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Primary, Secondary and Tertiary EOR

Crude oil development and production in U.S. oil reservoirs can include up to three distinct phases: primary, secondary, and tertiary (or enhanced) recovery.

During primary recovery, the natural pressure of the reservoir or gravity drive oil into the wellbore, combined with artificial lift techniques (such as pumps) which bring the oil to the surface.

But only about 10 percent of a reservoir’s original oil in place is typically produced during primary recovery.

Secondary recovery techniques extend a field’s productive life generally by injecting water or gas to displace oil and drive it to a production wellbore, resulting in the recovery of an additional 20 percent of the original oil in place.

However, with much of the easy-to-produce oil already recovered from U.S. oil fields, producers have attempted several tertiary, or enhanced oil recovery (EOR), techniques that offer prospects for ultimately producing 30 to 60 percent, or more, of the reservoir’s original oil in place. Three major categories of EOR have been found to be commercially successful to varying degrees:

  • Thermal recovery, which involves the introduction of heat such as the injection of steam to lower theviscosity, or thin, the heavy viscous oil, and improve its ability to flow through the reservoir.Thermal techniques account for over 40 percent of U.S. EOR production, primarily in California.
  • Gas injection, which uses gases such as natural gas, nitrogen, or carbon dioxide (CO2) thatexpand in a reservoir to push additional oil to a production wellbore, or other gases that dissolve inthe oil to lower its viscosity and improves its flow rate. Gas injection accounts for nearly 60 percentof EOR production in the United States.
  • Chemical injection, which can involve the use of long-chained molecules called polymers to increase theeffectiveness of waterfloods, or the use of detergent-like surfactants to help lower the surface tensionthat often prevents oil droplets from moving through a reservoir. Chemical techniques account for aboutone percent of U.S. EOR production.
    Each of these techniques has been hampered by its relatively high cost and, in some cases, by theunpredictability of its effectiveness.

EOR Information and pictures courtesy of U.S. website