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Surfactant flooding

Also known as
chemical flooding, surfactant flood, ASP flooding, surfactant-polymer flooding

Surfactant flooding is a chemical enhanced oil recovery method in which a surfactant solution is injected into a reservoir to lower the interfacial tension between oil and water, often to ultralow values, so that oil trapped by capillary forces can be displaced toward producing wells.

What is surfactant flooding?

Surfactant flooding is a form of chemical enhanced oil recovery. After waterflooding, much of the remaining oil is held in the rock’s pores as droplets trapped by capillary forces. A surfactant solution, injected as a slug and pushed through the reservoir, lowers the interfacial tension between oil and water enough to free those droplets, which can then coalesce into a moving bank of oil.

How it works

Trapped oil moves only when viscous forces overcome capillary forces, a balance described by the capillary number. Because interfacial tension sits in the denominator of that ratio, reducing it to ultralow values, below about 10-2 mN/m, raises the capillary number by the orders of magnitude needed to mobilize residual oil.

Designers tune the surfactant formulation to the reservoir’s oil, brine salinity and temperature, typically aiming for the salinity at which the system forms a middle-phase microemulsion with the lowest interfacial tension.

Variants

  • Surfactant-polymer (SP) flooding. A polymer is added, usually behind the surfactant slug, to thicken the injected water and improve sweep efficiency, so the surfactant contacts more of the reservoir.
  • Alkaline-surfactant-polymer (ASP) flooding. Alkali is added as well. It reacts with acidic components in some crude oils to form natural surfactants in place and can reduce the amount of surfactant lost to the rock.

Challenges

  • Surfactant adsorption onto rock, which depletes the slug as it travels.
  • Reservoir conditions, particularly high temperature, high salinity and hard water, which can degrade or precipitate surfactants.
  • Cost, since surfactants are used in large volumes.
  • Produced emulsions, which can be harder to separate at the surface.

Many surfactant floods have been piloted, and a smaller number applied at field scale, with outcomes depending strongly on the reservoir and the design.

Frequently asked questions

Waterflooding pushes oil with water and maintains pressure but leaves oil trapped by capillary forces. Surfactant flooding adds surfactant to lower interfacial tension so that trapped oil can also be displaced.

Alkaline-surfactant-polymer flooding combines alkali, which generates natural surfactants from acidic oils and reduces adsorption; synthetic surfactant, which lowers interfacial tension; and polymer, which improves sweep.

  • 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.

  • Interfacial tension IFT

    Interfacial tension (IFT) is the energy per unit area, or force per unit length, at the boundary between two liquids that do not mix, such as oil and water. Surfactants lower it by adsorbing at that boundary.

  • 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.

  • Surfactant adsorption

    Surfactant adsorption is the attachment of surfactant molecules to a surface, such as reservoir rock, soil, pigment or fabric. It is how surfactants do much of their work, but in oilfield and soil applications it also removes surfactant from solution before it reaches its target.

  • Microemulsion

    A microemulsion is a clear, thermodynamically stable mixture of oil, water and surfactant, often with a co-surfactant, that forms spontaneously. Its oil and water domains are only nanometers to tens of nanometers across, much smaller than the droplets of an ordinary emulsion.

Sources

  1. surfactant flooding; ultralow interfacial tension, SLB Energy Glossary
  2. Chemical flooding, SPE PetroWiki
  3. Enhanced Oil Recovery, L. W. Lake, Prentice Hall, 1989; reprinted by the Society of Petroleum Engineers