Page 27 - PVT Property Correlations

8 PVT Property Correlations

reservoir oil phase and how much comes from the reservoir free gas phase is
accomplished through R s function as in black-oil models. Tracking the
amount of surface oil (stock-tank oil) that comes from the reservoir gas
phase is accomplished through the R v function.
For gas condensate systems, R v is the same as surface condensate gas
ratio since the condensate dropout in the reservoir is usually assumed to be
immobile and will not be produced. For volatile oil systems, R v is the recip-
rocal of R s above the bubble point. Below the bubble point pressure, the res-
ervoir oil produces surface oil and surface gas. B o tracks the relation of the
volume of reservoir oil and the surface oil from the reservoir oil. R s tracks
the gas that comes out of the reservoir oil. In addition, the free gas that is
formed in the reservoir below bubble point pressure produces both conden-
sate (oil) and gas on surface, since it is gas condensate fluid. B g tracks the
relation between the gas volume in the reservoir and on surface. R v tracks
the liquid volume on surface to the surface gas produced (both these quanti-
ties are generated from the reservoir gas).
Like black-oil models, MBO models assume that surface oil and gas
maintain the same composition at any pressure. For most reservoir and pro-
duction engineering calculations, MBO models are adequate for handling
volatile oil and gas condensate calculations under depletion and secondary
recovery (El-Banbi et al., 2000; Fevang et al., 2000). Processes that result in
significant compositional changes are not suitable to be modeled with the
MBO approach.

Compositional Models

Compositional PVT models track the surface production of hydrocarbon
components. The components could be hydrocarbon components, impurities
(i.e., methane, ethane, hydrogen sulfide, carbon dioxide, etc.), or pseudo
(lumped or grouped) components. The main assumption in compositional
models is that the components (or pseudocomponents) can come from both
the reservoir gas and oil phases. In most compositional models, surface-
produced water still comes from the reservoir water phase. The partitioning
of each hydrocarbon component between reservoir gas and oil phases is han-
dled with the use of equilibrium constants (K values). Equilibrium constants
for any component are defined as

Y i
K i 5 ð1:5Þ
X i
Equilibrium constants are functions of pressure, temperature, and compo-
sition. Correlations exist to calculate K values. However, the most accurate
source of K values is tuned EOS models. In general, the application of com-
positional PVT models requires availability of laboratory-measured PVT
experiments and construction of EOS models. Fig. 1.6 schematically depicts