---
title: "Enhanced Oil Recovery | Earth Systems Science"
description: "Enhanced oil recovery uses steam, gas, or chemicals to extract more crude oil after primary and secondary recovery, linking geology, energy, and sustainability."
canonical: "https://fiveable.me/earth-systems-science/key-terms/enhanced-oil-recovery"
type: "key-term"
subject: "Earth Systems Science"
unit: "Unit 20"
---

# Enhanced Oil Recovery | Earth Systems Science

## Definition

Enhanced oil recovery (EOR) is a set of methods used in Earth Systems Science to extract more crude oil from a reservoir after primary and secondary recovery. It usually involves steam, gas, or chemicals to move oil that is still trapped underground.

## What It Is

Enhanced oil recovery, or EOR, is the stage of oil extraction that comes after a well can no longer produce much oil on its own or with simple water flooding. In Earth Systems Science, it is studied as a human intervention in the geosphere that changes how fluids move through rock and how energy resources are managed.

Primary recovery uses the reservoir’s own pressure to push oil to the surface. Secondary recovery usually adds water or gas to keep pressure up and sweep more oil toward the well. EOR goes further by changing the physical properties of the reservoir, the oil, or both, so that oil left behind in tiny pore spaces can move more easily.

There are three broad EOR approaches. Thermal recovery injects steam or hot water to heat thick oil, which lowers viscosity and lets it flow. Gas injection, often using carbon dioxide, natural gas, or other gases, helps maintain pressure and can mix with the oil so it moves more easily. Chemical methods use surfactants or polymers to reduce surface tension, alter flow behavior, or improve how the injected fluid pushes oil through the rock.

The reason EOR works is that a reservoir is not an open tank. Oil sits inside porous rock, and some of it gets trapped by capillary forces, viscosity, and uneven flow paths. Even after a field has been developed, a lot of oil can remain underground because it is hard to dislodge from pore spaces. EOR targets that leftover oil instead of drilling a brand-new field.

Earth Systems Science connects EOR to more than just engineering. Steam injection uses large amounts of heat and water. Gas injection can involve carbon dioxide, which links EOR to carbon capture and storage when captured CO2 is put into oil reservoirs. That makes EOR a good example of how resource extraction, atmospheric chemistry, and geologic systems can overlap in one process.

## Why It Matters

EOR matters in Earth Systems Science because it shows how humans alter the geosphere to extend fossil fuel production, not just discover new deposits. The term comes up when you are comparing ways energy is extracted from Earth materials and when you are tracing how geology affects technology choices.

It also connects to sustainability tradeoffs. A field that uses EOR can produce more oil from an existing reservoir, which may reduce the need to open a new site. At the same time, the process can require more energy, more water, and more infrastructure. That makes EOR a good case study for weighing resource efficiency against environmental cost.

This term also sits right next to climate system topics. Some gas injection projects use captured carbon dioxide, so the same reservoir can be part of oil production and carbon storage discussion. That overlap makes EOR useful for thinking about how Earth systems are coupled, especially when one intervention affects both the subsurface and the atmosphere.

## Connections

### Primary Recovery

Primary recovery is the first phase of oil extraction, when natural reservoir pressure and simple well flow bring oil to the surface. EOR comes after that stage, when much of the easily accessible oil is already gone. If you know primary recovery, you can see why EOR exists at all, because it tries to recover the oil that primary methods leave behind.

### Secondary Recovery

Secondary recovery usually means injecting water or gas to maintain pressure and sweep more oil toward production wells. EOR is different because it changes the reservoir conditions more aggressively, such as heating the oil or changing fluid chemistry. In practice, secondary recovery often sets the stage for EOR by removing the easiest remaining oil first.

### Carbon Capture and Storage (CCS)

Some EOR projects use captured carbon dioxide as the injected gas, which links oil recovery to carbon management. That does not automatically make the process low-carbon, but it does show how CCS can connect to subsurface geology. In a class discussion, this relationship often comes up when comparing climate mitigation with continued fossil fuel extraction.

### [geological carbon sequestration](/earth-systems-science/key-terms/geological-carbon-sequestration)

Geological carbon sequestration stores carbon dioxide underground in rock formations, often in depleted reservoirs or deep saline aquifers. EOR can use similar subsurface spaces and similar injection technology, but the purpose is different: one aims to store CO2, the other aims to mobilize oil. The overlap is useful for understanding how the same geology can serve different human goals.

## On the AP Exam

A quiz or short-response question might ask you to explain why a reservoir still contains oil after pumping slows down. Your job is to connect that leftover oil to pore spaces, viscosity, and pressure, then identify EOR as the next step that makes the oil easier to move. If the prompt gives you a diagram of injection wells, look for steam, gas, or chemicals and explain what each one changes in the reservoir. On a case study, you may need to compare EOR with primary or secondary recovery and describe the tradeoff between getting more energy from the same field and increasing the environmental footprint. If carbon dioxide is mentioned, connect the process to carbon capture and storage, not just oil production.

## Enhanced oil recovery vs Secondary Recovery

Secondary recovery and enhanced oil recovery both use injection, so they get mixed up a lot. Secondary recovery mainly maintains reservoir pressure and sweeps oil toward the well, while EOR changes the oil or the rock more directly by heating, altering chemistry, or using miscible gases. If the method is just water flooding or basic pressure support, it is usually secondary recovery, not EOR.

## Key Takeaways

- Enhanced oil recovery is the phase of oil extraction that tries to get more crude oil out after primary and secondary recovery have already removed the easiest supply.
- EOR works by changing pressure, temperature, or fluid chemistry inside the reservoir so oil trapped in small pore spaces can move more easily.
- Thermal, gas injection, and chemical injection are the main EOR approaches, and each one fits different reservoir conditions.
- In Earth Systems Science, EOR is a good example of how human energy use interacts with geologic systems, water use, and carbon management.
- EOR can increase production, but it also raises questions about resource efficiency, environmental impact, and whether underground CO2 is being stored or only used to extract more oil.

## FAQs

### What is enhanced oil recovery in Earth Systems Science?

Enhanced oil recovery is a set of methods for getting more oil out of a reservoir after natural pressure and basic injection methods are no longer enough. It uses steam, gas, or chemicals to move oil that is still trapped in the rock. In Earth Systems Science, it is studied as a human intervention in the subsurface.

### How is enhanced oil recovery different from secondary recovery?

Secondary recovery usually keeps pressure up and pushes oil toward production wells with water or gas. Enhanced oil recovery goes further by changing the oil or the rock itself, such as heating the oil or altering its flow behavior. If a method only replaces pressure support, it is usually secondary recovery.

### Why does enhanced oil recovery use carbon dioxide?

Carbon dioxide can help oil flow by maintaining pressure and, in some reservoirs, mixing with the oil so it moves more easily. That is why some EOR projects are connected to carbon capture and storage. The same gas can sometimes be used for both oil recovery and underground storage, even though the goals are different.

### What are the main types of enhanced oil recovery?

The main types are thermal methods, gas injection, and chemical injection. Thermal methods heat thick oil with steam, gas injection uses gases like carbon dioxide, and chemical injection uses surfactants or polymers to change fluid behavior. Which one works best depends on the reservoir’s temperature, depth, and rock properties.

## Related Study Guides

- [20.3 Geoengineering and Earth system interventions](/earth-systems-science/unit-20/geoengineering-earth-system-interventions/study-guide/c9jjhNQfBWMI2lLe)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
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