Page 58 - Reservoir Geomechanics
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42     Reservoir geomechanics



                  2600







                  2800







                  3000
                Depth (m)






                  3200




                                   HYDROSTATIC
                                     PORE PRESSURE
                                        GRADIENT
                  3400







                  3600
                     40      42      44      46      48      50      52      54
                                               Pore pressure (MPa)
              Figure 2.11. Pore pressure measurements from an oil and gas field in the northern North Sea. Note
              the distinct, low gradient, hydrocarbon legs associated with reservoirs encountered in a number of
              wells.


                Hydrocarbon column heights can result in substantial overpressure at the top of reser-
              voir compartments, especially when appreciable amounts of buoyant gas are present.
              This was seen for the FB-A and FB-B OI sands in South Eugene Island in Figure 2.8a
              (Finkbeiner, Zoback et al. 2001) and as mentioned above can ultimately limit the size
              of hydrocarbon columns in some reservoirs. Figure 2.11 shows excess pore pressure at
              the top of hydrocarbon columns from a field in the North Sea.
                Centroid effects refer to the fact that relatively high pore pressure occurs at the top of a
              tilted sand body encased in shale. As shown in Figure 2.12, the pore pressure at the top of
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