A Transient Plunger Lift Model for Liquid Unloading from Gas Wells

Accompanied with liquid condensation, natural gas production wells suffer from liquid loading if the gas flow rate is insufficient to carry liquids to the surface. With continuous production, the reservoir pressure decreases due to reservoir depletion, resulting in decrease of gas flow rate and inability to carry liquid upward. Then, the produced liquid accumulates in the well bottom and creates a static liquid column, adding a backpressure against reservoir pressure and reducing gas flow rates until the well production ceases. Due to many advantages, such as low operation cost and prevention of paraffin deposition along wellbore, plunger lift has been widely used in gas wells for the removal of liquid column and rescuing dying gas wells from liquid loading. The existing plunger lift models in literature are imperfect due to either limited field applications or oversimplified assumptions, which lead to considerable prediction errors. Starting from Gasbarri and Wiggins (2001) dynamic plunger lift model, several components in the cyclic movement of a plunger can be identified with each comprising a set of specific governing equations, namely, plunger upstroke, gas blowout, plunger fall-down, pressure buildup etc. Considering the gas flows with plunger moving in the tubing, the new model accounts for the instant velocities during plunger rising and falling. Reservoir performance as a component is included in all stages of plunger lift processes. By solving the transient governing equation in each component of plunger lift iteratively, the new model outputs the plunger velocity/acceleration, pressure versus time, production rate versus time etc. Compared to previous plunger lift models, improvements have been made on the equations of plunger rising and falling velocities. The present model also accounts for different reservoir performances. Oil and water cases from previous studies are used to evaluate the present model which provides more accurate and reasonable predictions of plunger rising and falling velocities.