enhanced oil recovery, Oil & Gas Glossary

What Is Enhanced Oil Recovery?

Enhanced oil recovery (also called EOR, tertiary recovery, or improved oil recovery) is a broad category of techniques applied to oil reservoirs after primary (pressure depletion) and secondary (waterflooding) recovery to extract additional crude oil that cannot be produced by conventional means. EOR methods work by reducing oil viscosity, altering rock wettability, improving areal and vertical sweep efficiency, or reducing interfacial tension between oil and formation water. Primary and secondary recovery typically recover 25-40% of the original oil in place (OOIP); EOR is estimated to recover an additional 30-60% of OOIP that conventional methods leave behind.

Key Takeaways

EOR is divided into three main categories: thermal methods (steam injection, in-situ combustion, SAGD), chemical methods (polymer, surfactant, alkaline flooding and combinations), and miscible/immiscible gas injection (CO2, hydrocarbon gas, nitrogen).
EOR produces approximately 3 million barrels per day globally — roughly 3% of world oil production — with thermal EOR dominating due to the massive heavy oil resources in Canada (SAGD) and California (cyclic steam).
Screening criteria differ sharply by method: thermal EOR requires viscous oil at shallow depth; CO2 miscible flooding requires light oil at depth below the minimum miscibility pressure; polymer flooding requires moderate permeability and moderate viscosity oil.
CO2 EOR holds particular strategic importance because injected CO2 is partially retained in the reservoir as a byproduct of the flooding process, enabling simultaneous hydrocarbon production and carbon sequestration.
EOR projects typically produce oil at higher lifting costs than primary or secondary methods, and their economics are highly sensitive to oil price; many EOR pilots that were uneconomic at $50/bbl become attractive at $70-80/bbl.

How Enhanced Oil Recovery Works

Primary recovery exploits natural reservoir energy — fluid expansion, dissolved gas drive, gravity drainage, and aquifer support — to push oil to the wellbore. As reservoir pressure declines, production rates fall and eventually become uneconomic. Secondary recovery, most commonly waterflooding, injects water to maintain reservoir pressure and physically displace oil toward producing wells. Waterflooding can be highly effective in homogeneous, water-wet reservoirs but is limited by sweep efficiency: water preferentially invades high-permeability streaks, channels through the reservoir, and bypasses significant volumes of oil trapped by capillary forces in low-permeability pores or isolated by heterogeneity. By the end of a waterflood, the remaining oil saturation in swept zones is typically 25-35% of pore volume — oil held by surface tension that water pressure alone cannot mobilize.

EOR addresses these limitations through fundamentally different displacement mechanisms. Thermal methods reduce oil viscosity by heating the reservoir: heavy oils that are essentially immobile at reservoir temperature flow readily at 200-350°C. Chemical methods reduce the interfacial tension between oil and water (surfactants), increase water viscosity to improve mobility ratio (polymers), or generate surfactants in situ by reacting with naphthenic acids in the crude (alkaline flooding). Miscible gas flooding — primarily CO2 — achieves miscibility with the crude oil under sufficient pressure, eliminating interfacial tension entirely and allowing the CO2-oil mixture to flow as a single phase toward producing wells. Each mechanism addresses a different physical barrier to oil mobilization, and the choice of EOR method is governed by matching the method's mechanism to the reservoir's specific characteristics.

The distinction between EOR and the related term IOR (improved oil recovery) reflects different scopes: EOR specifically refers to methods that change the fluid-rock interaction physics (thermal, chemical, miscible), whereas IOR is a broader term that includes EOR plus reservoir management improvements such as infill drilling, horizontal wells, improved waterflood pattern optimization, and production logging to shut off water channels. Tertiary recovery was an older synonym for EOR that fell from favor because EOR techniques are increasingly applied before or concurrent with secondary methods rather than strictly afterward.

Fast Facts: Enhanced Oil Recovery

Global EOR production: Approximately 3 million bbl/day (roughly 3% of global oil output)
Additional recovery potential: 30-60% of OOIP estimated recoverable by EOR vs. 25-40% by primary + secondary alone
Dominant EOR method by volume: Thermal (steam) — accounts for over 50% of global EOR production
CO2 EOR in the U.S.: Approximately 300,000 bbl/day, primarily Permian Basin and Gulf Coast fields
SAGD production: Canada's oil sands produce approximately 1.5 million bbl/day via SAGD and cyclic steam stimulation
Polymer flooding: Largest chemical EOR method by production; widely deployed in China's Daqing field (200,000+ bbl/day incremental)
Minimum oil price for EOR economics: Generally $45-70/bbl for mature thermal projects; $60-90/bbl for new chemical or CO2 floods
CO2 retention in reservoir: Typically 40-60% of injected CO2 is permanently sequestered per barrel of oil produced

Field Tip:

A common mistake in EOR screening is applying thermal methods to reservoirs that are too deep or too hot. Steam injection becomes uneconomic below approximately 3,000 feet because heat loss in the injection well reduces steam quality at the formation to near zero, and the capital cost of superheated steam generation cannot be offset by recoverable reserves. Similarly, CO2 miscible flooding requires reservoir pressure above the minimum miscibility pressure (MMP), which increases with depth and oil API gravity; if reservoir pressure has declined below MMP and pressure maintenance through water injection is not feasible, CO2 flooding operates in immiscible mode with substantially lower incremental recovery. Always calculate MMP before screening CO2 EOR candidates.

Thermal EOR: Steam Injection, SAGD, and In-Situ Combustion

Thermal EOR increases reservoir temperature to reduce oil viscosity. Cyclic steam stimulation (huff-and-puff) injects steam into a production well, shuts in for a soak period, then returns to production; this cycle repeats until incremental production per cycle is uneconomic. Steam drive injects steam continuously through dedicated injectors while oil is produced from offset producers. Steam-assisted gravity drainage (SAGD) uses two horizontal wells stacked vertically; steam injected into the upper well heats a growing steam chamber so that oil drains by gravity into the lower production well. SAGD is the dominant production method in the Canadian oil sands and recovers 50-70% of OOIP from bitumen reservoirs with essentially zero primary recovery. In-situ combustion (fire flooding) ignites a portion of crude in the reservoir to drive a thermal front toward producers; technically effective but operationally complex, it sees limited commercial deployment.

Chemical and Miscible Gas EOR Methods

Chemical EOR methods target the capillary forces that trap residual oil after waterflooding. Surfactant flooding reduces oil-water interfacial tension from approximately 30 mN/m to less than 0.01 mN/m, mobilizing trapped oil droplets. Polymer flooding adds high-molecular-weight polymers to injection water, improving the mobility ratio and reducing viscous fingering. Alkaline-surfactant-polymer (ASP) flooding combines all three mechanisms and has achieved incremental recoveries of 15-25% OOIP in mature waterflooded fields. The primary barriers are chemical cost, adsorption onto reservoir rock, and the need for careful laboratory optimization to the specific crude oil-brine-rock system.

CO2 miscible flooding achieves miscibility with the reservoir crude under sufficient pressure, eliminating interfacial tension and enabling essentially complete displacement of oil from swept volumes. The minimum miscibility pressure (MMP) for most light oils ranges from 1,200 to 3,500 psi. CO2 is sourced from natural CO2 fields, industrial point sources (ethanol plants, gas processing), or captured from combustion. The CO2 EOR industry in the United States uses approximately 70 million tons of CO2 per year. The integration of CO2 EOR with carbon capture and storage (CCS) infrastructure is increasingly cited as a pathway to producing oil with reduced carbon intensity, though lifecycle accounting of these claims is actively debated.

Enhanced oil recovery is also referred to as:

EOR — the universal industry abbreviation used in technical, commercial, and regulatory contexts worldwide
tertiary recovery — the historical term for EOR applied as a third phase after primary and secondary; now disfavored because EOR is increasingly applied alongside or before secondary methods
IOR (improved oil recovery) — a broader term that encompasses EOR plus infill drilling, horizontal wells, and reservoir management improvements that do not change fluid-rock interaction physics
advanced recovery — a marketing-oriented term used by some operators and service companies, particularly in the context of mature field rejuvenation

Frequently Asked Questions About Enhanced Oil Recovery

What percentage of a reservoir's oil can EOR recover?

EOR incremental recovery depends heavily on the method and reservoir characteristics. CO2 miscible flooding in favorable light-oil reservoirs can recover an additional 10-20% of OOIP beyond what waterflooding achieves. Polymer flooding typically delivers 5-15% incremental OOIP. SAGD applied to oil sands bitumen — where primary recovery is near zero — can recover 50-70% of OOIP. Combined ASP flooding in optimized laboratory and field conditions has demonstrated 20-30% incremental recovery in some cases. The wide range reflects the critical importance of matching EOR method to reservoir-specific geology, fluid properties, and economic conditions.

How does CO2 EOR contribute to carbon sequestration?

During CO2 flooding, injected CO2 contacts reservoir oil, dissolves into it, and causes oil swelling and viscosity reduction that drives oil to producing wells. A portion of the injected CO2 is produced back at the production wells and is separated, compressed, and reinjected — a recycling process that continues until CO2 breakthrough into producers becomes too high. CO2 that dissolves permanently into residual oil or brine, or is trapped in pores after the flood, remains sequestered in the reservoir. Studies of mature CO2 EOR fields suggest 40-70% of injected CO2 is permanently stored per unit of oil produced, though this varies by reservoir geology, injection strategy, and monitoring methodology. Proponents argue that CO2 EOR allows sequestration costs to be partially offset by oil revenue; critics note that the produced oil is eventually combusted, releasing more CO2 than was stored.

What are the main screening criteria for selecting an EOR method?

EOR screening uses a hierarchy of reservoir and fluid properties to identify candidate methods. Oil viscosity is the primary discriminator: viscosities above 100-1,000 cP favor thermal methods; viscosities below 10 cP with sufficient API gravity favor CO2 miscible flooding. Reservoir depth affects both thermal and miscible methods: depths below 3,000-4,000 feet make steam injection impractical due to heat loss, while depths above 2,500-3,000 feet are generally needed for CO2 to achieve miscibility pressures. Permeability determines whether chemical EOR is technically feasible: polymer molecules require permeability above roughly 10-50 mD to propagate through the formation without unacceptable plugging. Temperature affects chemical stability: many surfactant and polymer formulations degrade above 200-250°F, limiting chemical EOR to moderate-temperature reservoirs. Formation heterogeneity, clay content, and wettability must also be characterized through laboratory corefloods before committing capital to a full-field EOR project.

Why Enhanced Oil Recovery Matters in Oil and Gas

Discovered fields contain trillions of barrels of remaining oil that primary and secondary methods will leave in the ground. EOR recovers a substantial fraction of that stranded resource from fields where infrastructure is already sunk, often at lower per-barrel cost than finding new frontier reserves. As conventional fields mature, EOR becomes a strategic necessity for sustaining global supply.

Enhanced Oil Recovery: EOR Methods to Extract Residual Crude After Primary and Secondary Production