Page 207 - Geology and Geochemistry of Oil and Gas

176 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

pressure increase, the retrograde phenomena cause dissolution in the gas of heavier
hydrocarbons (up to C 15 ). Such accumulations become gas-condensate accumula-
tions. The assignment of an accumulation to the free gas (dry) or gas-condensate
category is highly important for the production purposes.
In order to prevent the loss of condensate in the reservoir, gas-condensate ac-
cumulations must be developed with pressure (desirably also temperature) mainte-
nance. The boundary between the dry gas and gas-condensate accumulation is
3
3
tentative and is set most often at 20–250 cm of condensate per 1 m of gas. There is
an easy transition from gas to condensate (or vice versa) type of accumulation with
changing temperature and pressure. When the price of gas depends on its heating
value, a more detailed classification based on the condensate content can be prepared
for practical purposes.
The produced gas always contains some water. The water may be the water of
condensation (i.e., water dissolved in gas together with the heavier hydrocarbons),
and may be carried by the flowing gas. There are numerous sources of the conden-
sation water: (1) the capillary and ‘‘submelted’’ water (see Chapter 4); (2) beyond-
the-contour water; (3) bottom water; and (4) water coming from the water-saturated
lenses and interbeds within the gas-bearing formations. Even a slight decrease in the
pore throats of clastic reservoir rocks may create water blocks. Removal of water
blocks may occur as the reservoir conditions change during the development and
production.
The water may be retained for a long time within the fine-grained hydrophilic
lenses or interbeds; however, the water may begin to enter the flowing gas as the
pressure differential between the lenses (interbeds) and producing formation in-
creases. Mathematical models have been developed for calculating such phenomena.
As the temperature increases (due to increasing depth of burial or, rarely, due to
technological reasons) the interfacial tension decreases, which may result in an in-
crease in production water-cut. Water coning and fingering can occur in the water-
drive gas accumulations, i.e., accumulations underlain by water. Certain gas pro-
duction techniques must be designed to prevent this from occurring.
The nature of gas accumulation’s contact with the underlying fluids is of the
utmost importance. If the accumulation is totally underlain by water, it is called a
single-phase gas accumulation (although two phases, liquid and gas are actually
present). At shallow depth with a temperature of 60–801C and pressure below
10 MPa, the accumulation has a rather clear-cut gas–water contact. When the tem-
o
perature increases to 120–130 C and if the gas enters the contact zone at a speed
greater than that of its dissolution in water, a gas–water emulsion forms (in the
transition zone). Sometimes, such a transition zone can constitute the entire accu-
mulation. Although the emulsions have been known for a long time, the emulsion-
type accumulations as a separate type was first identified by Kontorovich et al.
(1975, pp. 426–427).
As the temperature further increases beyond 1201C and the pressure increases
beyond 20 MPa (critical pressure of water ¼ 21.8 MPa), the water solubility in gas
drastically increases (retrograde evaporation and a decrease in interfacial tension).
These conditions are unfavorable for the preservation of gas–emulsion or gas–water