7.1 Introduction

As discussed in chapter 6, during the lifting of the liquid slug in intermittent gas lift, gas breakthrough at the bottom of the slug results in fallback, i.e. part of the liquid volume falls back and is not produced to the wellhead. Fallback is a natural phenomenon caused by the great difference in density between the liquid and gas phases making the gas bubble below the liquid slug penetrate the liquid. In a deeper well, it can also happen that fallback consumes the total starting slug length and no liquid is produced at the wellhead. Reduction or elimination of fallback by inserting a free piston called plunger between the gas and liquid slugs, therefore, can greatly improve the efficiency of intermittent gas lift. Such installations are very similar to plunger lift installations used since the 1920s to produce oil wells or remove liquids from gas wells.

Plunger lift proper (not covered in this book) is a kind of artificial lift utilizing the energy of formation gas to periodically lift liquid slugs accumulated at the bottom of the tubing string. Thanks to the open installation used, formation gas is trapped in the well's annulus until casing pressure builds up to a level sufficient to lift the liquid slug to the surface. Gas from the annulus is injected at the bottom of the tubing string, below the special plunger rising along with the liquid slug, and this prevents an excessive amount of fallback to occur. Intermittent operation is...