Page 109 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 48

The producing gas oil ratio can be split into two components as shown in fig. 2.3, i.e.

R = R s +(R−R s)

The first of these, R s scf/stb, when taken down to the reservoir with the one stb of oil,
will dissolve in the oil at the prevailing reservoir pressure to give B o rb of oil plus
dissolved gas. The remainder, (R − R s) scf/stb, when taken down to the reservoir will
occupy a volume

scf rb
(R R ) × B g = (R R ) B g (rb. free gas / stb) (2.1)
−
−
−
s
s
stb scf
and therefore, the total underground withdrawal of hydrocarbons associated with the
production of one stb of oil is
(Underground withdrawal)/stb = B o + (R − R s) B g (rb/stb) (2.2)

The above relationship shows why the gas formation volume factor has the rather
unfortunate units of rb/scf. It is simply to convert gas oil ratios, measured in scf/stb,
directly to rb/stb to be compatible with the units of B o. While B g is used almost
exclusively in oil reservoir engineering its equivalent in gas reservoir engineering is E,
the gas expansion factor, which was introduced in the previous chapter and has the
units scf/rcf. The relation between B g and E is therefore,

rb 1
B g = (2.3)
scf 5.615E

thus B g has always very small values; for a typical value of E of, say, 150 scf/rcf the
value of B g would be .00119 rb/scf.