improved oil recovery, Oil & Gas Glossary

What Is Improved Oil Recovery?

Improved oil recovery (IOR) is a broad term encompassing all methods used to increase the fraction of oil recovered from a reservoir beyond what primary drive mechanisms alone would produce, including both secondary recovery (waterflooding and pressure maintenance) and tertiary or enhanced oil recovery (EOR) methods such as thermal, chemical, and miscible gas injection, as well as production optimization techniques including infill drilling, horizontal wells, artificial lift improvements, and reservoir surveillance. IOR is sometimes used interchangeably with EOR but strictly includes a wider range of practices that do not necessarily alter fundamental displacement mechanisms.

Key Takeaways

The global average oil recovery factor is approximately 35%, meaning roughly two-thirds of oil discovered remains in the ground; IOR techniques can raise this to 45-55% in favorable reservoirs.
IOR is the broader category encompassing all post-primary recovery; EOR is a subset specifically involving injection of fluids or energy to alter displacement mechanisms or fluid properties.
Secondary recovery (waterflooding and gas injection for pressure maintenance) accounts for the largest volume of IOR production globally and is the standard practice in most mature fields.
EOR methods — thermal, chemical, and gas injection — are more capital-intensive and are applied selectively based on reservoir characteristics, fluid properties, and economic conditions.
IOR investment timing relative to field maturity is critical: secondary recovery should ideally begin before reservoir pressure declines significantly, while EOR is often applied in later field life when primary and secondary production are declining.

How Improved Oil Recovery Works

Primary recovery relies on natural reservoir energy: solution gas drive, gas cap drive, water drive, gravity drainage, or compaction drive. As reservoir pressure depletes, production rates decline and eventually become uneconomic. The fraction of oil recovered by primary means alone typically ranges from 5-30% of original oil in place (OOIP), depending on reservoir drive mechanism, fluid properties, and rock characteristics. The remaining oil is trapped by capillary forces, bypassed due to reservoir heterogeneity, or left in low-permeability zones that natural pressure cannot drain effectively.

Secondary recovery addresses the pressure depletion problem by injecting water or gas to maintain or restore reservoir pressure and physically displace oil toward producing wells. Waterflooding is the most widely applied IOR method globally; it is relatively inexpensive, uses widely available injection fluid, and can recover an additional 15-25% of OOIP beyond primary recovery in favorable reservoirs. Gas injection for pressure maintenance, using either natural gas, nitrogen, or CO2, is used where water availability is limited or where gas injection provides better sweep or miscibility benefits.

EOR methods go further by changing the properties of the displacing fluid, the reservoir fluids, or both, to improve microscopic displacement efficiency (how much oil is mobilized in swept zones) and macroscopic sweep efficiency (what fraction of the reservoir volume is contacted). Thermal methods (steam flooding, steam-assisted gravity drainage (SAGD), in-situ combustion) reduce oil viscosity in heavy oil reservoirs. Chemical EOR (polymer flooding, surfactant/polymer flooding, alkaline-surfactant-polymer (ASP) flooding) improves displacement by reducing interfacial tension or improving mobility ratio. Miscible gas flooding, particularly CO2 flooding, dissolves into the oil and reduces viscosity and interfacial tension, enabling near-complete displacement within the swept volume.

Fast Facts: Improved Oil Recovery

Global average recovery factor: ~35% of OOIP by primary and secondary means
IOR potential: 45-55% OOIP recovery achievable in many reservoirs with IOR
Largest IOR method by volume: Waterflooding (secondary recovery)
Most common EOR method globally: Thermal (steam) for heavy oil; CO2 miscible flooding for light oil
CO2 EOR global capacity: Over 150 active CO2 EOR projects, primarily in the US Permian Basin
Polymer flood improvement: Typically adds 5-12% OOIP recovery over waterflood alone
SAGD application: Alberta oil sands, producing from reservoirs with bitumen viscosity of 100,000+ cP
IOR surveillance tool: 4D (time-lapse) seismic to monitor fluid movement and waterflood sweep

Field Tip:

The single biggest determinant of waterflood success is mobility ratio: the ratio of water mobility to oil mobility. A mobility ratio greater than 1 means water moves faster than oil through the reservoir, leading to early water breakthrough and poor sweep. Adding polymer to injection water reduces water mobility, lowering the mobility ratio toward or below 1 and significantly improving areal and vertical sweep efficiency. Polymer flooding is most cost-effective in moderate-to-high permeability reservoirs where polymer can be injected without excessive injection pressure.

IOR vs. EOR: Understanding the Distinction

The terms IOR and EOR are often used interchangeably in industry literature, but the technical distinction matters for classification and investment decisions. EOR strictly refers to methods that alter the fundamental physicochemical mechanisms of oil displacement: thermal methods reduce viscosity, chemical methods reduce interfacial tension or improve mobility ratio, and miscible gas floods eliminate the capillary pressure barrier between injected fluid and reservoir oil. These methods target the residual oil saturation remaining after conventional waterflooding.

IOR is the broader umbrella that includes EOR but also encompasses secondary recovery (waterflooding, gas injection), production optimization (artificial lift, wellbore stimulation, deliquification), and reservoir management practices (infill drilling, horizontal wells, pattern modification, conformance control). An infill drilling program that adds wells to improve drainage of unswept reservoir compartments is IOR but not EOR. A polymer flood that improves waterflood sweep efficiency is both IOR and EOR. Understanding this distinction is important when evaluating project economics, since production optimization and secondary recovery typically offer lower cost per incremental barrel than tertiary EOR methods.

Economic Considerations and Field Maturity

The timing and sequencing of IOR investments relative to field maturity significantly affects project economics. Secondary recovery (waterflooding) is most efficient when initiated before primary recovery has depleted reservoir pressure below bubble point, because maintaining oil above bubble point avoids gas breakout that reduces oil relative permeability and complicates subsequent EOR. Many major fields worldwide were waterflooded from early in their production life, a practice that substantially increased total recovery compared to fields where waterflooding was initiated after primary recovery had already depleted pressure.

EOR projects require extensive technical screening before commitment. Key screening criteria include reservoir temperature and depth (thermal EOR), oil viscosity (determines which EOR method is applicable), reservoir permeability and heterogeneity (affects sweep and injectivity), and existing infrastructure (water handling, gas recycling, CO2 supply). CO2 EOR projects in particular require access to CO2 supply infrastructure, either from natural CO2 reservoirs, industrial capture sources, or anthropogenic CO2 captured from power plants or industrial facilities. The convergence of EOR economics with carbon capture utilization and storage (CCUS) policy incentives has renewed interest in CO2 flooding as a pathway to both incremental oil recovery and permanent CO2 sequestration.

Improved oil recovery is also referred to as:

Enhanced oil recovery (EOR) — technically a subset of IOR encompassing thermal, chemical, and miscible gas methods, but often used interchangeably with IOR in industry practice
Tertiary recovery — historical term for EOR methods applied after primary and secondary recovery, now somewhat outdated as EOR is sometimes applied earlier in field life
Incremental recovery — production attributable to IOR methods above what the waterflood or primary depletion baseline would produce

Frequently Asked Questions About Improved Oil Recovery

What is the typical incremental recovery from waterflooding vs. EOR?

Waterflooding (secondary recovery) typically adds 15-25% of OOIP recovery beyond what primary depletion achieves, depending on reservoir heterogeneity, mobility ratio, and flood pattern design. EOR methods applied after waterflooding typically target the residual oil saturation remaining after sweep, adding another 5-15% of OOIP depending on the method and reservoir characteristics. CO2 miscible flooding in light oil carbonate or sandstone reservoirs can recover 8-15% additional OOIP. Polymer flooding in moderate-permeability sandstones typically adds 5-12% OOIP. Thermal methods (SAGD) in heavy oil reservoirs can achieve total recoveries of 50-70% OOIP from reservoirs that would otherwise recover only 5-10% by primary means.

How does 4D seismic support IOR surveillance?

4D (time-lapse) seismic involves acquiring repeated 3D seismic surveys over a producing field and comparing changes in the seismic response over time. Changes in fluid saturation and pressure as waterflooding progresses alter the acoustic impedance of reservoir rocks, producing detectable differences in the seismic signal. 4D seismic can identify unswept reservoir compartments where injected water has not yet reached, channels or high-permeability streaks causing premature water breakthrough, and fault compartments isolating portions of the reservoir from the flood pattern. This information guides infill drilling, pattern modification, and conformance control decisions to maximize IOR sweep efficiency.

Why is CO2 EOR often linked to carbon capture and storage?

CO2 miscible flooding requires large, continuous volumes of CO2, and the CO2 injected into the reservoir is largely retained underground when wells are eventually abandoned — making CO2 EOR a form of geological carbon storage. Industrial sources of CO2 (power plants, ethanol plants, natural gas processing facilities, cement and steel plants) can supply the injection volumes needed, and policy frameworks in the United States (45Q tax credit), Canada, and other jurisdictions provide financial incentives that improve the economics of CO2 EOR when the CO2 comes from anthropogenic sources. The combination of incremental oil revenue and carbon credit revenue has made CO2 EOR an increasingly attractive component of carbon capture, utilization, and storage (CCUS) strategies.

Why Improved Oil Recovery Matters in Oil and Gas

With the global average recovery factor at approximately 35%, the resource left in discovered fields by conventional production methods dwarfs what is still being found through new exploration. IOR represents the single largest opportunity to grow recoverable reserves without the cost and risk of frontier exploration. Even a one-percentage-point improvement in global average recovery factor equates to tens of billions of barrels of additional recoverable oil from existing fields. As easy-to-find conventional resources become scarcer and exploration drilling costs rise, IOR investment in mature fields offers an increasingly competitive path to reserve replacement and production growth for oil and gas companies worldwide.