Page 171 - Well Logging and Formation Evaluation

Reservoir Engineering Issues 161

and that the drawdown is such that samples are taken above the bubble
point.
A fundamental problem with PVT sampling and measurement is the
fact that the end point of a process whereby the gas is liberated from oil
is dependent on whether the gas is kept in close proximity with the oil
during the liberation or is removed. In a reservoir, the gas typically
remains in contact with the oil and will follow the pattern of behavior as
indicated by the pressure-temperature (P-T) diagram. However, during
production in a well, the gas will become isolated from the oil. This iso-
lation changes the phase behavior of the oil left behind, modifying the
P-T diagram. Hence, unless any experiment models the true processes
undergone by the gas and oil during migration to the wellbore, produc-
tion, and gas/oil separation at surface, it cannot accurately predict the
actual production of gas and oil from a field.
The parameters that are measured during PVT analysis consist of the
following:

• The oil formation volume factor (B o). B o is defined as the volume of
reservoir liquid required to produce one volume unit of stock tank oil
(i.e., oil at standard surface conditions). Depending on how “gassy” the
oil is (i.e., how high the gas/oil ratio [GOR] is), B o typically varies
between 1.0 and 1.5.
• The gas formation volume factor (B g). B g is the reservoir volume of
one volume unit of gas at standard conditions. Rather confusingly,
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3
while in SI units B g is normally given in reservoir m /standard m , in
field units it is usually quoted as reservoir barrels/standard cubic feet.
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Typical range (for SI units) is 0.004–0.06r.m /st.m .
• The solution gas/oil ratio (R s ). R s is defined as the volume units of gas
that evolve from B o reservoir volume units of oil when the oil is trans-
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ported to surface conditions. It has the units of standard m gas/stan-
3
dard m oil, or standard cubic feet/standard stock tank barrel (stb).
Where there is no free gas being produced from the reservoir, this is
the same as the GOR. However, GOR is a term that is used to describe
well production behavior, unlike R s , which is essentially a laboratory
measurement.

If the reservoir pressure falls below the bubble point, free gas will be
produced in the reservoir. Since the gas is more mobile than the oil, this
will lead to a dramatic rise in the gas produced at surface, although in