Abstract

Tracking the fall of the plunger down the tubing can be used to optimize the operation of plunger lifted wells. Acoustic fluid level instruments can be used on plunger lifted wells to acquire a series of plunger/fluid level soundings and/or to record the acoustic signal produced as the plunger falls down the tubing. Five different data acquisition and analysis methods can be used to monitor the position of the plunger, as the plunger falls down the tubing during the controller's shut-in time period. The acquired data is used to determine the

  1. fall velocity of the plunger

  2. depth to the plunger and

  3. time for the plunger to fall to fluid.

Results acquired from field case studies from 15 sessions at various wells are used to correlate the various construction features of different types of plungers with their fall velocity. Some construction features cause a plunger to fall rapidly through the tubing, while other features cause the plunger to have a slow fall velocity.

By accurately measuring the plunger fall velocity, the proper shut-in time for the plunger lift installation can be determined. The plunger trace measurements will ensure that the plunger has reached the fluid at the bottom of the tubing by the end of the shut-in period. Setting the well's controller to have the shortest possible shut-in time period to allow the plunger to fall to bottom can maximize oil and gas production from plunger lift installations.

Introduction

Some wells produce gas with a small amount of liquid. The gas is usually produced only up the tubing and is normally not produced up the casing annulus. The produced gas carries the liquid into the tubing and the produced liquid generally accumulate in the tubing. If the gas velocity up the tubing is above the Turner critical velocity, then the liquid will be carried with the gas to the surface. If the gas velocity up the tubing is below the critical velocity, then the produced liquid will accumulate in bottom of the tubing1. Gas and liquid flow from the formation will decrease or even stop, if enough liquid is allowed to accumulate in the bottom of the well. Backpressure on the formation increases as the height of the accumulated liquid increases and eventually flow from the formation will cease when the backpressure on the formation is equal to the static pressure of the reservoir. Artificial lift methods to produce the accumulated liquid vary. Sometimes pumping units are used dewater gas wells by pumping the liquid to the surface, but plunger lift is the technique that is most frequently used to lift the liquid accumulated at the bottom of the tubing of low productivity gas wells to the surface.

Plunger lift is a low cost method for lifting liquids (water, condensate and/or oil) from gas and oil wells. This system reduces the cost of operating a well compared to other artificial lift methods, because the formation pressure supplies the energy used to lift the liquids. During plunger lift operations, repeated cycles of surface gas flow and surface gas shut-in occurs. During shut-in the gas flow down the flowline is stopped when the surface control valve is closed. This allows the plunger to fall down to the bottom of the tubing. After a pre-determined amount of time the surface flow valve opens and the tubing is connected to the low-pressure flowline. This reduces the pressure in the tubing above the liquid column and the pressure below the plunger lifts the plunger and most of the liquid above the plunger to the surface. During this process the bottomhole pressure is reduced and this allows additional gas to flow from the formation and casing annulus and be produced at the surface. The plunger operation cycle is continually repeated to produce the well.

An operator can produce the well more efficiently if the plunger fall rate, plunger location, and time the plunger takes to fall to the liquid and bottom of tubing are determined. Five different data acquisition and analysis methods can be used to monitor the position of the plunger, as the plunger falls down the tubing during the shut-in time period. An acoustic liquid level instrument is used to determine the distance from the surface to the top of the plunger during the shut-in portion of plunger lift operational cycle. The distance to the plunger and the rate of fall can be measured when the plunger is above the liquid. When the plunger enters the liquid, the acoustic pulse reflects from the top of the liquid so that the distance to the liquid level is measured. This paper discusses the procedures used to apply five different data acquisition and analysis methods to track the fall of the plunger, and gives examples of the data collected by each method.

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