What is Oil API Gravity
In the early years of the petroleum industry, the American Petroleum Institute (API) adopted the API gravity (°API) as a measure of the crude oil density or specific gravity (SG). The API gravity is calculated from the following equation:
°API= (141.5/SG) – 131.5
The API scale for gravity was adapted from the Baumé scale, developed in the late 18th century to be used in hydrometers for measuring even small differences in the specific gravity of liquids, using water as a reference material in these devices.
The equation above shows that lighter with lower specific gravity has higher API gravity.
Crude oil is generally lighter than water. Water with SG of 1 has an API gravity of 10. Therefore, liquid hydrocarbons with lower SGs have higher API gravities.
The API of crude oils varies typically between 10 and 50, with most crude oils falling in the range of 20-45. Using API gravity, the conventional crude oils can be generally considered as light (°API>30), medium (30>°API>22), and heavy (°API<22).
For example, Brent crude oil has an API gravity of 38.3 and WTI has an API of 39.6. This means that WTI is slightly lighter than Brent. However, both Brent and WTI fall into the light oil category.
Classification of Crude Oil Based on API Gravity
1. Light Crude Oil: API gravity above 31.1°.
– Light crude is highly valued because it contains a higher proportion of hydrocarbons that can be easily refined into gasoline, diesel, and other high-demand products. It is less dense and viscous, making it easier to transport and refine.
2. Medium Crude Oil: API gravity between 22.3° and 31.1°.
– Medium crude oil is a versatile type of oil that can be processed into a wide range of products. It strikes a balance between the lighter and heavier crudes in terms of ease of handling and refining.
3. Heavy Crude Oil: API gravity between 10.0° and 22.3°.
– Heavy crude oil is denser and more viscous, making it more challenging to produce, transport, and refine.
4. Extra Heavy Crude Oil: API gravity less than 10.0°.
– Extra heavy crude, also known as bitumen or tar sands, is extremely dense and viscous. It is often blended with lighter hydrocarbons or processed using special techniques like thermal recovery or upgrading to make it suitable for refining.
Importance of API Gravity in the Oil Industry
1. Market Value: Crude oil with a higher API gravity is generally more valuable because it produces a greater yield of high-demand products like gasoline and diesel.
2. Refining Process: The API gravity of crude oil determines the complexity of the refining process. Light crudes require simpler and less energy-intensive processes, while heavy crudes need more sophisticated refining techniques, such as cracking and coking, to convert them into usable products.
3. Transportation: Lighter crudes are easier and cheaper to transport because of their lower viscosity. Heavier crudes often need to be heated, blended with lighter oils, or diluted with other substances to facilitate transportation through pipelines.
4. Production Challenges: Extracting and producing heavy and extra heavy crudes pose significant technical challenges. These oils often require enhanced recovery techniques, such as steam injection or solvent extraction, to flow to the surface.
API Gravity in Global Oil Markets
– West Texas Intermediate (WTI): A light crude oil with an API gravity of approximately 39.6°. WTI is a benchmark for oil prices in North America and is known for its high quality and ease of refining.
– Brent Crude: With an API gravity of around 38.3°, Brent crude is slightly heavier than WTI but is still considered a light crude. It serves as a global benchmark for oil prices.
– Arab Light: A medium-light crude with an API gravity of about 33.4°, Arab Light is one of the primary export grades from Saudi Arabia.
– Maya Crude: A heavy crude oil from Mexico with an API gravity of about 22°. Maya crude is more challenging to refine and is typically sold at a discount compared to lighter crude.
This article is adapted from a post by Reservoir Solutions and other sources.
