23 - API And Crude Oil Density...A Quick Guide To API Gravity
Like any other material, Crude Oil has a set of properties that defines its chemical and physical characteristics. These properties form the basis of comparison between several crude oil blends. Depending on the purpose of comparison, some properties prevail more than others. For instance, viscosity, wax content, sulfur, and acidity are important to crude oil transportation and refining, while API Gravity is important for crude oil trading and pricing.
What Is API Gravity?
API Gravity is a measure of how heavy or light crude-oil is compared to water (density of crude oil / density of water). The API Gravity measurement is a positive number that is inversely correlated to crude-oil specific gravity (the lighter the crude-oil, the higher the API Gravity)
How is API Gravity Calculated?
To calculate API Gravity, first measure Crude Oil specific gravity at 16 C⁰ ( 60 F⁰) then plug the number into the following equation:
API gravity = (141.5/Crude Oil Specific Gravity) – 131.5
What is the API Gravity Unit Of Measurement?
The API gravity is unitless (as the case with specific gravity). However, it is popular to refer to API gravity in degrees.
API Gravity Controversy
In 1916, The U.S. National Bureau of Standards adopted Baumé scale for crude oil density measurement, API followed suit. Few years later, it was concluded that the value of 141.5 is not accurate, consquently it has been corrected to 140. However; API decided to keep the old value as the API Gravity scale became widely used in the industry.
What Are The Crude Oil Classification?
API classifies crude oil into 4 categories depending on their API Gravity range:
Light – API > 31.1
Medium – API between 22.3 and 31.1
Heavy – API between 10 and 22.3
Extra Heavy – API < 10.0 (heavier than water)
The determination of whether a crude-oil blend is light or heavy is not always according to its API Gravity number. In some occasions, categorization depends on the region from which the oil is extracted. For example the Iraqi Basrah Heavy oil has an of API Gravity of 23 (which place it as medium) yet it is being traded as heavy oil.
Some Crude Oil Blends and Benchmarks
Medium: Hibernia, Terra Nova, White Rose
Light: Albian Prem, Syncrude Sweet
Medium: Bow River
Heavy: Western Canadian Select (WCS), Albian Heavy, Cold Lake
Extra Heavy: Athabsca Bitumen
Light: Brent, WTI, OPEC Basket
Medium: Iraqi Basrah, Dubai Crude
Heavy: Angolan Kuito, Venezuelan Laguna, Mexican Mayan, California Kern River
Light oil is usually preferred as it is usually less expensive to extract, easier to transport, and contains a higher percentage of hydrocarbons that can be converted to fuel such as gasoline and diesel.
Barrels to Metric TonsAPI Gravity can be used to convert between two popular units of measurements:
Barrels for volume
Metric Tons for weight
The following equation offers a quick and easy conversion between between the two units:
1/[(141.5/(API + 131.5) x 0.159]
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