Lift string – column of pipes used to lift reservoir fluids (oil, gas, water) to the surface during well completion, flow and gas lift production. Normally up to 114 millimeter diameter tubing is used as lift string, and in big wells – casing strings.

Lift strings are run into the wells to the upper perforation or to the pay top. Mostly single strings are used, and sometimes – multi strings with concentric and eccentric hanging of parallel strings. Multi strings may be one-and-a-half strings (inner tubing is shorter then outer tubing), double strings (if the casing string is not air-tight or in hydrogeology big-diameter wells), triple (or more) strings for dual completion of wells. Lifting/injection of liquid may be either through inner or outer string. Lift string protects production string from reservoir media impact and allows for wells completion by way of gradual decreasing the density of the filled liquid (replacing mud with water, oil, etc.), for aeration and for killing the blowing well by injecting high-density fluid (water, mud).