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When hydrocarbons are first recovered, the typically high reservoir pressure ensures that these substances naturally flow to the production wells and then gush to the surface. With increasing exploitation, however, the pressure in the reservoir drops. Procedures for remedying this involve artificially increasing the pressure in the natural gas reservoirs. This can be done, for example, by injecting nitrogen (N2) into a gas pocket. The process can also be done with other inert gases: in the USA, carbon dioxide (CO2) from underground sources is also used as a cost-effective variant.
Natural gas extraction
Depending on the geological conditions, some of the injected nitrogen can mix over time with the natural gas during recovery. From a purely technical viewpoint, this is not a problem – natural gas contains a certain amount of N2 anyway. However, if the proportion of nitrogen exceeds a particular level, the natural gas is no longer viable for industrial use. This can make it necessary to remove the nitrogen when treating natural gas. Two benefits result from this: the transport volume is decreased and the calorific or fuel value is increased.
In order to store or transport natural gas in liquid form (LNG), the nitrogen content should be below one percent. To separate the N2 from the recovered gas stream, so-called "Nitrogen Rejection Units" (NRUs) are used. These units cool the nitrogen-rich gas until part of it liquefies. At temperatures between minus 140 and minus 180 degrees Celsius, so-called fractionating columns separate almost all of the nitrogen from the natural gas. This reduces the proportion of nitrogen to under one percent.
Important reference in the natural gas business
The separated natural gas stream can then be used, for example, as fuel for the turbines that power the liquefaction process. The largest Australian oil and gas company, Woodside, uses this nitrogen rejection solution at its Pluto and Xena natural gas fields near Karratha, off the country's west coast. The natural nitrogen content there is over six percent, so a Linde NRU was integrated when Woodside planned its natural gas liquefaction plants. At the end of 2011, the plant successfully went into operation.
Due to the world's growing demand for natural gas, it can be assumed that increasing numbers of natural gas fields with high nitrogen content will be tapped and that Linde technology for N2 separation will become more important. This is why Linde experts see the NRU in Australia as an important reference project in the natural gas business. With the new plant, Linde is well positioned for future orders in this promising market.
At a glance
Nitrogen is often present in natural gas and reduces its purity and quality.
Cryogenic processes can efficiently remove nitrogen
Design, plant construction and commissioning of NRUs
Treatment of pipeline gas in LNG plants or after Enhanced Oil / Gas Recovery
Nitrogen extraction plant for Pluto LNG in Karratha, Australia for Woodside Burrup Property, Ltd
Raw gas capacity of 78,000 Nm³/h