Page 142 - PVT Property Correlations

Chapter 6

Volatile Oils

Volatile oils (similar to gas condensates) are usually found in deeper reser-
voirs and higher temperature reservoirs. When volatile oil reservoirs are
found in under-saturated state (initial reservoir pressure is higher than bubble
point pressure), the fluid is essentially oil under reservoir conditions. As with
black oils, when a volatile oil reservoir fluid is produced to surface, gas
(which was originally dissolved in the oil) comes out of solution and
becomes a free gas. There are several noticeable differences between black
oils and volatile oils (McCain, 1993). Fig. 6.1 represents a typical volatile oil
phase diagram.
The differences between black oils and volatile oils are as follows:
In both volatile oil and black oil fluids, the reservoir temperature is less
than the critical temperature. However, the reservoir temperature is closer
to the critical temperature in volatile oil reservoirs than it is in black oil
reservoirs.
The volatile oil reservoirs produce initially with higher producing
gas oil ratio (GOR) than black oil reservoirs.
The stock-tank oil is usually colored in volatile oil fluids, and darker in
black oil fluids.
Iso-volume lines in volatile oil fluids are shifted upward, while in black
oil fluids they may be evenly spaced.
Gas associated with volatile oil is gas condensate, while gas associated
with black oil can usually be approximated with the dry gas concept (or
the wet gas concept, when the amount of solution gas is relatively high).
The proximity of iso-volume lines to each other, in volatile oils, implies
that a small pressure drop in the vicinity of the well will release large
amounts of gas around the wellbore. El-Banbi and McCain (2001), through
detailed compositional simulation, showed that the pressure drop can result
in large compositional and saturation changes around the wellbore. The large
amount of gas released around the wellbore can also reduce the oil relative
permeability and may lead to choking the well with excessive gas.

PVT Property Correlations. DOI: https://doi.org/10.1016/B978-0-12-812572-4.00006-0
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