displacement efficiency, Oil & Gas Glossary

What Is Displacement Efficiency?

Displacement efficiency (also called microscopic displacement efficiency or Ed) is the fraction of movable hydrocarbons that are actually displaced from a swept pore volume when an injected fluid — water, gas, polymer, or solvent — contacts the reservoir rock. It measures how effectively the injected fluid sweeps oil or gas from the pores it physically touches, and is one of two components that combine to determine a reservoir's overall recovery factor.

Key Takeaways

Displacement efficiency (Ed) multiplied by volumetric sweep efficiency (Ev) equals the overall recovery factor (ER = Ed × Ev), making it a fundamental parameter in reservoir engineering and EOR design.
Waterflooding achieves displacement efficiencies of 50–80% in favorable reservoirs; the remaining oil is trapped by capillary forces at the grain scale and is not contacted by injected water.
Residual oil saturation (Sor) defines the theoretical ceiling for water displacement — the oil that cannot be mobilized at a given water/oil interfacial tension regardless of how much water is injected.
Miscible flooding can drive Ed toward 100% by eliminating the interfacial tension between injected fluid and reservoir oil, removing the capillary trapping that limits conventional waterfloods.
Core flood laboratory tests at reservoir conditions are the standard method for measuring Ed and residual saturations before committing to a full-field EOR program.

How Displacement Efficiency Works

When an injection fluid enters a reservoir pore network, it does not displace every hydrocarbon molecule it contacts. At the microscopic scale, oil is retained in small pore throats and crevices by capillary pressure — the interfacial tension between water and oil that holds droplets in place against the viscous forces of the flowing flood. The fraction of pore-volume hydrocarbons that the injection fluid successfully mobilizes and moves toward a production well is the displacement efficiency. Measured in decimal or percentage form, Ed of 0.70 means 70% of the oil originally occupying the flooded pore volume has been produced; 30% remains behind as residual oil.

Engineers differentiate between microscopic displacement efficiency — the pore-scale phenomenon described above — and macroscopic or volumetric sweep efficiency, which describes how much of the total reservoir bulk volume is contacted by the injected fluid at all. Overall recovery combines both: a waterflood might achieve Ed = 0.75 in the zones it sweeps, but if vertical and areal sweep efficiency together reach only Ev = 0.60, the total recovery factor is just 0.45 (45% of original oil in place). Improving one without the other yields diminishing returns, which is why modern EOR programs typically target both mechanisms simultaneously.

Gas injection presents a different displacement challenge. In immiscible gas floods, gravity override causes the injected gas to rise to the top of the reservoir and bypass lower oil-bearing intervals, collapsing volumetric sweep. Even in swept zones, the gas–oil mobility ratio is highly unfavorable — gas is many times less viscous than oil — so viscous fingering allows gas to channel through oil rather than piston-displacing it. These factors combine to make immiscible gas Ed significantly lower than waterflood Ed in most sandstone reservoirs, though gas performs well in naturally fractured carbonates where gravity drainage is the recovery mechanism.

Fast Facts: Displacement Efficiency

Symbol: Ed (also written E_D)
Range: 0.0 (no displacement) to 1.0 (complete displacement)
Typical waterflood Ed: 0.50–0.80 in favorable sandstones
Limiting factor: Residual oil saturation (Sor), controlled by capillary number
Miscible flood Ed: Can approach 1.0 when IFT is eliminated
Laboratory method: Core flood test at reservoir temperature and pressure
Combined formula: ER = Ed × Ev (recovery factor = displacement × sweep)
Key log indicator: Resistivity and saturation logs calibrated to core data

Field Tip:

Before approving an EOR project, request a core flood test that measures both waterflood residual oil saturation (Sorw) and the proposed EOR agent's residual saturation (Sorm). The difference between these two numbers — the incremental oil mobilization — is the direct measure of improved Ed. If Sorm is not meaningfully lower than Sorw at realistic injection conditions, the EOR process cannot improve displacement efficiency and the economics will not work regardless of favorable sweep.

Pore-Scale Mechanisms Controlling Displacement

Three pore-scale forces govern displacement efficiency: viscous forces (which drive injected fluid through pores), capillary forces (which trap oil in small pore throats), and gravity (which separates fluids by density). The dimensionless capillary number (Nc = viscous force / capillary force) quantifies the balance. At the low capillary numbers typical of conventional waterfloods (10^-7 to 10^-5), capillary trapping dominates and residual oil saturations are high. EOR methods raise the capillary number by increasing viscous forces (polymer floods, higher flow rates) or reducing interfacial tension (surfactant floods, miscible floods), allowing more oil to be mobilized from pore throats that would otherwise trap it permanently.

Viscous fingering is a separate pore-scale instability that reduces displacement efficiency when the injected fluid is less viscous than the reservoir oil. The Buckley-Leverett fractional flow theory describes how an unfavorable mobility ratio causes injection fluid to bypass oil rather than form a stable displacement front. Polymer flooding addresses this by increasing the effective viscosity of the injection water, improving the mobility ratio and creating a more uniform flood front that contacts more oil before breakthrough at the producing well.

EOR Methods That Improve Displacement Efficiency

Miscible flooding — whether CO2 injection at or above minimum miscibility pressure (MMP), hydrocarbon miscible injection, or enriched gas flooding — is the most direct way to improve Ed because miscibility eliminates the oil–water interface entirely. With zero interfacial tension, the capillary number becomes effectively infinite and residual oil saturation drops toward zero in contacted pore volumes. The Permian Basin's prolific CO2 EOR programs in formations such as the San Andres and Grayburg dolomites rely on this principle, with Ed values approaching 0.90–0.95 under miscible conditions compared to 0.55–0.65 under immiscible waterflood.

Surfactant-polymer (SP) and alkali-surfactant-polymer (ASP) flooding reduce IFT through chemical means without requiring high pressure for miscibility. By lowering IFT by two to three orders of magnitude, these methods shift capillary numbers high enough to mobilize residual oil. Thermal methods — steamflooding and in-situ combustion — improve Ed by reducing oil viscosity and thermally cracking heavy components, lowering the viscosity contrast that otherwise causes bypassing. Each method's improvement in Ed must be weighed against its incremental cost to determine whether the additional recovery justifies the investment.

Displacement efficiency is also referred to as:

microscopic displacement efficiency — the most technically precise term, emphasizing that this parameter operates at the pore scale rather than the reservoir scale
pore-scale displacement efficiency — used interchangeably with microscopic displacement efficiency in academic literature
Ed — standard engineering symbol used in reservoir simulation and reserve calculations
local displacement efficiency — term encountered in older SPE literature, particularly in the context of polymer flooding studies

Frequently Asked Questions About Displacement Efficiency

What is the difference between displacement efficiency and sweep efficiency?

Displacement efficiency (Ed) measures how much oil is removed from the pore volume that the injected fluid actually contacts — it is a pore-scale, microscopic measurement. Sweep efficiency (Ev) measures what fraction of the total reservoir bulk volume is contacted by the injected fluid at all — it is a reservoir-scale, macroscopic measurement accounting for areal heterogeneity, vertical stratification, gravity segregation, and well patterns. Overall recovery factor equals the product of both: ER = Ed × Ev. An excellent displacement process in the laboratory can still deliver poor field results if sweep efficiency is low due to reservoir heterogeneity or poor well placement.

Why can't conventional waterflooding displace all the oil?

Even after extensive waterflooding, a fraction of the original oil — typically 20–50% of pore volume — remains trapped as isolated droplets and ganglia in small pore throats. This residual oil saturation (Sor) persists because capillary pressure forces exceed the viscous driving force of the waterflood at normal injection rates. The oil is not bypassed by the water front; it is contacted but physically immobilized by interfacial tension. Reducing Sor requires either eliminating the oil–water interface (miscible flooding), dramatically reducing IFT (surfactant flooding), or reducing oil viscosity enough to change the force balance (thermal methods).

How is displacement efficiency measured in the laboratory?

Core flood tests are the standard measurement method. A preserved or restored core plug from the reservoir is flooded at reservoir temperature and overburden pressure with a representative brine or the proposed EOR fluid. Effluent oil and water volumes are tracked through the flood to calculate saturations at each flood stage. The ratio of oil produced to initial movable oil in the core gives Ed. Ideally, tests are conducted at reservoir wettability conditions using reservoir fluids, since both wettability (which controls how oil coats grain surfaces) and fluid properties significantly affect the measured residual saturation and therefore the displacement efficiency.

Why Displacement Efficiency Matters in Oil and Gas

Displacement efficiency sits at the heart of secondary and tertiary recovery economics. Global conventional waterflood recovery factors average 35–40% of original oil in place, meaning 60–65% of discovered reserves are left behind — much of it stranded by capillary trapping that a higher Ed could address. As operators exhaust primary and secondary production from mature fields, improving Ed through EOR becomes the primary lever for extending field life and recovering reserves already delineated by existing wells. Even a 5-percentage-point improvement in Ed across a large waterflood can add millions of barrels of technically recoverable resource without drilling a single new well, fundamentally changing the economics of mature field development.