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Tank bottoms

Also known as
tank sludge, tank bottom sludge

Tank bottoms are the sludge that accumulates at the bottom of oil storage tanks: a mixture of heavy hydrocarbons, paraffin, asphaltenes, emulsified water, sand, corrosion products and other solids. They reduce storage capacity and must be periodically removed and processed or disposed of.

What are tank bottoms?

Crude oil and produced fluids stored in tanks are never perfectly clean. Over time, heavier and denser materials settle out and collect at the bottom:

  • Paraffin waxes and asphaltenes.
  • Water and stable emulsions, often called basic sediment and water.
  • Sand, silt and clay carried from the well.
  • Iron sulfide and other corrosion products.
  • Scale and other solids.

The result is a thick sludge layer that grows until it is removed.

Problems caused by tank bottoms

  • Lost storage capacity.
  • Oil trapped in the sludge, which is lost value if the sludge is disposed of.
  • Off-spec oil sales, when sludge contaminates oil drawn from the tank.
  • Corrosion under deposits at the tank floor.
  • Safety hazards during cleaning, including hydrogen sulfide and pyrophoric iron sulfide.
  • Disposal costs.

How tank bottoms are handled

  • Manual cleaning, with crews entering the tank after it is emptied and ventilated. This is slow and requires confined-space safety procedures.
  • Mechanical cleaning, with pumps, nozzles and centrifuges.
  • Chemical treatment, using solvents, demulsifiers, dispersants and surfactants to break emulsions, release oil from solids and fluidize sludge so oil can be recovered and sold.
  • Processing and disposal of remaining solids and water under applicable regulations.

Preventive treatment to keep paraffin, asphaltenes and solids moving can slow buildup and is part of broader flow assurance.

Frequently asked questions

Often, yes. Treating sludge to break emulsions and release oil from solids can recover saleable oil and reduce the volume of waste.

It varies with the fluids stored, throughput and treatment programs, from months to years.

  • Emulsion

    An emulsion is a mixture of two liquids that do not dissolve in each other, such as oil and water, in which one is dispersed through the other as small droplets. Most emulsions separate over time unless an emulsifier stabilizes the droplets.

  • Paraffin deposition

    Paraffin deposition is the buildup of solid wax on tubing, flowlines, tanks and other equipment when crude oil cools below the temperature at which its long-chain paraffin hydrocarbons start to crystallize. Deposits restrict flow and can plug a well or pipeline.

  • Iron sulfide FeS

    Iron sulfide (FeS) is a black deposit that forms when hydrogen sulfide reacts with iron, often from corroding steel, in oil and gas wells, pipelines and tanks. It causes scale, stabilizes emulsions, promotes corrosion and can be pyrophoric, igniting on exposure to air.

  • Flow assurance

    Flow assurance is the discipline of ensuring that oil, gas and water keep flowing without interruption from the reservoir to the point of sale. It covers preventing and removing deposits such as paraffin, asphaltenes, scale and hydrates, and managing emulsions, corrosion and solids.

Sources

  1. Oil and gas exploration and production wastes, U.S. Environmental Protection Agency
  2. Tank cleaning and sludge handling, American Petroleum Institute (API)