Page 187 - Geology and Geochemistry of Oil and Gas

156 FORMATION OF HYDROCARBON ACCUMULATIONS

1977; Chekalyuk, 1986; Parker et al., 1986; Simonenko, 1988; Beletskaya, 1990;
Sariapo, 1993). The solubility was established for most mixtures (mostly binary,
however). It is important to mention that the high-molecular-weight compounds
found in crude oils are poorly soluble in water. The increase in solubility of liquid
hydrocarbons in water with increasing temperature becomes appreciable only above
160–1801C. Separation of liquid hydrocarbons from solution may occur when
temperature declines or the water salinity increases greatly.
The molecular solubility may play a significant role in the expulsion (and
migration) process only when the rocks are subjected to high temperature and
pressure (T 4120–1801C, p420 MPa). Water-soluble hydrocarbons can travel
together with water for a long distance and time. They can be ‘‘salted-out’’ only
after the solubility limit is exceeded. The total saturation of water with hydrocarbons,
however, is observed only in the immediate proximity of oil–water interface.
The liquid hydrocarbons are highly soluble in the colloidal-emulsion or micellar
form. This solubility is at least one order of magnitude higher than that in the case of
true solutions. Beletskaya (1990) stated that:
(1) Emulsions (water-in-oil) form spontaneously, due to the heat motions of
molecules.
(2) In order for the emulsion to form, there is no need to overcome the micelle-
formation critical barrier (enough of surfactants and emulsifiers are present in
the dispersed organic matter per se); however, an alkaline environment is
required.
(3) Inverse emulsions (water-in-oil) may form with the asphaltene-type surfactants;
such inverse emulsions have the external hydrocarbon phase (hydrophobic), are
floating in water, and when moving through rocks are capable of desorbing and
dissolving paraffins and asphaltenes. These emulsions are stable over a wide
range of pressure and temperature.
The emulsions are electrically charged and electrolytically active. Practically no
studies of the interaction between emulsions and both natural and artificial
electromagnetic fields have been conducted (Starobinets et al., 1986). The major
obstacles for the movement of colloidal solutions are the size of the particles
(0.1–0.001 mm) and the electrostatic difficulty for the movement of negatively
charged particles (the surfaces of clays are negatively charged). According to
Eremenko and Chilingar (1996), the particles move unobstructed through the pores
where the pore throat (or canal) diameter is at least three times greater than the
particle size. When micelles form, the size of the resulting globules may be
comparable or even greater than the diameter of pore throats. At the same time, as
the water-in-oil emulsions, the gas-in-water emulsions can form.
Demulsification may occur upon changes in the water composition and pH and
when the emulsifier is catagenetically altered. The break-up of an emulsion may
cause the primary accumulation even within the source rocks. Owing to the low
permeability of rocks to emulsion, the latter can serve as a barrier.
The ‘‘submelted’’ water layers (see Chapter 4) are affected by the surface
forces and have different properties compared with free water. The difference is
first of all in their higher solving capability and their lower polarity. According to