Page 114 - PVT Property Correlations

92 PVT Property Correlations

The well stream volume is the volume of fluid produced from the well
before undergoing separation in the first stage separator.
When reservoir pressure exceeds the dew point pressure, both CGR and
condensate yield are theoretically constant. With reservoir depletion, the
pressure declines below the dew point pressure, and producing CGR and
condensate yield decline as well. Few correlations are available to estimate
the changes in producing CGR as function of reservoir pressure. These corre-
lations usually depend on initial CGR, specific gravity of first stage separator
gas, API gravity of the stock-tank condensate, and reservoir pressure. One
correlation (England, 2002) estimates the condensate yield at initial condi-
tions (or at dew point pressure and above) from simple data. The correlation
was developed for particular regions. Other correlations presented in
Appendix B, Gas Correlations Formulae are used to calculate the CGR
decline with decreasing pressures below the dew point pressure.
Although not in agreement with the correct definition, in practice, CGR
is sometimes reported as the volume of condensate collected in the stock-
tank to the volume of gas produced from the first stage separator. We notice,
however, that the majority of the gas is produced in the first stage separator.
This is the amount of gas that is usually measured accurately (because it is
often sold or used in the field for gas lift or power generation). Chapter 4,
Wet Gases, contains methods to calculate the additional gas produced from
second stage separator and stock-tank by use of correlations, when not avail-
able from field measurements.

Natural Gas Liquids
NGL are also called plant products. In operation of gas condensate reser-
voirs, field separation can recover high value condensates in the stock-tank.
Additional liquids can also be recovered if the separator gas is processed
through a gas plant. NGL refers to the liquids that are extracted from natural
gas after further processing of the gas in a gas plant. It is a process of frac-
tionation that results in production of simple hydrocarbons from the hydro-
carbon mixture known as natural gas. NGL refers to ethane, propane, normal
butane, iso-butane, and pentane and heavier hydrocarbons. These quantities
are measured in “Gallons Per Thousand” or GPM. They can be also mea-
sured in barrels per million cubic feet of gas (bbl/MMscf).
To calculate how much GPM can be obtained from a specific gas, the
composition of the gas must be known. The GPM of every component is cal-
culated from any of the following equations:

y i M i
GPM i 5 19:65 gallons=Mscf ð5:2Þ
ρ
oi