Which stage of oil recovery is described as after secondary recovery, using tertiary methods?

• A. Primary recovery
• B. Secondary recovery
• C. Quaternary recovery
• D. Tertiary recovery

Answer: (D)

After primary production, where natural reservoir energy drives oil to the well, and after secondary recovery, which uses an injected drive like water or gas to continue producing, the next stage employs enhanced techniques to recover additional oil. This is tertiary recovery, also called enhanced oil recovery, and it relies on methods such as thermal (steam), chemical (polymers, surfactants), or gas (CO2) injections to improve displacement and sweep efficiency. In standard classifications, tertiary follows secondary, making it the appropriate description for the stage described. Quaternary recovery exists in some discussions, but the widely taught sequence for this context is primary, secondary, then tertiary.