Enhanced oil recovery (EOR) is a set of techniques used to extract crude oil that remains trapped in a reservoir after conventional methods have already taken what they can. Conventional drilling and water flooding typically leave 60% to 70% of the oil still underground. EOR methods can pull out an additional 5% to 15% of the original oil in place, extending the productive life of fields that would otherwise be abandoned.
Why Standard Methods Leave Oil Behind
Oil production happens in stages. During primary recovery, the natural pressure inside a reservoir pushes oil to the surface on its own. As that pressure drops, production slows, and operators move to secondary recovery, which usually means injecting water or natural gas to restore pressure and push more oil toward production wells. Water flooding is cheap and effective, especially offshore where seawater is readily available.
Even after secondary recovery, the majority of oil stays locked in the rock. Some of it clings to pore surfaces because of the way oil and water interact at a molecular level. Some sits in pockets the injected water never reached. EOR targets this leftover oil using heat, specialized gases, chemicals, or even microbes to change the physical properties of the oil or the reservoir itself.
Gas Injection
Gas injection is the most widely used EOR approach. Carbon dioxide is the preferred gas because it’s relatively inexpensive, requires lower pressure to work effectively, and doubles as a way to store a greenhouse gas underground. Nitrogen and natural gas are also used depending on reservoir conditions.
The key distinction is whether the injected gas actually dissolves into the oil. In miscible injection, CO2 mixes completely with crude oil under high enough pressure, reducing the surface tension between oil and rock to nearly zero. The two fluids essentially become one, making the combined liquid thin enough to flow freely through tiny pores. Recovery from miscible CO2 flooding typically adds 4% to 12% of the original oil in place.
When reservoir pressure is too low or the oil is too heavy for full mixing, operators use immiscible injection instead. The CO2 doesn’t dissolve into the oil, but it causes the oil to swell and become less viscous, making it easier to move. Immiscible flooding can actually recover up to 18% of the original oil in place in heavy oil reservoirs, because the starting point for recovery is so much lower.
Thermal Recovery
Thermal methods are designed for heavy, tar-like crude that barely flows at reservoir temperature. The basic idea is straightforward: heat the oil until it thins out enough to move.
In cyclic steam stimulation (CSS), steam is injected into a well, left to soak for days or weeks, then the same well is used to pump out the now-thinned oil. This cycle repeats until returns diminish. Steam-assisted gravity drainage (SAGD) takes a different approach, using two horizontal wells stacked one above the other. Steam flows continuously into the upper well, creating a growing chamber of heat. Softened oil drains by gravity into the lower well, where it’s collected. SAGD generally recovers more oil than CSS because the steam chamber expands more effectively through the reservoir over time.
Chemical Injection
Chemical EOR uses solutions of polymers, surfactants, and alkaline agents, either alone or combined, to coax trapped oil out of rock.
Polymers thicken the injected water, creating a more uniform wall of fluid that sweeps through the reservoir instead of finding the path of least resistance and leaving oil behind. By raising the viscosity of the water closer to that of the oil, polymers force the displacing fluid into low-permeability zones that plain water would skip entirely. Polymer flooding typically recovers about 8% of the original oil in place, at an added cost of $8 to $16 per incremental barrel.
Surfactants work at a smaller scale. They reduce the tension between oil and water, loosening oil droplets that cling to rock surfaces. Surfactants can also create tiny emulsions, dispersing oil into droplets small enough to slip through pore spaces. Larger droplets, interestingly, can plug high-flow channels and redirect the flood into unswept areas, improving overall coverage.
Alkaline agents are often added alongside surfactants. They react with naturally acidic compounds in crude oil to generate soap-like substances in the reservoir itself, amplifying the surfactant effect. Alkalis also act as sacrificial agents, preventing expensive surfactants from being absorbed by the rock before they can do their job. When all three chemicals are combined (alkaline-surfactant-polymer, or ASP flooding), the result is improved performance at both the microscopic and reservoir-wide scale.
Microbial Recovery
Microbial enhanced oil recovery (MEOR) uses bacteria and their metabolic byproducts to mobilize oil. Microbes injected into a reservoir can produce gases, solvents, and natural surfactants that reduce oil viscosity and alter how fluids move through rock. Some bacteria selectively plug high-permeability channels, redirecting flow toward untouched oil. Others break down waxy deposits that restrict flow near the wellbore.
MEOR is attractive because it’s relatively cheap and environmentally low-impact compared to injecting industrial chemicals or large volumes of CO2. Field trials have reported a success rate of roughly 80%, though the technique works best in shallower, lower-temperature reservoirs where the microbes can survive and reproduce.
CO2 Storage as a Side Benefit
CO2-based EOR has an unusual environmental dimension. During operations, injected CO2 cycles back and forth: some breaks through into production wells and is captured, separated, and reinjected. At any given point during active flooding, about 50% of the CO2 is retained in the reservoir while the rest circulates through the recycling loop.
The more meaningful number comes at the end of a project’s life. When a CO2-EOR operation is completed, more than 95% of all purchased CO2 remains permanently stored in the geological formation. This has made CO2-EOR a practical bridge between the oil industry and carbon capture goals. The oil revenue helps offset the cost of capturing and transporting CO2, while the geology that trapped oil for millions of years now traps carbon dioxide.
Cost and Practical Challenges
EOR is inherently more expensive than conventional production. Every method requires additional infrastructure, whether that’s steam generators, CO2 pipelines, chemical mixing facilities, or specialized injection wells. These techniques tend to be economical only in larger fields and when oil prices are high enough to justify the investment.
One of the biggest practical hurdles is the time lag between spending money and seeing results. Capital goes in years before incremental oil shows up at the surface. Polymer flooding at its best can cost as little as $2.42 per incremental barrel in favorable onshore fields, but chemical and thermal methods in more challenging reservoirs cost significantly more, and the payoff timeline stretches longer.
Water management adds another layer of complexity. EOR operations produce enormous volumes of water mixed with oil and chemicals. This produced water must be treated before it can be reinjected or discharged. Advanced treatment processes can recover and reuse 70% to 80% of the water, but the infrastructure required is substantial. In some fields, daily water requirements reach millions of cubic meters, making efficient recycling essential to both economics and environmental compliance.
How Much More Oil EOR Actually Delivers
The practical ceiling depends on the reservoir and the method. For light to medium crude, EOR typically adds 5% to 15% of the original oil in place beyond what primary and secondary recovery achieved. Heavy oil reservoirs generally land at the lower end of that range because the oil is harder to mobilize even with advanced techniques. Across the global oil industry, EOR represents a significant opportunity: even a few percentage points of additional recovery from existing fields translates to billions of barrels that don’t require discovering and developing new reserves.