A property characterizing agility of the fluid under pressure gradient and quantified as a value of reservoir permeability normalized by dynamic fluid viscosity:

M = \frac{k}{\mu}

where

	formation permeability to a given fluid (which vary for different fluids)
	dynamic fluid viscosity

In multiphase flow the concept of total fluid mobility is not well-defined as phases may have different mobilities and flow quite independently from each other, having different pressures, moving at different velocities and segregated in space.

In most popular case of a 3-phase Oil + Gas + Water fluid model with relatively homogeneous 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:

M = k_{air} \cdot \left[M_{rw} + \left( 1 + \frac{R_s \, B_g}{B_o} \right) \cdot M_{ro} + \left( 1 + \frac{R_v \, B_o}{B_g} \right) \cdot M_{rg} \right]

\left<\frac{k}{\mu} \right> = k_{air} \cdot \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]

In case of 2-phase Oil + Water fluid model with regular to small values of (when Perrine model makes the most practical sense):

M = k_{air} \cdot \left[ M_{rw} + M_{ro}  \right]

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

See also