Page 136 - Geology and Geochemistry of Oil and Gas

PHYSICAL PROPERTIES OF NATURAL GASES 107
used: a dimensionless value equal to the ratio of gas density to the air density. The air
3
density under standard conditions is 1.293 kg/m .
Although many authors believe that the buoyancy forces (Archimedes principle)
are the main cause of gas movements and distribution in the accumulations, other
forces also cause fluid movements in the reservoir rocks. Pressure in the gas portion
of an accumulation is higher than in a similar water-filled trap by the amount of the
surplus pressure, which is equal to
p surplus ¼ ðr r ÞgH (6.5)
w
g
where r is the water (or oil) density, r the gas density, g the gravitational ac-
w
g
celeration, and H the height of accumulation column. For precise calculations, one
must take into account changes in the gas density depending on the height of ac-
cumulation column and the gas composition.
As the above equation shows, the surplus pressure is proportional to the density
difference and the height of accumulation column. The surplus pressure should not be
confused with abnormal, or abnormally-high formation pressure (AHFP). The rea-
sons for their emergence and existence are different. The surplus pressure is caused by
the existence of the accumulation with fluids of different density. On the other hand,
AHFP is an expression of the elastic energy accumulated in a limited volume; its
existence is defined by the relaxation conditions and is not associated with the presence
or absence of a hydrocarbon accumulation. Strictly speaking, when determining
AHFP, the surplus pressure should be subtracted although it is usually impractical.

6.3.2. Combustion heating value

The heat-generating capacity is the amount of heat (e.g., in joules) that is released
3
during the total combustion of a unit volume (e.g., 1 m ) of a natural gas. Part of the
combustion heat is expended to evaporate the water contained in the flue gas and the
water contained in the gas itself (if any). Thus, the uppermost (gross) Q up and
lowermost (net) Q low heating values are distinguished. The difference is the latent
heat of vaporization of water vapor in the flue gas. The hydrocarbon heat-generating
capacity grows as the molecular weight increases.

6.3.3. Compressibility of natural gases

Compressibility takes into account the deviation of the behavior of natural gases
from that of ideal gases. In estimating gas reserves it is important to compare the gas
volumes at reservoir conditions to those at standard conditions. Theoretically, the
van der Waals’ equation may be used for the natural gases:
2
ðp þ a=V ÞðV bÞ ¼ RT (6.6)
where p is the pressure in MPa, T and V are the absolute temperature (1C) and the
3
volume (cm ) of 1 g-mol of the gas, respectively, and R is the gas constant (8.32 J/
2
mol T). The a/V value (‘‘internal pressure’’) accounts for the mutual attraction of
molecules, and b is the volume of molecules.