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N2 is injected into natural gas reservoirs to artificially increase pressure – which results in more yield from the well. Called "Enhanced Gas Recovery" (EGR), this process can also be accomplished with other inert gases. In the USA, carbon dioxide (CO2) sourced from underground wells is an economical alternative.
Pilot project for CO2 injection in Germany
Gaz de France Produktion Exploration Deutschland GmbH (GDF-PEG), which owns gas reservoirs at Altmark in Saxony-Anhalt, Germany, has just started a pilot project to test this. The goal of the project is to answer all open questions about CO2 injection. In concrete terms, GDF-PEG is investigating whether the use of carbon dioxide increases recovery rates from the gas field. The Linde subsidiary Linde Engineering Dresden was responsible for planning and constructing the CO2 injection system here.
In contrast, nitrogen can be recovered virtually anywhere from atmospheric air – via an air separation plant. Physically, there are also differences between the two gases: CO2 requires more compression than N, which is why greater amounts of it are needed to create high pressure in natural gas reservoirs. This means that the EGR process using compressed nitrogen is more energy-efficient than the CO2 variant.
To maintain the pressure in a reservoir, another technology has also been developed: part of the recovered natural gas is fed back underground. This process is used by the Abu Dhabi National Oil Company (ADNOC) at its Habshan gas field, for example.
But because natural gas is urgently needed to create power, ADNOC and Linde have founded the joint venture Elixier. In 2011, this joint venture constructed two major air separation plants in Mirfa, a coastal city in Abu Dhabi, which produce circa 670,000 cubic metres of nitrogen per hour. From Mirfa, the nitrogen will be transported to its inland Habshan field via a 50-kilometre long pipeline and used for enhanced recovery of gas and oil reserves.
The project is also strategically important: ADNOC has access to circa 90 percent of Abu Dhabi's total oil and gas reserves. These are considered to be the fourth-largest in the world.