In laymen's language, viscosity is the measure of how thick a fluid is. A more viscous material will flow slowly compared to a less viscous material. The following statements makes the idea of viscosity even clearer.
- Water is less viscous than molasses.
The common unit used for viscosity is centipoise, denoted by "cp".
Viscosity is the result of molecules of the system interacting with each other. More strongly they interact, less freely they can move, causing higher viscosity. The amount of molecular interactions, in turn, is caused by types of molecules, energy they possess and proximity of the molecules.
For a given temperature, viscosity of crude oil varies with pressure. Above bubble point pressure, viscosity increases as pressure increases. Due to pressure the molecules tend to come closer increasing interactions among them. This results in higher viscosity.
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| Packed Hall = High Pressure = Difficult to move around = High Viscosity | Sparse Hall = Low Pressure = Easy to move around = Low Viscosity |
One the other hand, below bubble point pressure, viscosity increases as pressure decreases. At lower pressure, the lighter gas molecules escapes from the system, allowing the more heavier molecules to interact with each. This results in higher viscosity.
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