Gas Lift injects gas into the well flow. The downhole reservoir pressure falls off to the wellhead due to the counter pressure from weight of the oil column in the tubing. Thus a 150 MPa reservoir pressure at 1600 meters will fall to zero wellhead pressure if the specific gravity is 800 kg/m2. (0,8 times water). By injecting gas into this oil, the specific gravity is lowered and the well will start to flow. Typically gas in injected between casing and tubing, and a release valve on a gas lift mandrel is inserted in the tubing above the packer. The valve will open at a set pressure to inject lift gas into the tubing. Several mandrels with valves set at different pressure ranges can be used to improve lifting and start up. Ill: Schlumberger oilfield glossary Gas lift can be controlled for a single well to optimize production, and to reduce slugging effects where the gas droplets collect to form large bubbles that can upset production. Gas lift can also be optimized over several wells to use available gas in the most efficient way.