Weatherford Laboratories provides a variety of methods for the accurate determination of °API gravity. As one of the basic properties of crude oils, this is often the first test performed when an oil arrives at the laboratory.
°API Gravity is a simple technique that provides insight into the quality of an oil. It is a measure of how heavy or light a petroleum liquid is compared to water and is a dimensionless quantity but is referred to in degrees (°). Water is defined as having an °API gravity of 10. The °API gravity scale is an inverse measure of a petroleum liquid's density relative to water (also known as specific gravity). Samples with an API above 10 are lighter (or less dense) and would float on water. Samples with an API below 10 are heavy (or more dense) than water and would sink.
°API gravity values of most petroleum liquids fall between 10 and 70.
°API Gravity by pycnometer
- requires only 2 to 5 ml of oil
- can be performed quickly
A pycnometer (also known as a specific gravity bottle) accurately measures the specific gravity of a sample by referencing the weight of a volume of oil against the weight of the same volume of water using an analytical balance and a known temperature. This is then temperature corrected to the reference temperature of 60°F.
The formula for this calculation is:
° API = [141.5/specific gravity at 60°F]-131.5
Techniques for °API determination:
- °API Gravity of heavy oils by toluene dilution
- °API Gravity of oils by densitometer
- °API Gravity of oils by micro capillary tubes
- °API gravity predicition by programmed pyrolysis
Different techniques are available for samples that have minimal sample amounts or require special handling (high H2S, heavy crudes that don't flow, etc.)
The value of °API gravity:
- Light crude oil has an °API gravity higher than 31
- Medium crude oil has an °API gravity between 22 and 31
- Heavy crude oil has an °API gravity below 22
- Extra heavy crude oil has an °API gravity below 10
- °API gravity may be used to calculate barrels of oil per metric ton
Sample issues that may affect °API gravity:
- Entrained water
- Entrained solids
- Drilling fluids
Sample preparation is extremely important to ensuring high quality data and must be performed prior to measurement. A variety of techniques are available depending on the specific sample issue. These can include, but are not limited to, centrifugation, ultrasonic treatment, warming and de-watering.