Plunger lift is normally used on low pressure gas wells with some condensate, oil or water, or high gas ratio oil wells. In this case the well flow conditions can be so that liquid starts to collect downhole and eventually blocks gas so that the well production stops. In this case a plunger with an open/close valve can be inserted in the tubing. A plunger catcher at the top opens the valve and can hold the plunger, while another mechanism downhole will close the valve. The cycle starts with the plunger falling into the well with its valve open. Gas, condensate and oil can pass though the plunger until it reaches bottom. There the valve is closed, now with a volume of oil, condensate or water on top. Gas pressure starts to accumulate under the plunger and after some time pushes the plunger upwards, with liquid on top, which eventually flows out of the wellhead discharge. When the plunger reaches the wellhead plunger catcher, the valve opens and allows gas to flow freely for some time while new liquid collects at the bottom. After some preset time the catcher will release the plunger, and the cycle repeats.  

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