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178 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS
Gas in a gas cap can behave differently (see Chapter 6):
(1) At low and moderate pressure, the gas solubility in oil is governed by the
Henry’s law. 16
(2) At higher temperature and pressure (60–801C and 8–10 MPa), retrograde phe-
nomena begin to occur, and the gas in a gas cap is converted into the gas-
condensate state. As mentioned in Chapter 6, the boundary beyond which the
retrograde phenomena occur is determined not just by the temperature and
pressure, but also by the chemical composition of oil and gas.
As the basin subsides, the nature of the gas–oil and oil–water contacts changes.
Clear (distinct) contacts are replaced by transition zones. The transition zones show
the high oil content in gas or water; then, they are replaced with emulsions which, in
turn, are replaced by gas–oil solutions involving the entire accumulation.
The issue of the phase state of fluids in the transition zone is still somewhat open.
It is possible that the emulsion forms only when the fluids enter the borehole (due to
the turbulent flow of fluids when they enter the well). Such accumulations have been
called ‘‘the transitional stage accumulations’’ (Vasilyev et al., 1966).
A further subsidence (temperature of about 280–3501C) results in the unlimited mu-
tual solubility of hydrocarbons and water. The upper gas–oil (or vapor–oil) transitional
zone merges with the lower, water–oil transitional zone, forming a single accumulation.
The unlimited mutual solubility is an indication that the fluids are in the super-
critical state or close to it. A drastic increase in the mutual solubility of the fluids that
have been insoluble in each other before occurs on approaching the critical point.
This is observed in chemically different oils and water even at 120–1501C and be-
comes pronounced at 1801C.
Oil accumulations that are undersaturated with gas may approach (through the
vapor phase) the described accumulation type. Apparently, they should be called
‘‘the critical-state accumulations’’. The liquid phase and vapor phase merge at the
critical point so that one phase cannot be distinguished from the other. Also, in-
terfacial tension disappears, diffusion stops, density and composition fluctuations
appear, relative permeabilities significantly decrease (there are no phases, just mol-
ecules of different sizes and composition), etc.
The boundaries of such a system are determined only by the available space, by
the thermodynamic conditions, and by the direction of migration (the availability of
migration avenues and by the system’s energy).
Inasmuch as the critical-state fluid system is characterized by the absence of
distinct phases and of the gravity separation, the major premises of the gravity
hypothesis of the formation of hydrocarbon accumulations and its practical appli-
cations are negated. The hydrocarbons may not move updip, toward the anticlinal
crests. The absence of phases and of interfacial tension makes the concept of the
hydrodynamic trapping unrealistic. The absence of capillary pressure converts some
seals into reservoirs.
16
According to the Henry’s law, the amount of a gas absorbed by a given volume of liquid at a given
temperature is directly proportional to the pressure of gas. This law, which was first formulated in 1803 by
the English physician and chemist William Henry, holds only for dilute solutions and low gas pressures.