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Approximation of Material Balance Pressure @model for slightly compressibility flow:

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anchor

...

p
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p(t)  = p_i + \frac{\Delta Q(t)}{V_\phi \cdot c_t}
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anchorDQ
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\Delta Q = -  \frac{B_o - R_s \, B_g}{1- R_s \, R_v} \cdot  \, Q^{\uparrow}_O + \frac{ B_g - R_v \, B_o}{1- R_s \, R_v} \cdot \, \left( Q^{\downarrow}_G - Q^{\uparrow}_G + Q^{\downarrow}_{GCAP} \ \ \right) + B_w \, \left( Q^{\downarrow}_W - Q^{\uparrow}_W + Q^{\downarrow}_{WAQ} \ \  \right)


where

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body\Delta Q(t)

Cumulative Voidage Replacement Balance (CVRB) over time 

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bodyt

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bodyV_\phi = V \cdot \phi_i

initial drainage volume of the main pay (excluding the aquifer and gas cap)

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body\phi_i = \phi(p_i)

initial porosity

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body--uriencoded--c_t = c_\phi + c_o \, s_%7Boi%7D + c_g \, s_%7Bgi%7D + c_w \, s_%7Bwi%7D

total compressibility

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bodyc_\phi

pore compressibility 

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body--uriencoded--s_%7Bwi%7D

initial water saturation

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body--uriencoded--s_%7Bgi%7D

initial gas saturation

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body--uriencoded--s_%7Boi%7D

initial oil saturation:

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body--uriencoded--s_%7Boi%7D = 1 - s_%7Bwi%7D - s_%7Bgi%7D

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bodyc_o, \, c_g, \, c_w

fluid compressibility of water phaseoil phase and gas phase


The equations 

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anchorp
 and
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anchorDQ
 are often used in express assessment of thief water production share
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body--uriencoded--\Omega%5e%7B\uparrow%7D_W = Q%5e%7B\uparrow%7D_%7BW,%7B\rm true%7D%7D \, / \,Q%5e%7B\uparrow%7D_W
 and thief water injection share
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body--uriencoded--\Omega%5e%7B\downarrow%7D_W = Q%5e%7B\downarrow%7D_%7BW,%7B\rm true%7D%7D \, / \,Q%5e%7B\downarrow%7D_W
:

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anchorp
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p_i - p(t) = \alpha \cdot Q^{\uparrow}_O(t) + \beta \cdot Q^{\uparrow}_W(t) - \gamma \cdot Q^{\downarrow}_W(t)
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anchorconditions
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\alpha > 0, \quad \beta > 0, \quad \gamma > 0
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anchorPhi
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V_\phi = \frac{ 1 }{ \alpha \cdot c_t}  \cdot \frac{B_o - R_s \, B_g}{1- R_s \, R_v}
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anchoromega_up
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\Omega^{\uparrow}_W = \frac{Q^{\uparrow}_{W,{\rm true}}}{Q^{\uparrow}_W} = \frac{\beta}{\alpha}  \cdot \frac{1}{B_w} \cdot \frac{B_o - R_s \, B_g}{1- R_s \, R_v}
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anchoromega_down
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\Omega^{\downarrow}_W = \frac{Q^{\downarrow}_{W, {\rm true}}}{Q^{\downarrow}_W} =\frac{\gamma}{\alpha}  \cdot \frac{1}{B_w} \cdot \frac{B_o - R_s \, B_g}{1- R_s \, R_v}
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grouparax
Panel
bgColorpapayawhip
titleARAX

The MatBal equation 

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anchorMatBal
pageMaterial Balance Pressure @model
can be re-written as following:

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anchorMatBal_formula
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p = p_i + \frac{\delta Q}{c_\phi \, V_\phi} + \delta p_i
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anchorMatBal_formula
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\delta p_i =  \frac{ B_{og} \, F_{Oi} + B_{go} \, F_{Gi} + B_w \, F_W -1}{c_\phi}
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anchor1
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B_{og} = \frac{B_o - R_s \, B_g}{1- R_s \, R_v}
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anchor1
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B_{go} = \frac{ B_g - R_v \, B_o}{1- R_s \, R_v}

where

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body\delta Q

Cumulative Voidage Replacement Balance (CVRB) 

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[ Derivation of Slightly compressible Material Balance Pressure @model ]

[ Capacitance-Resistivity Model (CRM) @model ]