# Enhanced Oil Recovery (EOR)

**Source:** Buckhead Energy Oil & Gas Encyclopedia (https://www.buckheadenergy.com/learn) — canonical: https://www.buckheadenergy.com/enhanced-oil-recovery

Enhanced oil recovery (EOR) refers to techniques — such as waterflooding, gas or carbon-dioxide injection, and thermal or chemical methods — used to produce oil that remains in a reservoir after primary production has depleted its natural pressure, often extending the life of a mature field and, with it, the royalties it pays.

## Getting the oil that primary production leaves behind
Primary production — letting a reservoir's own pressure push oil to the surface — typically recovers only a fraction of the oil in place. A great deal remains trapped in the rock after the pressure fades. Enhanced oil recovery is the set of methods used to coax out more of that remaining oil, extending a field far beyond what primary production alone would yield.
EOR is usually described in stages beyond primary: secondary recovery, most commonly waterflooding or gas injection to re-pressure the reservoir and sweep oil toward producing wells; and tertiary recovery, more advanced methods like CO2 injection, chemical (polymer) flooding, or thermal (steam) techniques that change how the oil flows.

## Why old fields come back to life
EOR is why a field that seemed played out can produce for decades more. Classic examples are large legacy oil fields revived by waterfloods and CO2 floods that recover substantial additional oil long after primary production tailed off. For the minerals under such a field, that can mean royalties continuing — or even rising — well past when a purely primary decline would have ended them.
This is one reason a mature field is not automatically a dead one: an EOR project can reset the outlook for the whole reservoir.

## EOR usually needs unitization
Because these methods work on a whole reservoir — injecting fluids in one place to push oil toward wells elsewhere — they generally require the field to be operated as a unit. That is where unitization comes in: combining the many tracts over the reservoir into one unit so an EOR flood can be run coherently, with production shared by each owner's participation factor.
So EOR and unitization tend to go together: the recovery technique needs unified operation, and unitization provides the framework to share the results fairly.

## What it means for owners
For a mineral or royalty owner over a mature field, an EOR project is usually good news — it can extend and sometimes increase the production your royalty is paid on. It may also come with a unitization proposal that changes how your share is calculated (to a unit participation factor). Understanding both is useful when a field you own under is being flooded or unitized.
Buckhead Energy factors EOR potential and unit participation into how it values minerals over mature fields, since a field's second life can be substantial. This page is educational information, not financial advice.

## Frequently asked questions

**What is enhanced oil recovery?**
Techniques — such as waterflooding, gas or CO2 injection, and thermal or chemical methods — used to produce oil that remains in a reservoir after primary production has depleted its natural pressure. EOR often extends the life of a mature field and the royalties it pays.

**What are secondary and tertiary recovery?**
Secondary recovery, most commonly waterflooding or gas injection, re-pressures the reservoir to sweep oil toward wells. Tertiary recovery uses more advanced methods — CO2 injection, chemical (polymer) flooding, or thermal (steam) techniques — to recover still more oil by changing how it flows.

**Can enhanced oil recovery revive an old field?**
Yes. EOR is why fields that seemed played out can produce for decades more. Waterfloods and CO2 floods can recover substantial additional oil long after primary production tailed off, which can extend or even increase royalties for owners over the field.

**Why does enhanced oil recovery require unitization?**
Because EOR works on a whole reservoir — injecting fluids in one place to push oil toward wells elsewhere — the field generally must be operated as a unit. Unitization combines the tracts over the reservoir so the flood can be run coherently, with production shared by each owner's participation factor.

**Is enhanced oil recovery good for mineral owners?**
Usually yes. For minerals over a mature field, an EOR project can extend and sometimes increase the production your royalty is paid on. It may also come with a unitization proposal that changes how your share is calculated, to a unit participation factor.

## Related terms
- [Unitization](https://www.buckheadenergy.com/unitization)
- [Oil and Gas Production](https://www.buckheadenergy.com/oil-and-gas-production)
- [Decline Curve](https://www.buckheadenergy.com/decline-curve)
- [When a Well Stops Producing](https://www.buckheadenergy.com/what-happens-to-royalties-when-a-well-stops-producing)
- [Oil & Gas Encyclopedia — all terms](https://www.buckheadenergy.com/learn)

_Educational information only, not legal or tax advice. Buckhead Energy is a direct buyer of oil & gas mineral and royalty interests._