
- Potential for Hydrate Formation: In cold environments, the injected gas can combine with water to form hydrates, which can block valves and tubing.
- Gas Handling at Surface: The produced gas must be separated from the oil and water, and the injection gas must be recycled or processed, adding to surface facility costs.
Design and Optimization
Proper design of a gas lift system requires detailed analysis of reservoir characteristics, fluid properties, well geometry, and available gas pressure. Engineers use nodal analysis software to model the pressure drop in the tubing and determine the optimal injection depth and rate. Key design parameters include:
- Injection Gas Pressure: Must be high enough to overcome the hydrostatic head of the fluid column and the friction losses in the tubing.
- Injection Depth: The deepest possible injection point is usually preferred to maximize the reduction in hydrostatic head, but it must be above the perforations or the packer.
- Gas Injection Rate: Too little gas will not sufficiently reduce density; too much gas can cause excessive friction and reduce lift efficiency.
- Valve Spacing and Setting: Valves are spaced at intervals to allow for staged unloading and to optimize injection at the desired depth.
Usage Example
“After the reservoir pressure declined below the bubble point, the operator installed a continuous gas lift system with three injection valves set at depths of 2,000 ft, 4,000 ft, and 6,000 ft. By injecting 1.5 million standard cubic feet per day of natural gas at 1,200 psi, the well’s production rate increased from 50 barrels of oil per day (BOPD) to 250 BOPD.”
Industry Context
Gas lift is a mature and widely adopted technology, with applications in both onshore and offshore fields worldwide. It is often the preferred artificial lift method for deepwater and subsea completions because of its simplicity and reliability. In many fields, gas lift is integrated with gas processing and reinjection systems to optimize overall field production and gas utilization. Advances in valve technology, real-time monitoring, and automated control systems have further improved the efficiency and flexibility of gas lift operations, making it a critical tool for maximizing hydrocarbon recovery.