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anchorF
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{\bf u}  = -  M  \cdot ( \nabla p - \rho \, {\bf g} )

where

LaTeX Math Inline
bodyM

fluid mobility

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body\nabla

gradient operator

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body\rho

fluid density 

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body--uriencoded--%7B \bf g %7D

gravity vector pointing along Earth's Gravity Direction 


In single-phase flow the Darcy flow equation takes a following form:

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{\bf u}  = -  \frac{k}{\mu}  \cdot ( \nabla p - \rho \, {\bf g} )

where

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body

M =

k

/ \mufluid mobility

formation permeability

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body\

rho

mu

fluid
density 
viscosity


In multiphase flow the different phases 

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body

...

\alpha
move with different velocities 
LaTeX Math Inline
body--uriencoded--%7B\bf u%7D_\alpha
 and  Darcy flow equation is applicable for each phase independently:

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anchorZLYNK
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{\bf u}_{\alpha}  = -  \frac{k_{\alpha}}{\mu_{\alpha}}  \cdot ( \nabla p_{\alpha} - \rho_{\alpha} \, {\bf g} )

where

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bodyp_\alpha

phase pressure of the 

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body\alpha
-phase

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bodyk_\alpha

phase permeability of the 

LaTeX Math Inline
body\alpha
-phase


LaTeX Math Inline
body\rho_\alpha

fluid density of the 

LaTeX Math Inline
body\alpha
-phase

g %7Dgravity vector pointing along Earth's Gravity Direction 

LaTeX Math Inline
body\mu_\alpha

fluid viscosity of the 

LaTeX Math Inline
body\

nabla

gradient operator

...

alpha
-phase




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 phase pressure) the Darcy flow equation can be approximated with Perrine model of Multi-phase Mobility:

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