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130                                                          Reservoir Fluids


          sample which requires recombining to recreate the reservoir fluid. Both sampling
          techniques face the same problem of trying to capture a representative sample (i.e. the
          correct proportion of gas to oil) when the pressure falls below the bubble point.



          6.2.6.1. Subsurface samples
          Subsurface samples can be taken with a subsurface sampling chamber, called a
          sampling bomb, or with a formation pressure testing tool (RFT, MDT, RCI), all of
          which are devices run on wireline to the reservoir depth. These tools are further
          described in Section 6.3.6.
             The sampling bomb requires the well to be flowing, and the flowing bottom hole
          pressure (P wf ) should preferably be above the bubble point pressure of the fluid to
          avoid phase segregation. If this condition can be achieved, a sample of oil containing
          the correct amount of gas (R si scf/stb) will be collected. If the reservoir pressure is
          close to the bubble point, this means sampling at low rates to maximise the sampling
          pressure. The valves on the sampling bomb are open to allow the fluid to flow
          through the tool and are then hydraulically or electrically closed to trap a volume
                         3
          (typically 600 cm ) of fluid. This small sample volume is one of the drawbacks of
          subsurface sampling (Figure 6.24).
             Sampling saturated reservoirs with this technique requires special care to attempt
          to obtain a representative sample, and in any case when the flowing bottom hole
          pressure is lower than the bubble point, the validity of the sample remains doubtful.
          Multiple subsurface samples are usually taken by running sample bombs in tandem
          or performing repeat runs. The samples are checked for consistency by measuring
          their bubble point pressure at surface temperature. Samples whose bubble point lie
          within 2% of each other may be sent to the laboratory for PVT analysis.
             Samples taken from formation pressure testers (FPTs) can be taken immediately
          after drilling and do not require the well to be flowing. A small probe is sealed







                                                       sample chamber





                     Pressure
                                                                      p
                                                                       i
                                                                      p
                                                                       b
                                              p
                                               wf
                                                              r
          Figure 6.24  Subsurface sampling apparatus (after Dake,1978).
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