Free-piston pumping – a type of periodical gas lift with plunger piston. Used to remove fluid from gas well.

Plunger piston has a shape of long cylinder with either rigid telescoping seal or resilient seal and with axial channel, which may be shut-off by a valve.

When plunger piston is run-in-hole inside tubing, its valve is open and the seal is folded to reduce the resistance. After the piston hits the lower bumper, its valve closes, sealing elements unfold and the pressure of inflowing gas pushed the piston up to the wellhead transporting the fluid column above it.

After entering the wellhead control equipment (lubricator) the piston hits the upper bumper, the valve opens, and the piston is retained until the phase of wellbore fluids blow-out is finished. This freely moving piston inside the tubing shuts off the gas slug from the fluid column, thus preventing gas break-through into the fluid and fluid run-off along the tubing walls. This improves the production efficiency by lowering the working medium (gas, air) flow rate, and in some cases just reservoir energy is enough to transport fluid to the surface (when the well operates in periodical flowing mode).