IDENTIFICATION OF TYPE OF RESERVOIR FLUID - Petroleum Reservoir Fluid Property Correlations - page 200

Composition of Heptanes Plus
as an Indicator of Reservoir Fluid Type
This topic is primarily concerned with situations in which luid
property laboratory reports are not available, i.e., the composition of the
reservoir fluid is not available. However, some interesting observations
about the compositions of the five reservoir fluids can be determined
with the data set of figures 5-4 and 5-5.
Volatile oils and black oils
Table 2 of McCain (1994) gives a precise cut-off of 20 mol% heptanes
plus in the reservoir fluid between volatile oils and black oils. 9 This is
not correct.
As mentioned above, there is a range of initial producing gas-liquid
ratios, 1,500 to 1,900 scf/STB, in which the type of reservoir fluid is
indeterminate. There is also a range of compositions of heptanes plus in
which the type of reservoir fluid cannot be determined.
Figure 5-7, which is another subset of the data of figure 5-4, shows
that the composition of heptanes plus in the reservoir fluid must be
greater than 26.5 mol% for all of the initial producing gas-liquid ratio
data to be less than 1,500 scf/STB (black oils). And the figure shows that
the heptanes plus compositions must be less than 18.0 mol% for all of
the initial producing gas-liquid ratios to be greater than 1,900 scf/STB
(volatile oils). In the region from 18.0 mol% to 26.5 mol%, heptanes plus
the initial producing gas-oil ratios vary from less than 1,500 scf/STB to
greater than 1,900 scf/STB. Thus in this region, the type of oil cannot be
determined using heptanes plus composition.
Gas condensates and volatile oils
The same table in McCain (1994) indicates that the transition
between volatile oils and gas condensates is a composition of 12.5 mol%
heptanes plus in the reservoir fluid. Additional data have been obtained
and are included in figure 5-3. [This figure is similar to fig. 2 of McCain
(1994) with additional data added. 10 ] Figure 5-3 reveals a fairly precise
transition of 12.9 mol% heptanes plus. The vertical dashed line at 12.9
mol% heptanes plus separates those reservoir fluids that exhibited
bubblepoints (oils) from those fluids that exhibited dew points (gases).
Thus, if the composition of the reservoir fluid contains more than
12.9 mol% heptanes plus, the reservoir fluid is an oil. If the composition