Page 130 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 69

2.6 COMPLETE PVT ANALYSIS

The complete PVT analysis for oil, provided by most laboratories, usually consists of
the following experiments and calculations.

a) Compositional analysis of the separator oil and gas, for samples collected at the
surface, together with physical recombination, refer sec. 2.3(b), or; compositional
analysis of the reservoir fluid collected in a subsurface sample.

Such analyses usually give the mole fractions of each component up to the
hexanes. The hexanes and heavier components are grouped together, and the
average molecular weight and density of the latter are determined.
b) Flash expansion, as described in sec. 2.4 (table 2.1), conducted at reservoir
temperature. This experiment determines

− the bubble point pressure

− the compressibility of the undersaturated oil as
1 dv 1 dB
c =− o =− o (2.13)
o
v o dp B o dp
− the total volume v t of the oil and gas at each stage of depletion.

c) Differential liberation experiment as described in sec. 2.4 to determine

− E, Z, F and v o (as listed in table 2.2), with F and v o measured relative to the
unit volume of bubble point oil.

Alternatively, by measuring c during the last stage of the differential liberation,
b
d
the above data can be presented as
− E, Z, R − R (or just R ) and B (as listed in table 2.5), with R and
o
si
s
s
s
d
d
d
d
d
B measured relative to residual oil volume. In addition, the gas gravity is
o
d
measured at each stage of depletion.
d) Measurement of the oil viscosity at reservoir temperature (generally using the
1,3
rolling ball viscometer ), over the entire range of pressure steps from above
bubble point to atmospheric pressure. Gas viscosities are normally calculated at
reservoir temperature, from a knowledge of the gas gravity, using standard
5
correlations .
e) Separator tests to determine the shrinkage, c , and solution gas oil ratio, R , of
f b si f
unit volume of bubble point oil (1 barrel) when flashed through various separator
combinations (refer table 2.3). Instead of actually performing these tests, in many
cases the results are obtained using the phase equilibrium calculation
1
technique .
f) Composition and gravity of the separator gas in the above separator tests.