Mobility ratio

What is mobility ratio?

When water or another fluid is injected to push oil toward producing wells, the outcome depends on how easily each fluid moves through the rock. A fluid’s mobility is its effective permeability divided by its viscosity: the more permeable the rock is to it and the thinner it is, the more mobile it is.

Mobility ratio, M, compares the two:

M = (mobility of displacing fluid) / (mobility of displaced fluid)

Favorable and unfavorable ratios

• M at or below 1 (favorable). The injected fluid moves no faster than the oil ahead of it, so the displacement front advances fairly evenly, like a piston.
• M above 1 (unfavorable). The injected fluid moves more easily than the oil, so it forms fingers that race ahead through the oil, a process called viscous fingering. Water breaks through to producing wells early, leaving much of the oil behind.

Because water is usually much less viscous than oil, waterfloods of viscous oils tend to have unfavorable mobility ratios.

Why it matters

Mobility ratio is a main control on sweep efficiency in waterflooding and enhanced oil recovery. Methods to improve it include:

• Polymer flooding, which thickens the injected water to lower its mobility.
• Foams, which reduce gas mobility in gas injection.
• Thermal methods, which lower oil viscosity.

Mobility control addresses how well the injected fluid sweeps the reservoir. Lowering interfacial tension addresses a different problem: freeing oil trapped in the pores the fluid does reach.

Frequently asked questions