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Wax appearance temperature

Abbreviation
WAT
Also known as
WAT, cloud point (crude oil), wax appearance point

Wax appearance temperature (WAT) is the temperature at which wax crystals first begin to form as a crude oil cools. Below it, paraffin can deposit on surfaces. For crude oil it is also called the cloud point.

What is wax appearance temperature?

Crude oil contains long-chain paraffin waxes that stay dissolved while the oil is warm. As the oil cools, it reaches a temperature at which the first wax crystals appear. That is the wax appearance temperature, or WAT. For crude oils and other petroleum products it is also called the cloud point, because the first crystals make a clear oil look hazy.

Below the WAT, wax can come out of solution and stick to cold surfaces, the process known as paraffin deposition.

How WAT is measured

  • Visual cloud point, under ASTM D2500, in which the oil is cooled in steps and inspected for haze. It works only for oils clear enough to see through.
  • Differential scanning calorimetry, which detects the heat released as wax crystallizes.
  • Cross-polarized microscopy, which shows the first crystals directly.
  • Viscosity measurements, which change as wax begins to form.

Different methods give somewhat different values, so the method should be reported with the result.

WAT and pour point

WAT is the temperature at which wax starts to form. The pour point is lower: it is the temperature below which so much wax has formed that the oil stops flowing under test conditions. Between the two, the oil flows but is depositing wax and becoming more viscous.

Why WAT matters

Operators compare the WAT with the temperatures the oil will experience as it moves up the well, through flowlines and into tanks. Where temperatures fall below the WAT, wax will deposit, and flow assurance plans must address it with insulation, heat, chemical inhibitors, dispersants or mechanical removal.

Frequently asked questions

For crude oil, the terms are generally used interchangeably. The term cloud point is also used for nonionic surfactant solutions, where it means something different: the temperature at which the surfactant separates from water.

WAT is where wax first forms. Pour point is the lower temperature at which the oil no longer flows because enough wax has formed.

  • 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.

  • Pour point

    The pour point is the lowest temperature at which an oil will still flow under standard test conditions. Below it, wax crystals or high viscosity make the oil behave almost like a solid. For crude oil it is measured by methods such as ASTM D97 and ASTM D5853.

  • Cloud point (surfactant)

    The cloud point of a surfactant is the temperature at which a solution of a nonionic surfactant, such as an alcohol ethoxylate, turns cloudy on heating as the surfactant becomes less water-soluble and separates into a second phase.

  • 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. wax appearance temperature; cloud point, SLB Energy Glossary
  2. ASTM D2500, Standard Test Method for Cloud Point of Petroleum Products, ASTM International