A gas lift is a method used in oil and gas production to help bring fluids from the wellbore to the surface when the reservoir pressure is not sufficient for natural flow. Essentially, external gas is injected into the produced flow stream at some depth in the well bore to make the fluid column lighter and easier to lift.
The Mechanism of Gas Lift
Here's a breakdown of how the gas lift process works:
- Gas Injection: High-pressure gas, typically from an external source or reinjected produced gas, is injected down the annulus (the space between the production tubing and the casing).
- Entry into Flow Stream: This injected gas enters the production tubing through special valves, usually located at various depths along the tubing string. The primary point of injection is at some depth in the well bore, where it mixes with the oil, water, and any natural formation gas being produced.
- Fluid Column Lightening: When the injected gas mixes with the liquid hydrocarbons and water, it forms a mixture that is less dense than the original liquid column. Think of it like adding air bubbles to water – it makes the overall mix less heavy.
- Pressure Reduction: As stated in the reference, the additional gas augments the formation gas and crucially reduces the flowing bottom hole pressure. By reducing the pressure at the bottom of the well, a larger pressure differential is created between the reservoir and the wellbore.
- Increased Inflow: This reduced bottom hole pressure leads to increasing the inflow of produced fluids from the reservoir into the wellbore. The lighter, gas-charged mixture can then be lifted to the surface more easily by the remaining reservoir pressure or the artificial lift mechanism itself.
In simple terms, gas lift works by bubbling gas into the production fluid downhole, making the fluid column lighter and reducing the back-pressure on the reservoir, thereby encouraging more fluid to flow into the well and up to the surface.
Key Components
While the fundamental principle is injecting gas, the system requires specific components:
- Gas Source: A supply of high-pressure gas (e.g., compressors, gas pipeline).
- Injection System: Tubing (for production), casing (forming the annulus), and injection valves (mandrels and gas lift valves) that control where and how the gas enters the production tubing.
- Surface Controls: Equipment to regulate gas pressure and flow rates.
Why Use Gas Lift?
Producers utilize gas lift when:
- Natural reservoir pressure declines and is no longer sufficient for economic production.
- Other artificial lift methods (like pumps) are not suitable due to wellbore conditions (e.g., deviated wells, high sand production, high temperature).
- There is an available source of injection gas.
In summary, gas lift is an effective artificial lift technique that leverages the principle of reducing the density of the fluid column in the wellbore by injecting external gas, which in turn lowers bottom hole pressure and stimulates increased flow from the reservoir.
