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208                                                  Reservoir Drive Mechanisms


                                gas injector
                                gas injector

              oil producers
                                                           gas injector
                                                                  oil producers







                                        OWC
                       Steep Dip                           Shallow Dip
          Figure 9.5  Location of wells for gas cap drive.





           P i
                                                                         Produced
                                                                         Produced
                                                                           GOR
                                                                           GOR
                                Oil Rate
                                Oil Rate





                                                                          Reservoir
                                                                          Pressure

           R si                                                           Water Cut
                                                                          Water Cut




                                           Time (Years)
           First Oil                                                   RF = 20 - 50%
          Figure 9.6  Characteristic production pro¢le: gas cap drive.


          9.2.3. Water drive

          Natural water drive occurs when the underlying aquifer is both large (typically greater
          than ten times the oil volume) and the water is able to flow into the oil column, that
          is it has a communication path and sufficient permeability. If these conditions are satisfied,
          then once production from the oil column creates a pressure drop the aquifer
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