Main components of a Gas Lift System
Many types of equipment are used to produce a Gas Lift well. It is important to mention that a Gas Lift well easily accommodates a packer and a downhole safety valve. Hence, it is compliant with the dual barrier policy that any major oil & gas company follows.• To inject gas lift in a well, first you need to have a valve to regulate the flow of gas injected in your tubing-casing annulus. This valve is called a gas lift choke or a flow control valve. Once your gas lift is in your casing, it needs to find its way to the tubing. For that, you need to have gas lift mandrels equipped with gas lift valves. The combination of the two will allow communication betweenthe casing and the tubing.
- The conventional gas lift mandrels, for which the gas lift valve is screwed on the mandrel. Hence, replacing a valve requires pulling the entire completion out of hole.
- The side pocket mandrels, for which the gas lift valve is installed with a kickover tool using a slick line unit. The advantage of this system is that you can replace the valve in the mandrel without pulling out the entire tubing. You only need to pull the valve out of hole and run a new one.
- Dummy valves which act as a plug when you do not need to inject gas lift across your mandrel. This is often the case as a gas lift completion using side pocket mandrels can work for more than a decade. Hence, as the production conditions change (for instance due to reservoir pressure depletion over time), you need to adapt your gas lift design and may want to stop injecting gas lift across a specific mandrel
- Orifice valves are used as continuously open valves. One is usually installed in the lowest mandrel through which you want to inject your gas lift at the end of your unloading sequence. This is called the service valve. The most common type is square edge orifice, but you also have a fancier version where your orifice has a venturi shape (aka Nova valve by Schlumberger).
- Unloading valves which are calibrated to open and close under certain conditions. Injection Pressure Operated (IPO) valve is the most common type of unloading valve. In this case, the valve will open once the casing pressure has reached a certain value and will close once it decreased below another value.
First Gas Lift allows complying with downhole barrier safety policy that is mandatory in many regions and anyway implemented
and followed by major E&P companies. According to this policy, wells need to be equipped with a packer (to provide a downholebarrier to isolate the reservoir from the surface in the tubing-casing annulus) and a downhole safety valve (to provide such a barrier in the tubing) (figure 3). Pumping systems (ESP, PCP, rod pumps) are not good at handling gas and the presence of a packer forces the gas to go through the pump (while it can be vented through the tubing-casing annulus in packer-less wells). Systems using rods (PCP and rod pumps) are not easily compatible with downhole safety valves. On the other hand, Gas Lift works perfectly with both barriers. Gas lift is also the most flexible artificial lift method. It can handle a large range of API, gas-oil ratio, and has no limit on liquid rate or well depth. It is a very robust system that can handle harsh conditions like sand, H2S, or scale.
Gas lift is also very adaptative. A smartly designed Gas Lift mandrel spacing installed upon well completion is often used for more than a decade in the well. Gas lift design can be changed regularly to optimize well production. This light operation involves a slick line unit to pull the valves from the side pocket mandrel and replace them with new ones. It does not require a regular pull of the entire completion like the other artificial lift methods.
For all these reasons gas lift is now more and more regarded as the right artificial lift method for shale oil wells in North America. These wells that go through a very fast and significant liquid rate decline can benefit from the adaptivity of Gas Lift to become
more cost-effective.
