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.
Sometimes, depending on the hydrocarbon mix, gas that has been pumped from the well gets pumped back down into it.
This happens when the LPG, methane and so on won’t be commercialized.
Injecting it back, reduces or eliminates the need for flaring and is a more environmentally conscious action.
In other instances, the gas will be injected into the gas cap of the primary well head.
The choice of using a separate injection well, or the main well, will be a case by case basis.
Even when all of the hydrocarbons are roughly in once place, an injection of gas into the reservoir helps to maintain pressure that depletes as the levels do.
Sometimes gas injection will be used to help a new well ramp up after completion, other times it might be used towards the end of the life of the well.
Gas injections are often referred to as a secondary or tertiary enhanced recovery method.
The technique is more likely to be used in older wells, or tight wells where some extra help is needed.