Qingqi Zhao (Master of Science in Petroleum Engineering)
Transient Modeling of Plunger Lift for Gas Well Liquid Unloading
Conclusions and RecommendationsÂ
A natural gas production well is commonly accompanied by liquid condensate. As gas wells mature, production rate and bottom-hole pressure decrease, resulting in liquid loading since the gas cannot carry the liquids to the surface. As a result, the liquids are accumulated at the bottom hole, which increases hydrostatic pressure, reduces gas production, and eventually kills the well. As an economical artificial lift method, plunger lift can be used to remove liquids from high gas/liquid ratio (GLR) wells. Therefore, the well bottom-hole pressure can be maintained low and gas can be produced at a high flow rate. However, the transient flow behavior of plunger lift wells is not well understood. In this study, a transient mechanistic model is developed to simulate the entire dynamic process of plunger lift cyclically paced by a surface control valve.
Starting with the Gasban•i and Wiggins (2001) dynamic plunger lift model, four stages in the cyclic movement of a plunger can be identified and calculated by a set of specific governing equations for plunger upstroke, gas blowout, plunger fall-down, and pressure buildup. Considering the gas flows with a plunger moving in the tubing, the model can give the instant velocities during the rising and falling ofthe plunger. Vogel’s Inflow Performance Relationship (IPR) (Vogel, 1968) is used as the reservoir model to obtain the fluid flow from the reservoir to the well-bore. The plunger model is able to capture all the essential features of the plunger cycle: the plunger velocity/acceleration, pressure versus time, production rate versus time, etc. Compared with the previous models, the equations of the rising speed and the falling speed of the plunger are improved. Hydrocarbon mixture in the gas well is also considered in the modified model which provides more accurate and reasonable predictions of tubing and casing pressure. Future improvement of the plunger lift model can be accomplished by adding liquid leakage around the plunger, and liquid slug discharge dynamics.