Gas lift is a method of increasing oil well flow rates by injecting high pressure gas to reduce the weight of the liquid in the well column. It is compact and popular for offshore drilling, but other methods are cheaper for onshore wells. It can also be used in oil fields with high natural gas production.
Gas lift is a method of increasing the natural flow rate of an oil well by reducing the weight of the liquid in the well column through the injection of high pressure gas. The weight of the oil in the well column, together with the drag caused by the flow of viscous crude oil through the well system, must be overcome by the natural reservoir pressure to provide flow. Injecting gas near the bottom of the well column reduces the density of the oil and the total weight of the liquid within the well column. Gas lift systems are typically more compact and require less energy than other methods of increasing flow rates and are a popular solution for offshore drilling projects.
Most oil fields are under sufficient natural pressure to provide an economic flow rate when first drawn. As the oil is removed from the tank, however, the pressure drops and the flow rate slows or stops completely. Since this typically occurs before most of the oil has been removed from the reservoir, the remainder of the oil can be put to good use by reducing the well pipe downward pressure on the reservoir. This can be done by directly pumping the oil through the well pipe, replacing the missing oil in the reservoir with water or other liquids, or by reducing the weight of the liquid in the well pipe.
Gas is injected into the well pipe either through the well casing or directly through the production pipe. If gas is injected through the well casing, the gas inlet valve is usually placed in a mandrel, a sort of recess cut into the side of the production pipe. This allows the oil to flow into the pipe unimpeded by the gas injection apparatus and is generally favored in low volume wells. In larger wells, the gas lift system can be lowered directly into the production pipe without significantly affecting the oil flow.
In the case of most onshore oil wells, other methods of improving flow are simpler and cheaper than gas lift. It is primarily used on offshore drilling rigs where space is at a premium and the compact nature of the injection mechanisms is an advantage. It is also used in oil fields which produce a high volume of natural gas. The gas can be passed through a scrubber to purify and dry the gas on site, where it can be immediately injected into marginal producing oil wells. Once the gas is injected into a well, most of it is reclaimed to the surface and can be compressed and re-injected without much waste.
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