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The MatBal equation 

LaTeX Math Block Reference
anchorMatBal
  is often complemented by constant PI  model of Bottom-Hole Pressure (
LaTeX Math Inline
bodyp^{\uparrow}_{wf}(t)
 for producers and 
LaTeX Math Inline
bodyp^{\downarrow}_{wf}(t)
 for injectors):


LaTeX Math Block
anchorBHP_PROD
alignmentleft
p^{\uparrow}_{wf, k}(t) = p(t) - {J^{\uparrow}_k}^{-1} \cdot \frac{dQ^{\uparrow}_k}{dt}



LaTeX Math Block
anchorBHP_INJ
alignmentleft
p^{\downarrow}_{wf, \, j}(t) = p(t) -  {J^{\downarrow}_j}^{-1} \cdot \frac{dQ^{\downarrow}_j}{dt}


wherewhere

LaTeX Math Inline
bodyp^{\uparrow}_{wf, \, k}(t)

BHP in

LaTeX Math Inline
bodyk
-th producer

LaTeX Math Inline
bodyp^{\downarrow}_{wf, \, j}(t)

BHP in

LaTeX Math Inline
bodyj
-th injector

LaTeX Math Inline
bodyQ^{\uparrow}_k(t)

cumulative offtakes from

LaTeX Math Inline
bodyk
-th producer by the time moment
LaTeX Math Inline
bodyt

LaTeX Math Inline
bodyQ^{\downarrow}_j(t)

cumulative intakes to

LaTeX Math Inline
bodyj
-th injector by the time moment
LaTeX Math Inline
bodyt

LaTeX Math Inline
bodyJ^{\uparrow}_k

productivity index of

LaTeX Math Inline
bodyk
-th producer

LaTeX Math Inline
bodyJ^{\downarrow}_j

injectivity Index of

LaTeX Math Inline
bodyj
-th injector



In practice there is no way to measure the external influx 

LaTeX Math Inline
bodyQ^{\downarrow}_{GC}(t)
 and 
LaTeX Math Inline
bodyQ^{\downarrow}_{AQ}(t)
 so that one need to model them and calibrate model parameters to fit available data on production flowrates history and formation pressure data records. 

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