IDENTIFICATION OF TYPE OF RESERVOIR FLUID - Petroleum Reservoir Fluid Property Correlations - page 193

Initial Producing Gas-Liquid Ratio
as an Indicator of Reservoir Fluid Type
The term initial as used in this chapter, both in the text and also in the
titles of the figures, means “at a time when the average reservoir pressure
is above the dew point pressure or bubblepoint pressure of the reservoir
fluid.” The time period represented by initial may be a matter of days or
possibly years, depending on the difference between the initial reservoir
pressure and the saturation pressure of the reservoir fluid.
Also, the ratio of surface gas to stock-tank liquid is called gas-liquid
ratio when comparing oils with gas condensates because the stock-tank
liquid produced from a gas condensate is condensate rather than oil.
Thus the symbol will be GLR i . The surface gas includes the gas from all
separators and the stock tank. A correlation procedure given in chapter
3 may be used to estimate the stock-tank gas-liquid ratio if it is not
available from field data.
Volatile oils and black oils
Service company laboratory people state that the difference between
volatile oils and black oils is an oil formation volume factor of 2.0 res
bbl/STB when measured at bubblepoint pressure. If the measured oil
formation volume factor is greater than 2.0 res bbl/STB, the produced
gas is very rich and drops condensate in the gas meter, making gas
volume measurement difficult. Thus, the oil is considered to be a volatile
oil (volatile oils are associated with gas condensates rather than dry gases).
Oil formation volume factors at bubblepoint pressures less than 2.0 res
bbl/STB indicate black oils (black oils are associated with dry gases).
Figure 5-2 shows initial producing gas-liquid ratios (note that for
oils these are the same as solution gas-oil ratios at the bubblepoint, R sb )
and oil formation volume factors at bubble-point pressure, B ob , from
1,496 reservoir fluid samples with worldwide origins. Notice that if
the initial producing gas-oil ratios are less than 1,500 scf/STB, all oil
formation volume factors are less than 2.00 res bbl/STB, i.e., black oils.
If the initial producing gas-oil ratios are greater than 1,900 scf/STB,
all oil formation volume factors are greater than 2.00 res bbl/STB, i.e.,
volatile oils. Between these two values of initial producing gas-oil ratios,
some oil formation volume factors are less than 2.00 res bbl/STB and
some are greater. Thus, when the initial producing gas-oil ratios are
between 1,500 and 1,900 scf/STB, the type of reservoir fluid cannot be
determined with field production data.