Gas injection is also used for pressure maintenance of the reservoir and displacing the oil from injection towards production wells.
The gas used for injection is typically hydrocarbon based, but carbon dioxide (CO2) and nitrogen (or nitrogen-rich flue gas) can also be used.
The injected gas can be miscible, gas and oil dissolve in each other to form a more mobile liquid, or it could be immiscible, where oil and gas remain in their respective phases. Gas injection has good microscopic displacement, with miscible gas injection more so than immiscible.
In 2019, 22 fields on the NCS utilise gas injection. Typically, only immiscible gas injection is categorized as an IOR method, with the others being defined as enhanced oil recovery (EOR). On the NCS only immiscible gas injection is being used.
The Oseberg field is using both gas and water injection for pressure maintenance. The injection of large volumes of gas high in the structure of the main field is displacing oil quite effectively. Injection gas was previously imported from Troll East (TOGI) and Oseberg West. Now, produced gas is partially exported and partially re-injected. In 2012, Oseberg received the Norwegian Petroleum Directorate's IOR award for its efforts to increase recovery on the field using gas injection.