Geology Reference
In-Depth Information
As the vapor-gas mixture flows through the bottles, the liquid will drop
out from the gas solution and accumulate in the bottles. The amount of
liquid in each bottle will depend on the pressure decrease between the inlet
and outlet of the bottle. The condensation of liquid with declining pres-
sure will proceed over the entire domain of retrograde phenomenon until
the pressure of a minimum concentration of liquid in the gas is attained.
Thus, the vapor-gas mixture will become impoverished in condensate until
reaching practically dry gas composition.
Experiments conducted according to this scheme with retrograde con-
densation of oils showed that, with declining pressure, the mixture of the
methane, ethane and propane and the oil dissolved in it releases first the
least soluble high-molecular weight (heavy) oil fractions and then, with
further decline in pressure, the lighter fractions. This resulted in a series
of hydrocarbon liquids with declining specific gravity in the direction
of increasing pressure gradients (Velikovskiy and Yushkin, 1959; Dvali,
1963).
Based on the above described process, the formation of hydrocarbon
accumulations in the PS/RBS may be presented as follows:
A mixture of oil and variously generated (catagenetic and metagenic)
gases formed under high temperatures and pressures in the deeply-buried
Mesozoic and Paleogene-Miocene section moves through faults, fractures,
stratigraphic unconformities, mud volcano ejecta, contacts of diapirs with
permeable rocks, and other avenues of migration into the low-pressure zones
(Pliocene reservoir). This results in the condensation of vapors of dissolved
hydrocarbons according to the phenomenon of retrograde condensation. If
the moving fluids encounter favorable structures, they condense and form
a liquid-phase accumulation. The vapor-gas mixture relieved of its heavi-
est components migrates farther up the section. The condensation process
continues with the formation of lighter liquid hydrocarbon accumulations.
When a reservoir composed of successively situated traps is penetrated
by the vapor-gas mixture, a multilayered oil field forms, characterized by
an increase in oil specific gravity with depth. This is typical of the multilay-
ered fields in Azerbaijan and Turkmenistan.
The retrograde can occur due to a slow gas escape from the reservoir by
diffusion. Migration of gas can also occur rapidly due to mud volcano erup-
tion, formation of new faults due to tectonic movements, etc. Substantial
amounts of liquids may form in the reservoir over a short period of time
due to “emptying-up”, e.g., in producing gas-condensate fields under the
natural depletion regime with a significant retrograde condensation (and
loss) in the reservoir.
