Final answer:
Oxygen injection (D) is not used for enhanced oil recovery because it is not inert and poses a risk of fire or explosion.
Step-by-step explanation:
The type of inert gas injection that is NOT used for enhanced recovery in the context provided is D) Oxygen injection. In the context of oil and gas extraction, inert gas injection involves using non-reactive gases to displace oil towards production wells and improve the recovery of oil from a reservoir. Nitrogen and carbon dioxide are commonly used for this purpose; nitrogen is relatively cheap and readily available, and carbon dioxide can also help in oil recovery by dissolving in oil to reduce its viscosity.
Helium, while inert and serving various purposes in industrial processes, is not typically used for enhanced oil recovery due to its rarity and expense. Oxygen is excluded from this process because it is highly reactive, not inert, and can create a risk of fire or explosion when introduced into hydrocarbon-rich environments.