As production from the reservoir continues, gradually the energy of the reservoir depletes and it is no longer able to lift oil to the surface. At that point, Artificial Lift is employed which enables us to bring the oil from the depleted reservoir through the well to the surface. Approximately, 80% of the oil wells around the world are now on Artificial Lift and the most popular method employed is the Gas Lift.
In Gas Lift, a high pressure gas is injected into the well from the Casing - Tubing annulus. The injected gas reduces the density of the fluid above the point of injection in the well. Because of the overall reduced density, the fluid exerts less pressure on the reservoir and flow from the reservoir into the well can continue.
Gas Lift System can broadly be classified into two main categories:
- Continuous Gas Lift
- Intermittent Gas Lift
In this, gas is continuously injected into the well at the maximum possible depth, which depends on the injection pressure and the well depth. The gas mixes with the produced well fluid and decreases its density and hence the pressure gradient of the mixture from the point of gas injection to the surface. The decreased pressure gradient reduces the flowing Bottomhole Pressure (BHP) below the static BHP thereby creating a difference in pressure that allows the reservoir fluid to flow into the wellbore.
This is typically employed in wells with high production rate and high BHP.
 |
| Continuous Gas Lift |
Intermittent Gas Lift
As the name suggests, gas is injected periodically into the wellbore which displaces the fluid which accumulates as a slug in the well. When the high pressure gas is injected into the well , it rapidly expands and this expansion pushes the slug towards the surface. Because of the intermittent nature of the gas injection , the well produces less than a well with continuous gas lift.
The intermittent gas-lift method typically is used on wells that have low production rates.
 |
| Intermittent Gas Lift |
