The rising portion of plunger cycle makes use of some type of designed sealing mechanisms on the plunger. These sealing techniques reduce the amount of gas that bypasses (leakage) the surfacing plungers. When falling, many plungers have mechanisms designed to allow increased leakage or slippage enabling the plungers to fall faster.
Modeling techniques, for leakage about a rising plunger, shown in this paper are shown as analogous to orifice type flow restriction. For fast falling plungers the model is developed is similar to objects experiencing drag in a field of gas velocity. Data collected for this type of modeling is presented from suspension tests and confirmed with dynamic test data. Model ratification is done with some dynamic test data. Special two-piece and conventional plungers are hereby modeled from suspension and dynamic testing.
The results should help the operators to select specific plunger hardware for specific conditions and should assist in modeling plunger cycles.
Plunger lift is a common artificial lift method of producing liquids from a gas well to improve gas flow, usually without the addition of any outside energy or extra gas. The need for plunger lift arises as the reservoir pressure decreases and lower gas velocity fails to lift liquid from the well. The objective of plunger lift is to keep the wellbore free of liquids and associated pressure drop by lifting liquids on an intermittent basis to the surface. The feasibility of plunger lift is widely discussed in the literature[1–5].
This paper chiefly will deal with modeling and predicted results for plunger rise, plunger fall and also the lifting of liquid slugs over the plunger. The direct outcome of these results is to help assist the operators on the plunger hardware to be selected and in the set-up of the duration of the plunger cycles.
Gas slips upward around the plunger when it rises in the wellbore during the plunger cycle. The percentage of leakage5 compared to the gas production is relatively small. This leakage can gradually reduce the pressure under the plunger and reduce lifting energy if too much gas slips past. However, conventionally the plunger is designed with sealing contours or mechanisms to prevent the gas from underneath the plunger to leak to the liquid slug above it. Considered here also, is the modeling of the two-piece plunger and the conventional ‘sealing mechanism’ used with plungers for the rising and falling portion of the plunger cycle. In addition, modeling for lifting of the liquid slug is also provided showing how effects of the liquid slug size can affect the average rise velocity of the plunger along with other parameters. The details can be found in the following sub sections. These new considerations in modeling help provide a better understanding, of plunger cycles and operation. Data obtained from experimental runs from a test site was input into the model developed. The experimental data was obtained by suspension and dynamic testing. The results from the models were in the approximate range of measured data, thus validating the model.
Two-Piece Plunger. The plunger consists of a hollow cylindrical piston and a ball below. The hollow cylindrical piston could be changed in length, material used, thickness, size and number of grooves depending upon usage but is usually a fixed configuration with various materials available. The two-piece plunger cycle[5,6] typically requires about 5 to 10 seconds of shut-in time and the well is producing even when the plunger components are falling to the bottom of the tubing. The model developed is presented in Appendix A.
Fig. D-1 shows the various types of two-piece plungers currently being used in the industry and Fig. D-2 shows the mechanical components. The shifting rod seen in Fig. D-2 generally has a taper to it with large diameter towards the bottom of the rod. This helps facilitate holding the hollow cylinder at the top while the well is producing.