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Residual oil saturation

Abbreviation
Sor
Also known as
Sor, residual oil, trapped oil

Residual oil saturation (Sor) is the fraction of a rock's pore space that is still filled with oil after it has been swept by a displacing fluid such as water. This oil is trapped by capillary forces and will not flow under normal waterflood conditions.

What is residual oil saturation?

Saturation describes how the pore space in a rock is shared between fluids: oil saturation is the fraction of pore volume filled with oil. Residual oil saturation, written Sor, is the oil saturation left after a displacing fluid, usually water, has passed through the rock and no more oil will move. It is the oil that waterflooding cannot recover, however much water is injected.

Why oil is left behind

As water advances through rock, the continuous oil phase breaks up. Isolated droplets and clusters of oil become stranded in pores because capillary pressure at the narrow pore throats is greater than the pressure the flowing water can apply to push them through. This is capillary trapping. The amount of oil trapped depends on the rock’s pore structure, its wettability, and the interfacial tension between the fluids, and it varies widely from reservoir to reservoir.

Residual oil vs. remaining oil

The two are different. Residual oil is oil in rock that has been swept but is trapped. Remaining oil also includes oil in parts of the reservoir the water never reached, bypassed because of poor sweep efficiency. Different enhanced oil recovery methods target each: interfacial tension reduction for residual oil, and improved sweep for bypassed oil.

How it is measured

Residual oil saturation is estimated from laboratory core floods, from well logs run before and after injecting fluids, and from single-well chemical tracer tests, which measure how much oil remains in the rock around a well. Knowing it is essential for deciding whether an EOR project is worthwhile.

How residual oil saturation is reduced

Residual oil can be mobilized by raising the capillary number, mainly by lowering interfacial tension with surfactant flooding, or by changing wettability or the oil itself, for example with miscible gas that dissolves in and swells the oil.

Frequently asked questions

No. Residual oil is trapped in rock that has been swept by water. Remaining oil includes that trapped oil plus oil in unswept parts of the reservoir.

Yes, but not by more waterflooding alone. It takes a change in the forces that trap it, such as ultralow interfacial tension, altered wettability or miscible gas injection.

  • Capillary pressure

    Capillary pressure is the pressure difference across the curved boundary between two immiscible fluids, such as oil and water, in a narrow pore. It increases as pores get smaller and as interfacial tension rises, and it is a main force trapping oil in reservoir rock.

  • Capillary number Nc

    The capillary number is a dimensionless ratio of the viscous forces pushing a fluid through porous rock to the capillary forces holding it in place. In oil recovery, residual oil is mobilized only when the capillary number is raised far above normal waterflood values, usually by lowering interfacial tension.

  • Enhanced oil recovery EOR

    Enhanced oil recovery (EOR) is any technique that recovers oil left in a reservoir after natural pressure and waterflooding have done what they can, by changing the properties of the oil, the rock or the fluids between them. The main types are thermal, gas injection and chemical EOR.

  • Wettability

    Wettability is the tendency of a rock surface to be in contact with one fluid in preference to another when both are present, usually water or oil. Reservoir rock is described as water-wet, oil-wet, intermediate or mixed-wet, and its wettability controls how oil and water move through it.

Sources

  1. residual oil saturation, SLB Energy Glossary
  2. Residual oil saturation, SPE PetroWiki