Page 279 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 251

and (5) an inverted character of the hydrochemical profile with depth: the chemistry
of water changes from a calcium chloride and magnesium chloride type to a sodium
bicarbonate type (freshening of water with depth).

LABORATORY EXPERIMENTS
Little attention seems to have been given in the literature to artificially simulate
(experimental laboratory work) gravitational compaction and the chemistry of expelled
pore waters from deeply buried argillaceous sediments. Murray and Irvine conducted the
first investigation of pore-water chemistry from marine sediments in 1895 (Manheim,
1976). Soviet geoscientists became interested in the chemistry of pore waters from
Recent sediments in the 1930s. The work of Kryukov (1947) in developing effective
sediment squeezers is noteworthy in this regard. Chilingar and Knight (1960) conducted
experiments in the laboratory at high pressures. Sawabini and Chilingar developed
a high-pressure hydrostatic apparatus incorporating the effect of temperature at the
University of Southern California in Los Angeles (Sawabini et al., 1971). At Imperial
College of London, during the 1960s, a high-pressure uniaxial compaction device was
developed to study the influence of temperature and rate of loading on the pore-water
chemistry, progressive lithification, and fabric of clay sediments (Knill et al., 1976).
Brown conducted laboratory experiments in 1997.
Most of the dissolved salts present in the pore waters, which are trapped during
sedimentation, are squeezed out during the initial stages of compaction. Laboratory
results (Von Engelhardt and Gaida, 1963; Rieke et al., 1964; Chilingar et al., 1969;
Kryukov, 1971; Knill et al., 1976) showed that mineralization of expelled solutions
progressively decreases with increasing overburden pressure. These results led to the
conclusion that the concentrations of pore waters in shales should be lower than those in
associated sandstones. A corollary of this premise suggests that solutions squeezed-out
at the beginning of compaction should have higher concentrations than the pore waters
initially present in argillaceous sediments.

Early laboratory experiments

Von Engelhardt and Gaida (1963) found that for pressures between 30 and 800
kg/cm 2 (2.94-78.45 MPa) the concentration of electrolytes in pore waters of smectite
diminishes with increasing overburden pressure. At higher pressures up to 3200 kg/cm 2
(313.8 MPa), however, an increase in salt concentration within the remaining pore water
was observed by them. Von Engelhardt and Gaida (1963) explained this behavior as due
to the electrochemical properties of base-exchanging clays. If the pore water contains
an electrolyte, then the liquid immediately surrounding the clay particle will contain
fewer electrolytes than the liquid farther away from the double layer. Base-exchanging
clays suspended in electrolyte solutions adsorb a certain amount of fresher water,
which is bound in double layers around each clay particle. During compression,
the electrolyte-rich solution is removed and the water of the double layers, poor in
electrolyte content, is left behind. At higher compaction pressures (from 800 to 3200