Below a certain «critical velocity», liquids tend to migrate down the tubing and start to collect at the bottom.
For a while the well is able to unload the small slugs on it’s own. The surface indications are «heading» recorded on the sales chart. If no remedial measures are taken, the problem will worsen until the well loads up and dies.
Other indications of liquid loading problems are sharply decreasing production decline curves for both gas and liquids. Any well that must be «blown down» periodically is most certainly experiencing liquid loading.
The function of the natural gas plunger lift is to prevent these liquids from accumulating to the point that the well would die or require a lengthy shut-in period to recover.
The well is shut in when it is determined that loading is indeed occurring down hole. The well is opened up when casing pressure has built up enough to lift the accumulated liquids in the tubing along with the plunger as the gas breaks around the end of the tubing. This pressure and velocity must be great enough to overcome the sales line or separator pressure encountered on the trip to the surface.
Upon arrival of the plunger at the surface, the tubing string is completely free of liquids. At this point the formation encounters the least resistance to flow. Depending on the productivity of the well, high flow rates may be sustained by leaving the flow line open for some increment of time. This increment may be dictated by a certain pressure drop on the casing or observation of the sales chart to determine a time interval. The well should be shut in when loading is evidenced by a decline in differential on the sales meter. Then the cycle should be repeated.
Plungers are very effective even on low-pressure gas wells that have good productivity. It is necessary to cycle the plunger frequently removing very small amounts of liquids at a time. The good deliverability assures prompt recovery of casing pressure for the next cycle.
An increase in production can be expected from utilization of this system. The real benefit shows up in cumulative production and resumption of a normal decline curve.