Multistage Plunger-Lift Systems Can Provide Economical Alternative to Pumping Units

Abstract

Technology Update Plunger lift is a simple and economical artificial-lift (AL) system for wells that primarily produce gas. It is a widespread method of deliquefying wells with high gas-to-liquid ratios (GLRs), for lifting production from wells in which natural depletion has reduced flow rates below critical levels, or for producing from wells with high accumulations of solids, such as sand, salt, coal fines, paraffin, and scale. However, for some wells, plunger lift by itself may not add enough energy to the wellbore to produce gas in economic volumes. Deep gas and low-GLR wells, with ratios 1:1, may have lift requirements exceeding the capabilities of conventional plunger AL systems. Gassy oil wells often fall into the latter category. Frequently, pump jacks are installed on wells where conventional plunger lift is considered inadequate. Foaming methods to reduce the density of flowstream liquids may also be employed, but these are typically most effective in wells producing primarily water. A more effective strategy for increasing production from gassy oil wells and other wells where conventional plunger lift may be insufficient can be to install a multistage plunger-lift system. In these systems, a downhole tool is used to create multiple plunger-lift systems in one well. This multistage tool allows the liquid load to be lifted in sequences, a process that uses the well’s own energy to remove even large accumulations of liquids, or heavy liquids, efficiently. Multistage Plunger-Lift Tool The multistage tool (Fig. 1) is placed by wireline roughly 40–70% of the way down the tubing above an installed plunger-lift system, typically composed of a bottomhole bumper spring and a plunger above it. Then a second plunger is set on top of the tool. The system is operated like a conventional plunger-lift system. During the first sales cycle, the lower plunger carries fluids up the tubing and delivers them to the tool. They flow through the tool and are held above it by gas flow. Upon shut-in, the ball check in the tool engages, retaining the fluids until the upper plunger falls from the surface, settles through the liquids and lands at the tool. Simultaneously, the lower plunger falls back to the bottom. During the next sales cycle, the upper plunger delivers its fluids to the surface, while the lower plunger delivers more fluids to the tool. Both plungers work in tandem in subsequent cycles. In this way, the multistage tool acts like an intermediary standing valve. This process lifts smaller and more frequent liquid loads in stages, allowing the well to more efficiently use its own energy to remove liquids and increase productivity. The “nodding donkeys” visible at many wellsites demonstrate the popularity of using pump jacks as an AL method. Pump jacks (also known as sucker-rod pumps or beam-pumping units) are typically powered by fossil fuels or an electric motor. They require a significant upfront investment in equipment and installation, and maintenance costs can be considerable.