Natural Gas Underwater Extraction and Separation

Published: 17th May 2010
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Natural gas separation is a process used with underground and underwater extraction of natural gas in order to extract valuable methane from other, less useful, or possibly dangerous hydrocarbons and other components present in natural gas deposits. With extraction of natural gas from underwater deposits, it is particularly important to have on-site and relatively compact techniques to separate methane from raw natural gas, since unlike ground-based plants, offshore oil and gas drilling facilities may not have the luxury of large facilities nearby to process the gas.

These innovative new techniques often involve separation of methane before the natural gas even reaches the surface of the water. One technique involves trapping the raw gas inside of a gas bell after extraction, and allowing the gas to then separate from water or oil which may be present at the drilling or blow-out point. This mixture is then sealed and cooled in order to condense the gas beyond its dew point, and the resulting liquid mixture of gas condensate and other contaminants is fed into another vessel, known as a high pressure separator. This stage involves using high pressure to convert the raw hydrocarbons, as well as the water, back into their gaseous form, and sending them to a main gas compressor.

This condensate is then sent through a control valve, which is triggered in the main gas compressor by the temperature and pressure of the gas, where it undergoes a partial re-vaporization from the abrupt cooling after removal from the compressor. For thereason the hydrocarbons then go to a low pressure separator, where condensate is removed, and the remaining gas is sent to a booster compressor in order to ensure that flash vaporization does not reoccur. The reason for multiple compressions is to ensure that no condensate remains after the booster compression process, since the gas is then sent to a pipeline to a natural gas processing plant aboard the offshore drilling site.

At this point, the hydrocarbons must be separated from one another, since methane is the most useful hydrocarbon and easiest to transport, and heavier hydrocarbons may have different volatilities and present unique transport problems. Additionally, some water vapor and acid gases may remain in the mixture, so they are first removed in a preliminary raw processing plant via dehydration. Then, the hydrocarbons are separated from one another, usually using a series of membranes. A combination of synthetic membranes will allow methane to be sent to another pipeline, and then stored for later transport, and the heavier hydrocarbons are removed since they have heavier molecular weights. Theunique synthesis of separating techniques has made obtaining natural gas from remote undersea sources possible and allows increased production of natural gas.

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