Page 161 - Origin and Prediction of Abnormal Formation Pressures
P. 161

METHODS OF ESTIMATING AND PREDICTING ABNORMAL FORMATION PRESSURES     137
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                                              B\  A





                                 hi

                            aZZ
                            r                      i\\
                                 h2                I











                                                                  S   J
                                                        2o  2~
            Fig.  5-7.  Correction  of normal  compaction  curve  to  temperature  T1  at  depth  hi.  Abnormally  high  pressure
            zone  in  shale  is  dashed;  solid  line  above  h2  is  normal  compaction  trend,  AAt;  dashed  line  above  h2  is
            normal  compaction  curve  corrected  to  temperature  Tt  at depth  h 1 (BB ~ trend);  2a  is  the  trend  in  abnormally
            pressured  zone  without  correction  to  temperature  T1;  2b  is  the  trend  in  the  abnormally  pressured  zone  with
            correction  to temperature  T1. (Modified  after Dobrynin  and  Serebryakov,  1989,  fig.  56,  p.  115.)


               The  method  of normal  compaction  trend  was  used  in  hundreds  of  wells  in  various
            basins.  It  was  also  used  for  interpreting  seismic  data  in  studying  the  possibility  of
            predicting  anomalously high pressure intervals in the  sand-shale  sequences  of the West
            Kuban Depression, Russia (Dobrynin et al.,  1979).

            Method of compressional curves

               The  method  of  compressional  curves  has  also  been  used  in  estimating  abnormally
            high  and  abnormally  low  pressures  for  a  very  large  number  of  wells.  This  method
            is  based  on  the  use  of  compressional  curves  of regularly  compacted  rocks,  the  shape
            of  which  depends  on  the  difference  between  the  overburden  pressure  and  pore  fluid
            pressure.
               A  compressional  curve  is  defined  as  a  plot  of  several  physical  rock  properties,
            which characterize the compaction of rock, versus effective stress (effective stress is the
            difference between the total overburden pressure and the pore pressure).
               Compressional  curves  (semilogarithmic  plots  of  rock  properties  versus  depth)  are
            more  useful  for  estimating  the  abnormal  pressure  than  the  normal  compaction  trend,
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