Liquid loading has been an issue for mature gas wells due to declining reservoir energy. In mature fields, one of the methods used to mitigate this problem is plunger lift. However, the design of plunger lift may still question the premise of optimization. This paper presents a new model for the design of plunger lift for wells in gas reservoirs with significant water production. The proposed model allows for an efficient design of plunger lift by incorporating physics of fluid flow in the wellbore.
The wellbore/reservoir model presupposes presence of some liquid in the tubing and in the tubing/casing annulus, but most of the tubing and annular space is assumed to be filled with gas. Closure of the tubinghead valve initiates recuperation of reservoir energy, thereby allowing fluid influx to occur in the tubing and annulus. Eventually, accumulation of sufficient pressurized gas in the annulus lifts the plunger along with the liquid and gas on top of it. The model accounts for the pressure-volume (pV) work done by the pressurized gas in the annulus, including the energy needed to lift the liquid and gas on top of the plunger, and friction during the plunger movement. Because the dimensions and trajectory of the wellbore have such profound impact on the operability of plunger lift, operators can use this model by just providing the known input parameters to determine the design variables, target casing pressure and duration for the plunger cycle.