What is gas lift and when is it used?
Gas lift is a self-explanatory term being that gas is used to lift fluids from a well. This form of artificial lift can be used:
- in wells where the operator requires more production
- in wells that flow on their own, but need help in getting started
- in reverse flowing water disposal wells
- and most commonly in the Southern United States to unload fluids from gas wells
How does gas lift work?
Gas lift works by injecting gas beneath a column of fluid at a depth in the wellbore. This reduces the density of the column of fluid and helps to displace the fluid to the surface.
With the column of fluid from the point of injection to the top of the wellbore being lightened by gas, the reservoir can move more fluid and resources towards the surface. The primary goal of gas lift is to get gas as deep as possible and to keep injection there provided the well will allow.
How is this done?
- Gas is injected into the wellbore through gas lift valves and mandrels installed in the tubing string during completion. The valves have pre-set pressures so they open at the optimal time during the gas lift process to allow for the most amount of fluid to be recovered.
- A gas lift valve will remain in the closed position and not allow gas to move through it until sufficient opening pressure is exerted. Once the pressure around the valve is greater than the pressure inside the valve, it will open and let gas pass through.
- The gas then moves through the valve and through the injection port of the mandrel to arrive at the fluid it is to lift. From there it mixes with the fluid and helps to displace the fluid to the surface. The gas bubbles are small when they leave the mandrel and grow larger due to temperature change as they travel towards the surface.
Only one valve is open while gas lifting a well to ensure gas and pressure is as deep as possible, returning the largest column of fluid possible. If two valves are open, gas and pressure would be lost up hole. A valve opens and closes based on the pressure around it, the flowing pressure of the fluid, and where the valve is located in the well.
What are the types of gas lift equipment?
There are two types of gas lift valves and mandrels:
- Wireline retrievable equipment valves are installed in side pocket mandrels (previously installed in the tubing string during completion). These valves can be retrieved by running wireline through the tubing to the depth of the mandrel and retrieving / replacing these valves. No other wellbore intervention is needed.
- Conventional gas lift equipment (also referred to as tubing retrievable equipment) consists of a valve screwed directly onto a mandrel that is installed in the tubing string during completion.
The advantage of conventional equipment is that it is economical and allows for a full internal diameter of the tubing string. Conventional equipment is the choice of most operators in the United States due to availability and associated costs. The disadvantage of this choice of equipment is that it requires the entire tubing string to be pulled to replace non-functioning valves.
Why do operators choose gas lift?
Gas lift can be an economical, efficient, and favorable method of lifting fluids from a well if certain parameters can be met.
The performance of an installation is directly tied to:
- the amount of gas that can be injected
- the amount of pressure that can be sustained to inject with
- and the amount of energy wasted in the beginning because of pressures at the wellhead
A gas lift design is created before any equipment is assembled that will accurately tell an operator what expectations can be met with an installation. The gas lift design calculates the pressures at which the valves will need to be set at as well.
In gas shale plays across the United States and in liquid-rich plays as well, gas lift has become increasingly popular as an economical source of lifting fluids in a well.