Enhanced Oil Recovery (EOR) is a crucial component of modern petroleum production, aimed at maximizing the extraction of oil from reservoirs, particularly those in the later stages of production. As conventional oil fields mature, their production rates decline, making it imperative to employ advanced techniques to extract the remaining resources.

In this post, we’ll explore the fundamentals of EOR, including its significance in the energy industry, the various methods and techniques used, and the advantages it offers in optimizing oil recovery.

Let’s dive into the world of EOR and understand how it contributes to a more sustainable and efficient oil production process.

Getting started

Enhanced oil recovery is oil recovery by the injection of materials not normally present in the reservoir.

EOR is different from hydraulic fracturing, or “fracking,” the much-better-known practice of pumping high-pressure fluids underground to release more oil and gas. Fracking forces open new fissures in the rock, while EOR “scrubs” existing channels.

Oil is the dominant source of energy used worldwide; therefore, the application of enhanced oil recovery (EOR) methods on mature oil fields is mandatory. EOR methods are classified into three main categories: gas methods, chemical methods, and thermal methods.

Although all EOR methods activate some mechanisms leading to increased oil recovery, they are not suitable for all types of reservoirs and rock/fluid systems; therefore, screening criteria are defined for EOR methods based on successful experiences of applying EOR methods in oil fields with different fluid and rock properties.

Enhanced oil recovery (EOR) methods are also classified into five general categories:

Mobility control

The mobility ratio is a single term that describes the rate and efficiency of oil displacement by fluids that don’t mix (i.e.: oil & water). A mobility ratio of less than one is generally considered as favorable values, and values greater than one are considered not as favorable.


Processes such as steam stimulation, hot water injection, heat applied via ISC (In-situ Combustion), hot water, steam flooding, and steam-assisted gravity drainage (SAGD) are the most prominent thermal EOR techniques.


This EOR method relies on the injection of chemicals to boost oil recovery.  Conventional EOR chemicals are polymers, alkalis, and surfactants.  Polymers increase the viscosity of oil while reducing permeability to water in the reservoir.

Alkali reacts with crude oil in such a way that can lower water−crude oil interfacial tension (IFT), emulsify oil and water, change rock wettability, and solubilize interfacial films, all of which may lead to increased oil recovery.

Surfactant flooding is a well-established method of chemical EOR. This method has proven successful as it increases oil recovery through a combination of mechanisms. These include interfacial tension (IFT) reduction, wettability alteration, foam generation, and emulsification.


Miscible gas injection techniques involve the injection of a gas, such as CO2, CH4, or N2 into an oil reservoir. The injected gas works by immersing itself in the crude oil and forming a miscible mixture with the crude.

This process is beneficial as it increases pressure and decreases viscosity to promote improved flowability of the oil. Miscible gas injection involves a homogenous mixture of fluids where the fluids dissolve completely with other fluids.

Microbial-enhanced oil recovery (MEOR)

A comprehensive investigation of reservoir production mechanisms, fluid, and reservoir rock properties can lead to the design of efficient EOR processes. One of the potential processes is microbial-enhanced oil recovery (MEOR).

MEOR involves the use of bacteria, fungi, and other microbes in specialized formulations to mobilize oil and improve recovery from otherwise impossible or expensive scenarios.

Types of EOR

Oil production methods from oil fields are mainly divided into three categories:

Primary recovery

Natural forces (underground pressure) are sufficient to push oil to the surface.  Reservoir pressure is the key driver pushing the oil (permeability) from the pores (porosity) towards the well and to the surface. As oil is extracted, reservoir pressure drops and eventually will be too weak to continue to recover oil.

Secondary production

Man-made forces are being used for the first time to assist in getting oil to the surface.  Generally, this is done by injecting water or gas into the formation, increasing the formation pressure, and getting the oil moving through the pores again – or at a faster pace.

Tertiary recovery

Follow up to secondary recovery with different methods. It’s in Tertiary recovery that the phrase Enhanced Oil Recovery (EOR), comes into play. Thermal (superheated steam), gas injection, and water or chemical flooding.  At some point, any remaining oil is too difficult & expensive to produce.  The well has reached its end of life and is sealed.

Enhanced oil recovery techniques


Waterflooding is the most practiced method to produce oil in conventional reservoirs.  Injecting water helps to increase declining natural underground pressure and “sweep” the oil toward production wells

Gas injection

Gas flooding is the injection of hydrocarbon or non hydrocarbon components into oil reservoirs that are typically waterflooded to residual oil. Injecting gasses such as CO2 or natural gas to increase declining natural underground pressure to enhance oil production.

Chemical flooding

Chemical flooding involves injecting chemicals such as surfactants or polymers to reduce interfacial tension or increase the viscosity of the oil. Surfactant EOR improves the wettability of porous rocks allowing water to flow through them faster displacing more oil. Polymer is less effective on low-permeability rock structures.

Thermal methods

Thermal methods include the introduction of heat such as the injection of steam to lower the viscosity, 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.

Microbial EOR

Microbial EOR uses microorganisms to enhance oil recovery by altering the reservoir conditions.

MEOR (microbial enhanced oil recovery), a promising tertiary oil recovery method involves the utilization of native microbial species capable of producing various secondary metabolites which further enhances the recovery of oil through their plugging, viscosity, or interfacial tension reduction mechanisms.

The bacteria Clostridium produces biosurfactant that alters rock–fluid properties and increases oil detachment.

Main Chemical EOR Techniques

  • Surfactant Flooding boosts oil production by lowering interfacial tension, increasing oil mobility thus allowing better displacement of the oil by injected water. Surfactant EOR improves the wettability of porous rocks allowing water to flow through them faster displacing more oil.
  • Polymer Displacement increases the viscosity of water injected into the oil reservoir enabling it to exert more pressure on the oil without forcing its way past and simply flowing through. Because this method relies on increasing the viscosity of water it is less effective on low-permeability rock structures.
  • Alkaline Displacement relies on the chemical interaction of alkali, oil, and rock. When introduced to an oil field the alkaline agent reacts with the oil, forming surfactants that reduce interfacial tension. This allows oil to pass through porous rock more effectively.
  • Microbiological Treatment introduces specific microorganisms to an oil field which metabolizes some of the hydrocarbons, in turn producing byproducts that assist in oil recovery. These bi-products include solvents, acids, alcohols, bio-polymers, bio-surfactants, and gasses.

Advantages of Surfactant Flooding in Enhanced Oil Recovery

Recent developments in Surfactant Enhanced Oil Recovery have greatly reduced the surfactant concentration required for effective oil recovery. These new advances have actually come with an overall improvement for the environment and human health.

Some of the newest and most effective EOR surfactants are derived from plant resources such as sunflower oil, soy, and corn oil. A combination of thermal, gas, chemical, hydrodynamic, and other EOR methods are typically utilized to optimize recovery for any given oil reservoir’s characteristics.

One of the significant challenges that oil companies face during oil production is the low recovery factor (RF) of mature oil fields. A mature oilfield is one where production has reached its peak and has started to decline.

The average RF from mature oil fields around the world is between 20% and 40%. Operators have attempted several enhanced oil recovery (EOR) techniques that offer prospects for ultimately improving the recovery factor and accelerating oil production.


Enhanced Oil Recovery (EOR) is the application of various techniques to increase the amount of oil produced from a reservoir. Thermal, chemical, and microbial methods are among the most popular EOR technologies used today.

Each technique has its own advantages, drawbacks, and cost-effectiveness that make them suitable for different types of reservoirs under certain conditions. The combination of several techniques is often used to optimize the recovery of oil from a reservoir.

21st Century Energy Solutions Inc. is a leading provider of EOR services, providing a wide range of innovative and advanced technologies to maximize oil recovery. Our team of experts is committed to helping operators achieve their production goals through the use of cutting-edge technologies.

Contact us to learn more about how we can help you maximize oil recovery and improve production.