Page 289 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2
P. 289

456    Reservoir Engineering


                   should be measured to  calibrate the  instrument. Once the contour areas have
                   been planimetered, the net pay volume can be calculated in several ways. If  the
                   number of contour intervals is even, the volume can be calculated by  Simpson's
                   rule  [272]:



                                                                                 (5-197)

                   where V,  = reservoir volume, acre-ft
                          h  = contour interval, ft
                          yo = area  on top  of sand  minus  area  on base  of  sand  at  the highest
                              contour
                          yn = area  on top  of  sand  minus  area  on base  of  sand  at  the lowest
                              contour

                     When the number of contours is uneven, the volume can be found using the
                    slightly less accurate trapezoidal method

                                                                                 (5-198)
                      Once the reservoir volume is known, the oil in place, N, in stock tank barrels
                    is calculated by  an equation similar to that given earlier:

                          7,758V0 (1 - S, )
                      N=                                                         (5-199)
                                BO

                    where N  = reservoir oil initially in place, STB
                      7,758 = number of  barrels/  acre-ft
                         Vo = net producing reservoir volume, acre-ft
                         Bo = oil formation volume factor, RB/STB
                         S1  = interstitial water saturation, fraction
                          I)  = formation porosity, fraction

                      Similarly gas-in-place, G, in thousands of  standard cubic feet, is estimated by
                    an expression similar to that given earlier:

                          43,560Vg+(1 - S,)pT,
                      G=                                                         (5-200)
                                 ZTP,
                    where G = gas in place, scf
                         Vg = gas bearing volume of reservoir, acre-ft
                         ps = standard pressure, psia
                          T = reservoir temperature, degrees absolute
                         T,  = standard temperature base, degrees absolute
                          z  = gas deviation factor at reservoir conditions

                      Volumetric analysis yields the total hydrocarbon content of  the reservoir; this
                    figure must be adjusted by  a recovery factor to reflect the ultimate recoverable
                    reserves.  Recovery factors are based upon  empirical correlations, experience,
                    or analogy.
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