Paper: Predicting Plunger Lift Performance

Plunger lift excels at producing high GLR oil wells and removing liquid accumulations from gas wells. As reservoirs deplete and flowing rates decline, the gas phase becomes less efficient at lifting the liquid phase to the surface. Allowed to continue, the flowing gradient will become heavier until the well loads up with liquid and stops flowing. In gas wells, smaller tubing (siphon tube), a compressor, rod pumping, or plunger lift is installed to maintain flowing status. Neither the smaller tubing nor a compressor is a permanent solution. They increase the gas velocity initially so that the liquid will be carried out; however, at some time in the future the gas rate will again fall to an inadequate level and the liquid will not be removed from the well. Rod pumping is a permanent solution but an expensive one. Plunger lift provides a permanent solution like rod pumping, however, and at a lower price than any other method. Another alternative is a subsurface liquid diverter. A comparison of plunger lift and the subsurface liquid diverter will be presented in the section on intermittent gas lift. In oil wells, plunger lift may be installed in lieu of other types of artificial lift when the well stops flowing (if the GLR is high enough) or earlier in an effort to lighten the flowing gradient and increase draw-down. In most cases where plunger lift is applicable it will produce a well at a rate equal to or greater than that obtained by pumping, because a high GLR, while necessary for plunger lift, reduces pump efficiency due to gas interference. Once installed and operating, plunger lift can be expected to produce a well to depletion. As reservoir pressure (and thus maximum available casing pressure) declines, so do the producing rate and the need for a high casing pressure. There are a number of plunger lift wells operating with 70-100 psi casing pressure. There are no absolute maximum producing rates for plunger lift as there are none for flowing wells. The limiting producing rate in both cases is as much dependent upon the inflow performance of the well as it is upon its outflow performance. Thus plunger lift should not be automatically ruled out at high producing rates. If high rates are required, the larger diameter plungers should be considered. Plungers can also be used for paraffin removal and have been used in attempts to decrease GOR. A plunger does make a most efficient and economical paraffin scraper and several have been installed for this purpose alone. The attempts at GOR control are often unsuccessful. When the GOR does decrease, it is not due to decreased gas production, but rather increased oil production without an associated change in gas production. The economy of plunger lift is one of the most appealing factors. The capital and operating costs of other alternatives almost always exceed those of plunger lift. The total cost of installing plunger lift is $1500 to $3000.