Page 112 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 51

Mass of 1 stb of oil Mass of B o rb of oil
+ = +

R s scf dissolved gas dissolved gas in the
at standard conditions reservoir

or

lb lb
1(stb) ρ× o sc × 5.615 + R (scf) ρ× g sc

s
cu.ft
cu.ft
lb
×
= B (rb) ρ r o × 5.615
o
cu.ft
in which the subscripts sc and r refer to standard conditions and reservoir
conditions, respectively.

The gas density at standard conditions is

ρ sc = γ g × 0.0763 (refer equ. (1.30))

= 0.0511 lb/cu ft

Therefore,

(ρ o × 5.615) (R × ρ g )
+
s
ρ = sc sc
r o
B × 5.615
o
(52.8 5.615) (352 0.0511)
×
×
+
= = 47.37 lb / cu ft
1.1822 5.615
×
and the liquid oil gradient is 47.37/144 = 0.329 psi/ft.
2.3 COLLECTION OF FLUID SAMPLES
Samples of the reservoir fluid are usually collected at an early stage in the reservoir's
producing life and dispatched to a laboratory for the full PVT analysis. There are
basically two ways of collecting such samples, either by direct subsurface sampling or
by surface recombination of the oil and gas phases. Whichever technique is used the
same basic problem exists, and that is, to ensure that the proportion of gas to oil in the
composite sample is the same as that existing in the reservoir. Thus, sampling a
reservoir under initial conditions, each stock tank barrel of oil in the sample should be
combined with R si standard cubic feet of gas.
a) Subsurface sampling

This is the more direct method of sampling and is illustrated schematically in fig. 2.6.