Page 287 - Fundamentals of Reservoir Engineering
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OILWELL TESTING                                    224

                     the well and the fluid and pressure distribution to the surface would then be as shown
                     in fig. 7.36(b), in which a rise in the static tubing head pressure occurs due to phase
                     separation.

                               THP        PRESSURE                            THP   PRESSURE

                                                                            GAS
                            GAS
                                         VIRTUAL OIL
                                         GRADIENT
                                                                                  OIL


                                 OIL
                                                 DATUM                                  DATUM
                                     WATER                           MEASUREMENT DEPTH
                           MEASUREMENT DEPTH         TOP PERFS.                             TOP PERFS.

                                           (a)                                      (b)

                     Fig. 7.36  Extreme fluid distributions in the well; (a) with water entry and no rise in the
                                tubing head pressure, (b) without water entry and with a rise in the THP

                     Between these extremes, of course, there is an infinite range of possible fluid
                     distributions. The important thing is that the engineer should know the pressure
                     gradient in the wellbore fluid at the survey depth, which can only be obtained from the
                     static pressure - depth survey. If the well is completed as shown in fig. 7.3 (b), in which
                     for mechanical reasons it is not possible to conduct the buildup survey at the actual
                     reservoir depth, then, in order to relate the pressures measured in the borehole to the
                     datum level in the reservoir, it is first necessary to calculate the pressure at the top of
                     the perforations using the measured pressure gradient of the fluid in the borehole at
                     the survey depth, thus

                                          dp
                           p (perfs )  =  p +       × h
                                   m
                                          dz   (well )

                     and thereafter, use the calculated oil gradient in the reservoir to correct to the datum
                     plane.


                                           dp           dp
                           p (datum )  =  p +       × h −       × H
                                    m
                                           dz   (well )     dz   ( )
                                                          oil
              7.11   AFTERFLOW ANALYSIS

                     When a well is closed in for a pressure survey, the closure is usually made at the
                     surface rather than at the sandface. Because the fluids in the flow string have a higher
                     compressibility than in the reservoir, production will continue at the sandface for some
                     finite time after the surface production has ceased. The time lag between closing in the
                     well at the surface and feeling the effects of closure in the reservoir is, to a large
                     extent, dependent upon the mechanical design of the well. If the well is completed with
                     a packed off annulus the volume of fluids in the flow string is considerably smaller than
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