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titleDerivation

The pore-saturating fluid volume is related to total rock volume 

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bodyV_r
 and reservoir porosity 
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body\phi
 as: 

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V_f = V_r \phi


The total reservoir compressibility is going to be:

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c_t = -\frac{1}{V_r} \, \frac{\partial V_r}{\partial p}  = - \frac{\phi}{V_f} \, \frac{\partial}{\partial p} \left( \frac{V_f}{\phi}  \right) 


= - \frac{\phi}{V_f} \,  \left( V_f \, \frac{\partial}{\partial p} \frac{1}{\phi}  + \frac{1}{\phi} \frac{\partial V_f}{\partial p}  \right) 


=  \frac{1}{\phi} \, \frac{\partial \phi}{\partial p} - \frac{1}{V_f} \, \frac{\partial V_f}{\partial p}   = c_r + c_f

Some applications (like multi-phase pressure diffusion) account for the impact of cross-phase fluid exchange on the total compressibility which require some corrections to equation 

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anchorcf
:

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c_t(s,P) = c_r + c_w s_w +  c_o s_o +  c_g s_g  + s_o [ R_{sp} + (c_r  + c_o)  R_{sn} ] + s_g [ R_{vp} + R_{vn}(c_r + c_g) ]

where

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bodyR_{sn}, \; R_{vn}

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bodyR_{sp}, \; R_{vp}

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See Non-linear multi-phase pressure diffusion @model for derivation of 

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anchorc_t1
.

See also

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[Compressibility] [Total Compressibility]