Page 30 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

18 Reservoir Engineering

contact with the oil. As a result, the flash liberation is a constant-composition,
variable-volume process and the differential liberation is a variable-composition,
constant-volume process. For heavy crudes (low volatility, low API gravity oils)
with dissolved gases consisting primarily of methane and ethane, both liberation
processes yield similar quantities and compositions of evolved gas as well as
similar resulting oil volumes. However, for lighter, highly volatile crude oils
containing a relatively high proportion of intermediate hydrocarbons (such as
propane, butane, and pentane), the method of gas liberation can have an effect
on the PVT properties that are obtained. An example of differences in formation
volumes with flash and differential liberation processes can be seen in Figure
5-15 [19]. Actual reservoir conditions may be somewhere between these extremes
because the mobility of the liberated gas is greater than the oil, the gas is
produced at a higher rate, and the oil in the reservoir is in contact with all of
the initial solution gas for only a brief period [20]. Since volatile oil situations
are uncommon [20], many engineers feel the differential liberation process
typifies most reservoir conditions [ 191. For reservoir fluids at the bubblepoint
when a well is put on production, the gas evolved from the oil as the pressure
declines does not flow to the well until the critical gas saturation is exceeded.
Since the greatest pressure drop occurs near the wellbore, the critical gas
saturation occurs first near the well, especially if the pressure drop is large. In
general, differential liberation data is applicable if the reservoir pressure falls
considerably below the bubble-point pressure and the critical gas saturation is
exceeded in the majority of the drainage area, as indicated by producing gas-
oil ratios considerably in excess of the initial solution gas-oil ratio [17]. Flash
liberation data may be applicable to reservoirs where there is only a moderate
pressure decline below the bubblepoint, as indicated by producing gas-oil ratios

Figure 5-15. Comparison of measured and calculated composite oil volume [19].