Page 142 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 129

300

Rs (scf/stb) 200 P b
100

500 1000 1500 2000

1.3
Pressure (psia)
1.2
Bo (rb/stb) 1.1 P b

1.0
500 1000 1500 2000

Oil (rb/stb) B ob B
B o oi

Solution Gas R si R si
(scf/stb) R s

Liberated Gas
R - R
(scf/stb) si s 0 0
Figure 6.23 Solution GOR and formation volume factor vs. pressure.

physical properties – composition, density and viscosity. These values are used to
determine the initial volumes of fluid in place in stock tank volumes and the flow
properties of the fluid both in the reservoir and through the surface facilities, and to
identify any components which may require special treatment, such as sulphur
compounds.
Reservoir fluid sampling is usually done early in the field life in order to use the
results in the evaluation of the field and in the process facilities design. Once the
field has been produced and the reservoir pressure changes, the fluid properties will
change as described in the previous section. Early sampling is therefore an
opportunity to collect unaltered fluid samples.
Fluid samples may be collected downhole at near-reservoir conditions, or at
surface. Subsurface samples are more expensive to collect, since they require downhole
sampling tools, but are more likely to capture a representative sample, since they are
targeted at collecting a single-phase fluid. A surface sample is inevitably a two-phase