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Intermittent gas lift plunger application
The following article addresses intermittent gas lift plunger applications.
Overview
An important consideration related to intermittent gas lift operations is the injection-gas breakthrough and resulting loss of the liquid production per cycle from the injection gas penetrating the liquid slug during the time required to displace this slug to the surface. The produced-liquid slug can be a small fraction of the starting slug size because of injection-gas breakthrough. The losses are greater when the injection-gas pressure is low and the required depth of lift is near total depth in a deep well.
For example, a 12,000-ft well with a bottomhole flowing pressure of 300 psig and an available injection-gas pressure of only 450 psig can be gas lifted intermittently with the proper plunger. The well could not be gas lifted successfully from this depth without a plunger.
Intermittent gas lift plunger application
A typical plunger installation for intermittent gas lift operation is shown in Fig. 1. A plunger can be expected to decrease the injection-gas requirement for an intermittent gas lift installation from 30 to 70% depending on plunger construction, the depth of lift, injection-gas pressure, and adjustment of the injection-gas volume to the well before the plunger is installed.
Fig. 1-Plunger in an intermittent gas lift installation.
There is little if any liquid-slug recovery by intermittent gas lift from very deep wells with low injection-gas pressure unless a plunger is installed. The plunger provides a solid interface between the starting liquid slug and the displacing injection gas. The plunger practically eliminates liquid fallback as a result of gas penetrating the liquid slug. The increase in liquid recovery and the decrease in the injection-gas requirement per cycle from installing a plunger are minimal in an intermittent gas lift installation with small liquid slugs being lifted at an exceedingly high slug velocity in shallow wells. Another advantage of a plunger is that it reduces paraffin in a well with a paraffin problem. Plungers are installed in some wells for the sole purpose of keeping the tubing free of paraffin deposition.
A plunger can be installed in an existing tubing-retrievable conventional gas lift valve installation by wireline without pulling the tubing if there are no tight spots in the tubing. A standing valve and a bottomhole collar lock or a stop with a bumper spring can be installed with wireline tools. A standing valve normally is recommended but not required in wells with a low permeability. The bottomhole bumper spring is located immediately above the operating gas lift valve, and a standing valve is stationed below the valve. The remaining equipment is on the surface and includes a lubricator with a bumper spring and a plunger catcher mechanism. A plunger-arrival detector to shut in the tubing is not needed for an intermittent gas lift installation because the tubing is not shut in between injection-gas cycles.
A plunger ascent velocity of 800 to 1,000 ft/min is recommended for the most efficient lift. A plunger may stall or tend to stop and start at plunger velocities less than 350 to 400 ft/min. Plunger velocities in excess of 1,200 to 1,500 ft/min are not recommended because of possible damage to the plunger on arrival at the surface and an apparent tendency to bypass a thicker than normal liquid boundary on the tubing wall. Noting the times when a time-cycle controller opens and when the plunger arrives at the surface can approximate an average plunger velocity.
Conditions that favor the use of plunger in an intermittent gas lift
The addition of a plunger to an intermittent gas lift installation should be considered when:
- The available injection-gas pressure is low relative to the required depth of lift in a low-flowing-bottomhole-pressure well
- There is an excessive increase in wellhead tubing pressure as the liquid slug enters the flowline because of such factors as a small-inside diameter (ID) flowline, excessive number of bends at the wellhead, and flowline choke
- A paraffin-deposition problem exists
A plunger should increase the efficiency required of most intermittent gas lift installations. Because a plunger adds several moving parts, it can also increase the operating problems.
Conditions that prohibit the use of plunger in an intermittent gas lift
Well conditions that prohibit the use of a plunger are:
- A bore opening through surface wellhead and tree valves that differs from the tubing ID
- Excessive well deviation, which prevents a plunger from descending to its bottomhole bumper spring
- Tight spots in the tubing
- Appreciable sand production
- High-rate intermittent gas lift operations
The fall time required for a plunger to descend to the bottom bumper spring could reduce the maximum production from a high-cycle-frequency intermittent gas lift installation.
Specially designed plungers are available for wells with side-pocket mandrels. Plungers have worked in wells with a deviation near 50°, but the maximum deviation for a plunger operation depends on the construction of the plunger. The manufacturers should be able to provide the information related to their plunger operation in a deviated well.
There are numerous types of plunger sealing elements, bypass valves, plunger weights and lengths, and other features that may have been developed for unique applications. Some plungers are particularly applicable for gas lift and other types are not. Select the proper plunger to match the well conditions and application for trouble-free service and efficient operation.
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See also
Intermittent-flow gas lift installation design