Page 307 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 277

that the density of adsorbed water of Na-smectite varies with the amount of water
present in the clay. The highest densities of adsorbed water occur when the HzO/clay
weight ratio for Na-smectite is less than 0.5. The highest density values range up to
1.5, as reported by DeWit and Arens (1950), Mooney et al. (1951), and Mackenzie
(1958). The low-density values are slightly lower than one (1) as reported by Norrish
(1954), Anderson and Low (1958), and Cebell and Chilingarian (1972). The dissolving
capacity of water is inversely proportional to density (Fig. 10-33B). As the capillary is
squeezed during compaction, its radius is decreased and the water expelled first will
be the least bound, which is the more saline water close to the center of the capillary.
The results obtained by Chilingar and Rieke (1976) show that the total concentration of
expelled solutions goes through a maximum before starting to decrease with increasing
overburden pressure. The remaining adsorbed water poor in electrolytes is expelled at
higher overburden pressures until the concentrations of Mg 2+ and Ca 2+ ions start to
increase. This can be attributed, in these experiments at ambient temperature, to the
higher concentration of these ions in the water in close vicinity to the clay platelets.
Rieke and Chilingarian (1974) used this model to explain the relative salinities of
interstitial waters in well-compacted and undercompacted shales and their associated
sandstones, as shown in Fig. 10-34. Although a very simple model, it explains some of
the ambient-temperature, experimental results reported in this chapter.

Thermodynamic approach

Thermodynamic models combine the concepts of electrochemical equilibrium and
electroneutrality in compacting sediments. They are elaborate in determining the effects
of decreasing porosity (or void ratio) on the concentration of squeezed-out pore water
and the remaining pore water held in the sediments.

Bolt's pressure filtrate model
The model of Bolt (1961) is based on the way the cationic and anionic concentrations
vary in the vicinity of a charged clay particle. The model is applicable to a symmetric
electrolyte, such as NaC1 held in sediments with infinite initial void ratios (dilute
solutions). The concentration, C, expressed in equivalents of ions per unit of fluid
volume (mequiv/cm 3) of the expelled pore water is:
4Cok
C = (10-12)
(1 + k) 2
where k is the ratio of the negative-ion concentration toward the middle of the pores
divided by C, and k is presumed to vary from unity to zero during compaction
(Smith, 1977). Co is the initial concentration of solution in equilibrium with the clay in
mequiv/cm 3. The void ratio of the compacting clay is expressed as:

(l+k) qpmax (10-13)
e-- 1+ 1-k 4Co
where e is the void ratio, q is the cation exchange capacity expressed in mequiv/g, and
Pmax is the matrix density in g/cm 3. Elimination of k between Eqs. 10-12 and 10-13