Enhanced oil recovery
- Abbreviation
- EOR
- Also known as
- EOR, tertiary recovery, improved oil recovery
Enhanced oil recovery (EOR) is any technique that recovers oil left in a reservoir after natural pressure and waterflooding have done what they can, by changing the properties of the oil, the rock or the fluids between them. The main types are thermal, gas injection and chemical EOR.
What is enhanced oil recovery?
Most of the oil in a reservoir is never produced. Enhanced oil recovery, also called tertiary recovery, covers the methods used to recover more of it after conventional production has run its course. EOR works by changing something about the reservoir system that holds the remaining oil in place: making the oil flow more easily, making the injected fluid push it more effectively, or releasing oil trapped in the rock’s pores.
Primary, secondary and tertiary recovery
Oil production is often described in three stages. According to the U.S. Department of Energy:
| Stage | How oil is produced | Typical recovery of original oil in place |
|---|---|---|
| Primary recovery | Natural reservoir pressure and pumping | About 10 percent |
| Secondary recovery | Waterflooding or gas injection to maintain pressure and push oil | Brings recovery to about 20 to 40 percent |
| Tertiary recovery (EOR) | Heat, gas or chemicals that change how oil moves | Offers prospects of 30 to 60 percent or more |
The stages are not always applied in that order; some reservoirs start EOR early.
Why oil is left behind
After waterflooding, much of the remaining oil is held as isolated droplets and films in the rock’s pores. Two things keep it there:
- Capillary forces. At the narrow openings between pores, the interfacial tension between oil and water creates capillary pressure that traps oil droplets. The oil that remains is the residual oil saturation.
- Poor sweep. Injected water follows the easiest paths and bypasses parts of the reservoir, especially when the oil is much more viscous than water. See sweep efficiency.
EOR methods target one or both.
Main EOR methods
– Thermal EOR, mainly steam injection, heats heavy oil to reduce its viscosity so it flows. – Gas injection, most commonly carbon dioxide, which can mix with the oil, swell it and reduce its viscosity. Natural gas and nitrogen are also used. – Chemical EOR, which includes: – Polymer flooding, which thickens the injected water to improve sweep. – Surfactant flooding, which lowers interfacial tension, often to ultralow values, to release trapped oil. It is sometimes combined with alkali and polymer as ASP flooding. – Microbial enhanced oil recovery, which uses microorganisms or their products, including biosurfactants.
How surfactants mobilize trapped oil
Surfactants act on the forces that trap residual oil. By lowering interfacial tension, they raise the capillary number, the ratio of the forces pushing oil to the forces holding it, so trapped droplets can squeeze through pore throats. Some surfactants also shift the rock’s wettability toward water-wet, so that water can imbibe into the rock and displace oil from it. The main technical challenges are keeping surfactant from being lost by adsorption onto rock and keeping it stable in hot, salty reservoir brines.
EOR at Locus
Locus’s AssurEOR products are biosurfactant treatments for producing wells, which Locus describes as reducing interfacial tension and capillary pressure and restoring water-wet conditions to mobilize oil left after primary and secondary recovery.
Frequently asked questions
Secondary recovery maintains reservoir pressure and pushes oil toward producing wells, usually by injecting water. Tertiary recovery, or EOR, goes further by changing the properties of the oil or of the fluid-rock system, with heat, miscible gas or chemicals, to recover oil that waterflooding cannot move.
Thermal and gas injection methods account for most EOR production worldwide. In the United States, carbon dioxide injection is one of the most widely used methods.
EOR in shale and tight formations is an active area of development. Gas injection cycles and surfactant treatments that improve wettability and imbibition are among the approaches being applied, but they differ substantially from conventional reservoir floods.
Sources
- Enhanced Oil Recovery, U.S. Department of Energy, Office of Fossil Energy and Carbon Management
- enhanced oil recovery, SLB Energy Glossary
- Enhanced oil recovery, SPE PetroWiki