Fluid Viscosity

A Minimum at the Bubble Point

Oil viscosity is not a single number; it changes with pressure and is lowest at the bubble point. Above the bubble point, compressing the undersaturated oil raises its viscosity slightly. Below it, gas leaves solution and the heavier remaining oil becomes much more viscous as pressure drops.

Lighter Oil Flows Easier

The whole curve shifts with the oil's API gravity: a light, high-API oil is far less viscous than a heavy, low-API one. Gas viscosity is lower again, a fraction of the oil's, and rises gently with pressure.

Viscosity Is Mobility

Mobility, the ease with which a phase flows, is its relative permeability divided by its viscosity. A viscous oil is hard to move and is easily bypassed by injected water or gas. The contrast between oil and water viscosity sets the mobility ratio that decides how cleanly a waterflood sweeps, the subject of a later chapter.