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Transient Plunger-Lift Model Improves Prediction of Liquid Unloading From Gas Wells

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19211, “A Transient Plunger-Lift Model for Liquid Unloading From Gas Wells,” by Jianjun Zhu, SPE, Haiwen Zhu, and Qingqi Zhao, The University of Tulsa, et al., prepared for the 2019 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2019 International Petroleum Technology Conference. Reproduced by permission.
 Natural-gas wells suffer from liquid loading if the gas-flow rate is insufficient to carry liquids to the surface. Because of the technique’s many advantages, plunger lift has been used widely in gas wells for the removal of liquid columns and the rescue of dying gas wells from liquid loading. Existing plunger-lift models in the literature are imperfect either because of limited field applications or oversimplified assumptions. Several components in the cyclic movement of a plunger can be identified, with each comprising a set of specific governing equations. In this paper, a new model is described that considers the gas flow with the plunger moving in the tubing, and accounts for instant velocities during plunger rise and fall.
 Introduction
 A typical plunger-lift system mainly comprises a piston or plunger, a surface control valve (SCV), a catcher, a lubricator or shock spring, a bumper spring, and a plunger sensor. In addition, casing and tubing serve as key components during the plunger-lifting process. Existing plunger-lift models can be categorized broadly into static and dynamic models.
 Although many dynamic plunger-lift models have been studied in the literature, a universally validated model with thorough understanding of the transient behaviors of gas and liquid still is unavailable. Because of incomplete consideration of plunger dynamics or over-simplified assumptions, most existing models suffer from questionable applicability. This problem can be remedied by incorporating or proposing in-depth closure relationships and performing para-metric studies to find the most-suitable one. In this study, based on previous modeling frameworks, a new transient plunger-lift model for liquid unloading in gas wells is presented. Compared with previous models, improvements have been made in the equations of plunger rising and falling velocities. The present model also accounts for different reservoir performances, which provides more-accurate and -reasonable predictions of these velocities.
 Model Development
 The plunger-lift model can be divided into six components: plunger up-stroke, gas blowout, plunger fall, pressure buildup, gas flows above and below the plunger, and reservoir performance. Reservoir performance is a component included in all stages of the plunger-lift processes. In the energy-buildup stage, reservoir performance contributes to restoring energy below the plunger. Basic mass and momentum-conservation equations are used to derive the plunger-lift simulation model. Model descriptions, assumptions, and related equations are discussed in detail in the complete paper.
 Results and Discussions
 The calculated results from the proposed plunger-lift model are presented and discussed in the complete paper, which also presents the input parameters used in the model.