Secondary and tertiary recovery processes are typically defined as those that add external energy to the reservoir to recover additional oil or gas. The most common type of secondary recovery process is that of water flooding, in which water is injected into the reservoir and displaces oil toward the production wells. As the name implies, water flooding as a secondary process typically occurs after primary production, although this is not necessarily so. Other secondary and tertiary processes that are commonly used include gas injection (both miscible and immiscible), microemulsion and polymer flooding, in situ combustion, and steam flooding.
Tertiary recovery processes often use EOR technology. EOR processes are those that stimulate oil recovery by the injection of materials not normally present within the reservoir. EOR techniques allow the economic value of existing fields to be maximized through increased oil recovery and field life extension. There are also environmental benefits associated with the use of EOR, as described previously.