EOR stands for enhanced oil recovery, a set of techniques used to extract crude oil that conventional methods leave behind in underground reservoirs. After standard drilling and water flooding, roughly 60 to 80 percent of a reservoir’s oil typically stays trapped in the rock. EOR methods can push total recovery to 30 to 60 percent or more of the original oil in place, pulling out crude that would otherwise be permanently inaccessible.
Why Standard Methods Leave So Much Oil Behind
Oil production happens in stages. In the first stage, called primary recovery, natural underground pressure and pumps push oil up through the wellbore. This only gets about 10 percent of the oil out. In the second stage, operators inject water or gas to physically push more oil toward production wells, bringing total recovery to 20 to 40 percent.
That still leaves the majority of the oil locked in tiny pore spaces within the rock. The remaining crude is either too thick to flow, too tightly bound to rock surfaces by surface tension, or sitting in zones the injected water never reached. EOR is the third stage, designed specifically to change the physical or chemical properties of the oil or the reservoir to release what’s left.
Thermal Recovery: Using Heat
Thermal methods work by heating heavy, thick crude oil so it flows more easily. The most common approach is steam flooding, where steam is injected directly into the reservoir. The heat dramatically lowers the oil’s viscosity, turning what’s essentially tar-like sludge into something fluid enough to move through rock toward a production well. At higher temperatures, some lighter components of the oil actually vaporize and travel with the steam, creating a secondary recovery effect called distillation.
Thermal EOR is particularly useful for heavy oil deposits where the crude is simply too thick to respond to conventional water flooding. It’s energy-intensive, since generating and injecting steam requires significant fuel, but for the right reservoirs it unlocks oil that no other method can reach.
Gas Injection: CO2 and Other Gases
Gas injection is one of the most widely used EOR approaches, and CO2 flooding is the standout performer. When carbon dioxide is pumped into a reservoir at high enough pressure, it enters a supercritical state where it behaves like a gas but has the density of a liquid. In this form, CO2 blends thoroughly with crude oil, and the boundary tension between the two fluids drops to essentially zero. The oil swells in volume, becomes less viscous, and flows far more readily.
CO2 flooding outperforms other gas injection methods (like nitrogen or natural gas) because of how completely it mixes with crude oil. Three key processes happen simultaneously: the CO2 diffuses into the oil at a molecular level, it dissolves into the crude, and it disperses through the reservoir. The combined effect mobilizes oil that water flooding simply can’t touch. Operators can also use nitrogen or natural gas in reservoirs where CO2 isn’t available or practical, though recovery rates are generally lower.
Chemical Flooding: Surfactants, Polymers, and Alkalis
Chemical EOR uses carefully designed fluid mixtures to change how oil, water, and rock interact underground. The three main ingredients each play a distinct role.
- Surfactants reduce the surface tension between oil and water, much like dish soap breaks up grease. This allows water to make better contact with oil droplets and pull them free from rock surfaces. Surfactants can also create tiny oil-in-water emulsions, small enough to flow through pore spaces that larger oil droplets can’t pass through.
- Polymers thicken the injected water, creating a more viscous fluid front that sweeps evenly through the reservoir instead of channeling through the easiest paths. This forces fluid into previously bypassed zones where oil is still sitting. Polymer flooding typically recovers an additional 8 percent of the oil in place, at a cost of $8 to $16 per extra barrel.
- Alkaline agents react with naturally acidic compounds in crude oil to generate soap-like substances right inside the reservoir. These in-situ soaps mimic the effect of injected surfactants. Alkalis also act as sacrificial agents, preventing expensive surfactants from being absorbed by rock surfaces before they can do their job.
In practice, operators often combine all three in what’s called ASP flooding (alkaline-surfactant-polymer), where each component reinforces the others. The surfactants free trapped oil, the polymers ensure the fluid sweeps broadly, and the alkalis stretch the effectiveness of the surfactants further.
Microbial EOR: Using Biology
A newer category of EOR uses microorganisms instead of chemicals or heat. In microbial enhanced oil recovery, bacteria are either injected into the reservoir or stimulated to grow there by adding nutrients. These microbes produce useful byproducts: gases that repressurize the formation, biosurfactants that lower surface tension, and acids that dissolve rock to open flow paths.
The appeal is that microbial EOR is cheaper and more environmentally friendly than chemical or thermal methods. Based on hundreds of field trials worldwide, the technology has shown a success rate of roughly 80 percent. However, biological systems are harder to control than chemical ones, and scaling the technology to full industrial production remains a challenge. Much of the recent development work has happened in China, where field applications have been most extensive.
The Cost of Getting That Extra Oil
EOR is expensive compared to primary and secondary recovery. For CO2 flooding, the cost of the carbon dioxide alone adds $20 to $30 per barrel of oil produced. On top of that, operators need surface facilities to separate CO2 from the oil once it reaches the surface, compressors to reinject the gas, and time to repressurize aging reservoirs before production even begins. Each of these adds cost and delays revenue.
This means EOR projects live or die by oil prices. When crude is trading high, the economics work and operators invest in tertiary recovery. When prices drop, projects get shelved. The decision to pursue EOR is always a calculation: is the price of oil high enough, and expected to stay high enough, to justify the extra spending?
CO2-EOR and Carbon Storage
One of the more interesting developments in EOR is its overlap with climate goals. When CO2 is injected for oil recovery, more than 95 percent of the carbon dioxide used can be permanently stored in the reservoir. After the field stops producing, the site can be repurposed as a dedicated carbon storage facility.
This connection to carbon capture and storage (CCS) changes the economics and politics of CO2-EOR. If the carbon dioxide comes from industrial smokestacks or power plants, the process effectively buries emissions underground while producing oil. The net carbon balance depends on several factors: how much CO2 is stored per barrel produced, where the CO2 comes from, and how the produced oil is ultimately used. Lifecycle analyses show that CO2-EOR paired with carbon storage has a meaningfully lower carbon footprint than other EOR methods, though it still results in fossil fuel production.
For oil-producing countries looking to reduce emissions while maintaining output, CO2-EOR with storage offers a bridge. It keeps existing infrastructure productive while creating a financial incentive to capture carbon that would otherwise enter the atmosphere.