Enhanced oil recovery (EOR) refers to a set of techniques used to extract oil that conventional drilling and pumping methods leave behind in a reservoir. After an oil well’s natural pressure drops and basic water injection (known as secondary recovery) runs its course, roughly two-thirds of the original oil typically remains trapped underground. EOR methods can push total recovery to 30 to 60 percent or more of a reservoir’s original oil in place, according to the U.S. Department of Energy.
Why Standard Methods Leave Oil Behind
When a well is first drilled, natural underground pressure pushes oil to the surface. This is called primary recovery, and it usually captures only about 10 to 15 percent of the oil in the reservoir. Operators then move to secondary recovery, most often by injecting water into the formation to maintain pressure and sweep oil toward production wells. Even after this stage, a large share of oil stays locked in the rock’s tiny pore spaces, held there by friction, viscosity, and surface tension.
EOR is sometimes called tertiary recovery because it comes after those first two stages. Each EOR method works by changing the physical or chemical properties of the oil, the rock, or both, making it easier for trapped oil to flow. The three commercially proven categories are thermal recovery, gas injection, and chemical injection.
Thermal Recovery
Thermal recovery lowers oil’s viscosity (its resistance to flow) by introducing heat into the reservoir, most commonly by injecting steam. Think of it like warming up cold honey so it pours more easily. Once the heavy, thick oil heats up, it moves through the rock toward production wells far more readily than it would at reservoir temperature.
Steam injection is the most widely used form of thermal EOR. In a typical operation, high-pressure steam is pumped down an injection well, heating the surrounding formation. The process works especially well in reservoirs containing heavy crude oil that is too thick to flow on its own at natural temperatures. Some operations use a cyclic approach, alternating between injecting steam and producing oil from the same well, while others maintain continuous steam injection through dedicated wells.
Gas Injection
Gas injection works in two ways depending on the gas used and reservoir conditions. Injected gas can physically expand inside the reservoir to push oil toward production wells, or it can dissolve into the oil itself, thinning it out and helping it flow. The most common gases used are carbon dioxide (CO2), natural gas, and nitrogen.
CO2 injection is particularly effective and has become the most commercially significant form of gas EOR. When CO2 mixes with oil under the right pressure and temperature, it becomes “miscible,” meaning it blends with the oil into a single fluid phase. This dramatically reduces the oil’s viscosity and allows it to slip free of the rock pores where it was stuck. In cases where full miscibility isn’t achieved, the CO2 still swells the oil and reduces its thickness enough to improve flow.
CO2-based EOR also has a climate angle. The CO2 injected underground can remain permanently stored in the geological formation, a process known as carbon sequestration. Federal tax credits exist to incentivize this kind of geological carbon storage, making CO2 injection financially attractive beyond just the value of the recovered oil.
Chemical Injection
Chemical EOR uses specially designed substances mixed with water to overcome the forces trapping oil in rock. Two main types of chemicals are used: polymers and surfactants.
Polymers are long-chained molecules that thicken the injected water. Normal water injected during secondary recovery often flows through the easiest paths in the rock and bypasses pockets of oil. Polymer-thickened water moves more evenly through the formation, sweeping oil out of areas that plain water would miss. This doesn’t change the oil itself but makes the water push far more effective.
Surfactants work like detergents. Oil droplets in a reservoir cling to rock surfaces because of surface tension, much like a grease spot sticks to a countertop. Surfactants lower that surface tension, releasing the oil droplets so they can be carried along by injected fluids. Some operations combine surfactants, polymers, and an alkaline solution in what’s called an ASP flood, attacking multiple trapping mechanisms at once.
Chemical EOR tends to be more expensive per barrel than thermal or gas methods because the chemicals themselves are costly and must be carefully matched to each reservoir’s specific rock and fluid chemistry. However, it can unlock oil in reservoirs where heat or gas injection isn’t practical.
How Operators Choose a Method
The right EOR technique depends almost entirely on the reservoir’s characteristics. Thermal recovery is the go-to choice for heavy oil in shallow formations where steam can effectively heat the rock. Gas injection, particularly CO2, works best in lighter oil reservoirs at depths where pressure is high enough for the gas to become miscible with the oil. Chemical injection is typically considered for reservoirs with moderate temperatures where the chemicals remain stable and effective.
Operators also weigh infrastructure costs. Steam generation requires significant energy input and water supply. CO2 injection needs a reliable source of carbon dioxide, whether from natural underground deposits, industrial facilities, or direct air capture plants, plus a pipeline network to deliver it. Chemical floods require ongoing purchases of specialty chemicals and careful monitoring to ensure the injected mixture performs as designed underground.
Scale and Economic Impact
EOR projects operate across the United States and globally, with CO2 injection being particularly widespread in oil-producing regions with access to natural CO2 sources or industrial emissions. Thousands of EOR projects have been deployed worldwide, and the techniques collectively account for a meaningful share of domestic oil production.
The economics of EOR depend on oil prices, the cost of injectants (steam, CO2, chemicals), and how much additional oil a given reservoir will yield. When oil prices are high, projects that were marginally profitable become attractive. When prices drop, operators may scale back injection rates or pause projects entirely. The upfront investment for an EOR project can run into hundreds of millions of dollars for large-scale operations, but the payoff is access to billions of barrels of oil that would otherwise stay permanently underground.
Newer Approaches Under Development
Beyond the three established categories, researchers are exploring less conventional EOR techniques. Microbial EOR uses bacteria injected into reservoirs to produce gases, solvents, or biosurfactants that help mobilize trapped oil. The appeal is low cost and minimal environmental impact, though scaling the technique to full field operations has proven challenging.
Plasma pulse stimulation is an emerging technology being tested at the field trial stage. Instead of pumping large volumes of fluid underground, this approach generates ultra-fast plasma discharges within a specialized fluid to create shock pulses that fracture rock and open new flow paths. Researchers at the University of Houston have demonstrated that these pulses can create complex fracture networks in carbonates, sandstones, and shales while using drastically less water and fewer chemicals than conventional stimulation methods. The technology remains in early trials and is not yet commercially available, but it represents a direction that could eventually reduce EOR’s environmental footprint.
Nanoparticle-enhanced fluids are another area of active research. Tiny engineered particles can alter the wettability of rock surfaces (how easily oil releases from them) or stabilize foams used in gas injection, potentially improving recovery rates beyond what current methods achieve.