Summary
As an economical and efficient artificial lift method, plunger lift can be used to unload the accumulated liquids from the bottom of gas wells, which helps lower the bottomhole pressure, resulting in higher gas production rate. However, the transient flow behavior of the plunger-lift-aided production system is still not well understood due to the lack of a reliable and accurate prediction model. In this study, a transient mechanistic model is developed to simulate the comprehensive dynamic process of a plunger-lift system that is cyclically paced by a surface control valve.
Starting from the Gasbarri and Wiggins (2001) dynamic plunger-lift model, four stages in the cyclic movement of a plunger can be identified and calculated using a set of specific governing equations. Considering the gas flows with a plunger in the tubing, the model can calculate the instant velocities of the plunger during its rising and falling movement. The classical inflow performance relationship (IPR) is employed as the reservoir model to obtain the fluid flow rates from the reservoir to the wellbore. The proposed new model can capture the essential parameters of plunger-lift cycles, including plunger velocity/acceleration, tubing/casing pressure, production rates, etc. Compared to previous models, the predicted rising and falling speeds of the plunger are improved. The hydrocarbon mixture properties in the gas well are computed by a compositional model in this study, which provides more accurate and reasonable predictions of tubing and casing pressure.
Several parametric studies are presented in the paper. These studies will help to understand the influence of different parameters on the process of plunger lift. An appropriate combination of casing and tubing pressure should be taken into consideration. A reservoir coefficient term is introduced and defined. A larger reservoir coefficient will improve the ultimate profitability of the well by increasing the production rate at the beginning and accelerate the depletion of gas wells. If the gas/liquid ratio (GLR) is too low, liquid loading may be triggered. The parametric study shows that an adequate GLR is necessary for reliable plunger-lift performance.
