Gas lift systems are either continuous or intermittent flow. The majority of wells use continuous flow gas lift because it supplements in situ gas, which results in a natural, steady flow of formation fluids to the surface. Continuous gas injection requires a reliable source of high-pressure gas throughout the life of the well. However, in many cases, in situ gas sources decline before the zone is fully drained, forcing operators to turn to an outside source to maintain lift volume.
Intermittent flow gas lift periodically injects gas into the production string to displace slugs of fluid. The high-pressure gas expands rapidly after entering the tubing, which forces the fluid to the surface. In contrast to the steady flow created by continuous flow gas lift, intermittent flow results in slug flow, which may cause gas-and fluid-handling difficulties on the surface. In addition, the extreme pressure changes caused by the sudden injection of gas into the production tubing creates pressure surges near the producing zone, which may cause the onset of sand production in some formations.
Experts recommend the application of continuous flow gas lift for high-volume wells that have high-static bottomhole pressures (BHPs) and offshore wells that have strong waterdrive or for formations being produced via water-flood that have a high productivity index (PI) and high gas/oil ratios (GORs). Intermittent flow gas lift systems are typically not recommended for wells producing more than about 30 m3/d [200 bbl/d] or in wells that have high BHP but low PI or low BHP and high PI. As a consequence, many intermittent flow systems are deployed in depleted wells that were first using continuous flow systems and on gas wells that have begun to produce water.
Auto gas lift systems, also referred to as natural or in situ gas lift, have specific requirements and thus are used less commonly than traditional systems. Instead of using gas pumped down the casing annulus, these systems are supplied by reservoir gas caps or noncontiguous gas-bearing formations. Completion engineers design auto gas lift wells to include a completion design across the gas cap or gas zone that allows gas to be flowed into the production tubing. Operators control the gas flow rate with a specifically designed auto gas lift valve that has a flow area that can be adjusted from the surface.
Operators may use an auto gas lift system in the same way they would a more traditional continuous or intermittent system. But, because they eliminate certain equipment requirements, auto systems provide an advantage in capital savings and reduce weight on offshore platforms by eliminating the need for a compressor. Auto gas lift systems also allow the operator to produce associated gas without recompleting the well. And, because orifice openings in auto gas lift valves can be adjusted remotely, operators can control gas injection rates to accommodate changes in formation liquids production without an intervention.
The case for gas lift
Gas lift systems offer several benefits not available in other artificial lift systems. Gas lift valves set in side pocket mandrels may be easily and inexpensively removed and be replaced without a workover rig.
Gas lift side pocket mandrels are situated such that the pockets containing the valves are offset from the centerline of the tubing, which keeps the gas lift valves from the fluid flow and thus protects them from solids. Consequently, gas lift technology is a good option for hydraulically fractured wells or for wells completed in unconsolidated formations that routinely produce sand. Gas lift systems are applicable in high-angle, low-productivity and high-GOR wells.
Intermittent gas lift systems can also operate efficiently in low-pressure, low-productivity wells. As a consequence, to capture final production at the end of a well's life, operators may replace other artificial lift systems with gas lift systems until the bottomhole pressure becomes too low to lift the target fluid volumes, at which point operators typically switch AL systems to rod pump or electrical submersible pump systems.
Gas lift is an important artificial lift technique. When a reliable source of gas is available, gas lift systems, which are highly reliable, flexible and robust, can operate for the life of the well. The ability to change valves allows operators to adapt to production changes over the life of the well. As a consequence, gas lift is often the artificial lift method of choice for use in deepwater subsea wells in which continuous operation in high-pressure, high-flow-rate wells is critical.
Oilfield Review 2015.
Copyright © 2015 Schlumberger.
For help in preparation of this article, thanks to Jason Jones, Tyler, Texas, USA, and Eric Lovie, Singapore.