IDENTIFICATION OF TYPE OF RESERVOIR FLUID - Petroleum Reservoir Fluid Property Correlations - page 198

Wet gases and gas condensates
Dew points at reservoir temperatures have been observed in
laboratory studies of gas condensates with initial producing gas-liquid
ratios exceeding 1,000,000 scf/STB. However, at high initial producing
gas-liquid ratios, the amount of condensate left in the reservoir is very
small (less than about 1% of the pore volume). Apparently, nearly all gases
that release condensate at the surface also release some condensate in
the reservoir. Thus, these gases have dew points at reservoir temperatures
and are gas condensates.
There are very few, if any, true wet gases (some condensate at the
surface but no condensate in the reservoir). However, wet gas theory can
be applied to gas condensates that release small amounts of condensate
in the reservoir.
Rayes et al. showed that when the composition of heptanes plus in
the reservoir gas is less than 4 mol%, the volume of condensate released
into the reservoir is very small, and thus the gas can be treated as a wet
gas (although in theory, wet gases do not release any condensate in the
reservoir). 6 McCain (1994) indicated that gas condensates with initial
producing ratios greater than 15,000 scf/STB can be treated as if they
are wet gases. 7 However, no data was shown to confirm this value.
Figure 5-6 is another subset of the data of figure 5-4. It shows that
if the initial producing gas-liquid ratios are equal to or greater than
15,000 scf/STB, the compositions of the heptanes plus in the reservoir
fluids are definitely less than 4 mole percent. This confirms that gases
with initial producing gas-liquid ratios of this value and higher can be
treated as wet gases (although there surely will be a dew point and some
retrograde condensate formed in the reservoir).
Dry gases
Figure 5-5 shows that many, if not all, reservoir gases that produce
initial gas-liquid ratios much larger than 100,000 scf/STB have dew
points at reservoir temperature and will release some condensate in the
reservoir, and thus the gases are actually gas condensates. However, the
small amount of condensate at the surface, less than10 STB/MMscf, is
not enough to strongly affect the recombination calculations that are
used to determine the specific gravity of the reservoir gas. Thus, the
specific gravity of the surface gas can be used to calculate the properties
of the reservoir gas at reservoir conditions. So, for engineering purposes,
these gases can be treated as if they are dry gases (although in theory, dry
gases do not release any condensate at the surface).