Gas Lift

a device for raising liquid by means of the energy contained in compressed gas that is being mixed with the liquid. Gas lifts are generally used for raising petroleum from boreholes by using the gas coming out of the petroleum-bearing layers. Lifts are known in which atmospheric air is used for pumping a liquid, usually water; such lifts are called air lifts.

In a gas lift or air lift (see Figure 1), compressed gas or air is fed through pipe (3) and mixes with the liquid, forming a gas-liquid or air-water emulsion, which is lifted through pipe (2). The mixing of the gas and liquid takes place in the shoe (4) that connects the pipes. At the surface, the gaseous phase of the emulsion is separated from the liquid phase by the separator (1).

Figure 1. Diagram of an air lift: (1) separator, (2) pipe for raising emulsion, (3) pipe for air supply, (4) shoe; H is the height to which the water-gas mixture is raised, and h is the depth of submersion of the pipe

The operation of a gas lift is based on balancing the column of gas-liquid emulsion by a column of liquid, according to the law of communicating vessels. One of the vessels is the borehole or a reservoir, and the other is the pipe containing the gas-liquid mixture.

For static conditions, γ₁ h = γ_m(h + H), where γ₁, is the density of the liquid, γ_m is the density of the mixture, H is the height to which the gas-liquid mixture is raised, and h is the depth of submersion of the pipe. For γ_m < γ_l, h+ H > h; that is, with an increase in the depth of submersion of the shoe, the height to which the liquid may be raised is increased. The process of operation of a gas lift is accompanied by the phenomenon of the attraction of the liquid by bubbles of gas or air which, as they rise, expand and accelerate the movement of the gas-liquid mixture. The optimum speed of movement of the emulsion in the lower part of the pipe is 3 m/sec; in the upper part, 6-8 m/sec.

Gas lifts can raise water as high as 200 m and petroleum as high as 1,000 m at a rate of up to 500 m³/hr. The efficiency of gas lifts is 15 to 36 percent. In spite of the fact that more efficient technical means are available, gas lifts are still used for raising liquids.