A property characterizing agility of the fluid under pressure gradient with account of reservoir permeability and dynamic fluid viscosity:

M_f = \frac{k_f}{\mu_f}

where

	formation permeability to fluid "f"
	dynamic viscosity of fluid "f"

In multiphase flow the concept of fluid mobility is not well-defined as phases may flow quite independently from each other and have different dynamic fluid parameters (pressure and velocity).

But for relatively homogeneous multi-phase flow (phases may move at different velocities but occupy the same reservoir space and have the same pressure) the multi-phase mobility may be defined by Perrine model:

\left<\frac{k}{\mu} \right> = k_{air} \left[ \frac{k_{rw}}{\mu_w} + \left( 1 + \frac{R_s \, B_g}{B_o} \right) \cdot \frac{k_{ro}}{\mu_o} + \left( 1 + \frac{R_v \, B_o}{B_g} \right) \cdot \frac{k_{rg}}{\mu_g} \right]

and for a case of oil + water fluid :

\left<\frac{k}{\mu} \right> = k_{air} \left[ \frac{k_{rw}}{\mu_w} + \frac{k_{ro}}{\mu_o}  \right]

and this is when Perrine model makes the most practical sense.

See also