Page 285 - Origin and Prediction of Abnormal Formation Pressures
P. 285

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES          257
                           g/kg



                      mg/kg             C        l       ~




                 300  50  10            Br


















                  I00   0"   0               ,                             Ca~ §
                   CI/Br  Br-   0           250             5~30            750
                               Compaction  pressure,  kg/cm

            Fig.  10-16. Variation in  the  concentration of various ions in  interstitial solutions expelled from marine
            mud with increasing pressure at 80~  (Modified after Krasintseva and Korunova, 1968, fig. 3, p.  196. In
            Chilingarian et al., 1994, fig. 5-11, p. 127.)



            to an in-situ overburden pressure of 8.7 psi (60 MPa).  De-aired distilled water, under the
            head of a few centimeters, was then flushed through the two porous  stones.  The leachate
            (flushed  water)  was  collected  in  a  measuring  cylinder  and  analyzed  for  major  cations
            (Na +,  K +,  Mg 2+  and  Ca 2+)  at  regular  intervals  of  time.  After  leaching,  the  clay  was
            squeezed,  and  the  salinity  of the  expelled  pore  water  was  measured.  It  was  found  that
            as  a result of leaching,  the  salinity of the  squeezed  out pore  water was reduced from its
            original  value  of 26,700  to  1640  mg/1.  It is  interesting  to  note  that the  amounts  of Na +
            and K + extracted in the leachate are in excess of those present in the original pore water,
            whereas  the opposite  is true  for Ca 2+  and Mg 2+  (Table  10-8).  This  led  Kazi  and Moum
            (1972)  to  conclude  that  postdepositional  leaching  of marine  clays  is  manifested by  the
            migration  of high-valence  cations  from the pore  water towards  the  clay mineral  surface
            at the expense  of low-valence cations,  which move from the clay's  surface into the pore
            water.
               Chilingarian et al. (1973)  saturated a sample of smectite clay (API No. 25) in seawater
            for  a  period  of  seven  days.  The  sample  was  shaken  vigorously  twice  a  day.  Then  the
            supernatant  liquid  (leachate),  which  was  assumed  to  have  the  same  composition  as
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