Page 106 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 45

barrel of oil can be produced together with a volume of gas that greatly exceeds the
volume originally dissolved per barrel of oil above bubble point pressure.

Control in relating surface volumes of production to underground withdrawal is gained
by defining the following three PVT parameters which can all be measured by
laboratory experiments performed on samples of the reservoir oil, plus its originally
dissolved gas.

R s − The solution (or dissolved) gas oil ratio, which is the number of standard
cubic feet of gas which will dissolve in one stock tank barrel of oil when
both are taken down to the reservoir at the prevailing reservoir pressure
and temperature (units − scf. gas/stb oil).

B o − The oil formation volume factor, is the volume in barrels occupied in the
reservoir, at the prevailing pressure and temperature, by one stock tank
barrel of oil plus its dissolved gas (units – rb (oil + dissolved gas)/stb oil).

B g − The gas formation volume factor, which is the volume in barrels that one
standard cubic foot of gas will occupy as free gas in the reservoir at the
prevailing reservoir pressure and temperature (units − rb free gas/ssf gas).

Both the standard cubic foot (scf) and the stock tank barrel (stb) referred to in the
above definitions are defined at standard conditions, which in this text are taken as
60°F and one atmosphere (14.7 psia). It should also be noted that R s and B o are both
measured relative to one stock tank barrel of oil, which is the basic unit of volume used
in the field. All three parameters are strictly functions of pressure, as shown in fig. 2.5,
assuming that the reservoir temperature remains constant during depletion.

Precisely how these parameters can be used in relating measured surface volumes to
reservoir volumes is illustrated in figs. 2.2 and 2.3.

solution gas R scf / stb
si
p i
p
P +

T 1 stb oil
Phase diagram

rb ( oil + dissolved gas) / stb
B o
Fig. 2.2 Application of PVT parameters to relate surface to reservoir hydrocarbon
volumes; above bubble point pressure.