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Then use the following equality:

LaTeX Math Block
anchorrhophi
alignmentleft
d(\rho \, \phi) = \rho \, d \phi + \phi \, d\rho = \rho \, \phi \, \left( \frac{d \phi }{\phi} +  \frac{d \rho }{\rho}  \right) 
= \rho \, \phi \, \left( \frac{1}{\phi} \frac{d \phi}{dp} \, dp +  \frac{1}{\rho} \frac{d \rho}{dp} \, dp  \right) 
= \rho \, \phi \, (c_{\phi} \, dp + c \, dp) = \rho \, \phi \, c_t \, dp

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LaTeX Math Block
anchorS8TNB
alignmentleft
\rho \, \phi \, c_t  \cdot \frac{\partial (p)}{\partial t} + \nabla \, ( \rho \, {\bf u}) = 0
LaTeX Math Block
anchorqk
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\int_{\Sigma_k} \, {\bf u} \,  d {\bf A} = q_k(t)

where

LaTeX Math Inline
bodyc_t = с_\phi+ c


Let's assume Darcy flow with constant permeability 

LaTeX Math Inline
body--uriencoded--\displaystyle \frac%7Bdk%7D%7Bdp%7D = 0
and ignore gravity forces:

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LaTeX Math Block
anchortau
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\tau(t) = \int_0^t \frac{dt}{\mu (p_{BHP}(t )) \, c_t (p_{BHP}) } \, , \ \  p_{BHP} = p_{BHP}(t))}
 

to correct early-time transient  behaviour which turn equation

LaTeX Math Block Reference
anchorprePZ
 into:

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