Enhanced Oil Recovery (EOR)

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Enhanced Oil Recovery (EOR)

Enhanced Oil Recovery (EOR), also called tertiary recovery, refers to techniques used to extract oil that remains after primary and secondary recovery methods. Primary recovery relies on natural reservoir pressure and pumping; secondary recovery uses injected water or gas to maintain pressure. EOR alters reservoir conditions or oil properties to mobilize additional oil that these earlier stages cannot recover.

Key takeaways

  • EOR targets oil left after primary and secondary recovery (tertiary recovery).
  • Methods include gas injection (commonly CO2), thermal methods (steam or in‑situ combustion), and chemical/polymer injections.
  • EOR can extend the productive life of wells but is often costly and can have significant environmental risks.
  • Newer approaches such as plasma pulsing aim to lower environmental impact by avoiding injected gases or chemicals.

How EOR works

EOR techniques either change the physical properties of the oil (for example lowering viscosity) or alter reservoir pressure and flow patterns so trapped oil can move toward production wells. Because these methods are complex and expensive, operators typically deploy them only after primary and secondary recovery have been exhausted and only when economics—driven largely by oil prices—justify the additional cost.

Main EOR methods

  • Gas injection (miscible and immiscible)
  • Gases are injected to push oil toward production wells and to reduce oil viscosity. Carbon dioxide (CO2) is the most commonly used gas.
  • Recent advances enable CO2 transport and deployment as foams or gels, widening the range of reservoirs where it can be used.

  • While CO2 injection can also serve as a form of carbon storage in some projects, it raises environmental concerns related to greenhouse gases and leakage risks.

  • Thermal methods

  • Steam injection heats heavy oil, reducing viscosity so it flows more readily.

  • In‑situ combustion, or “fire flooding,” ignites part of the reservoir to heat adjacent oil and drive it toward wells. Both methods are energy intensive.

  • Chemical and polymer injection

  • Polymers and other chemicals are injected to improve sweep efficiency, reduce viscosity, and increase reservoir pressure.
  • These techniques can be effective but are often expensive and require careful reservoir management to avoid formation damage.

Uses, economics, and limits

EOR is mainly used to extend the life of proven and probable fields. Proven reserves are those with a greater than 90% likelihood of recovery; probable reserves generally have more than a 50% chance. Operators weigh the incremental recovery against high capital and operating costs—if oil prices or expected recoveries are too low, remaining oil may be left in place.

Environmental and operational constraints also limit deployment: injected substances can contaminate groundwater, and thermal or chemical operations can alter reservoir integrity. Regulatory, logistical, and surface infrastructure requirements further affect feasibility.

Emerging and lower‑impact techniques

Newer approaches seek to reduce environmental risks and operating costs. One example is plasma pulsing, a method developed to irradiate reservoirs with low‑energy pulses that reduce oil viscosity without injecting gases, heat, or chemicals. Such techniques are in early stages but may offer lower‑impact alternatives for certain reservoirs.

Conclusion

EOR encompasses several technical approaches to recover oil left after conventional methods. It can substantially increase recovery from mature fields but involves high costs and environmental trade‑offs. Decisions to deploy EOR hinge on reservoir characteristics, technology suitability, regulatory considerations, and oil market economics.