Page 25 - PVT Property Correlations

6 PVT Property Correlations

Hydrocarbon vapor Hydrocarbon liquid
Water phase
phase (gas phase) phase (oil phase)
Surface gas X X
Stock-tank oil (surface oil) X
Surface water X

FIGURE 1.4 Relation of produced fluids on surface and reservoir phases for black-oil models.

produced oil (stock-tank oil) and produced gas on surface to remain constant.
Despite the fact that, for most fluids, composition of produced fluids can
change when the reservoir pressure declines below the saturation pressure
(bubble point pressure for oils and dew point pressure for gases), the black-
oil model can adequately model many fluid types under reservoir processes
such as depletion and secondary recovery (e.g., water and casing-head gas
injection).
Fig. 1.4 is a schematic representation of the assumptions of the black-oil
model. Most production, reservoir, and surface facilities calculations deal
with surface fluids measured in the field (e.g., stock-tank oil, surface gas,
and surface water). The traditional black-oil model assumes that surface gas
comes from either the reservoir gas phase or reservoir oil phase, or both. Gas
exists in the reservoir gas phase as free gas, while it exists in the reservoir
oil phase as gas in solution. The black-oil model also assumes that the oil
produced on surface (stock-tank oil) comes from the reservoir oil phase; the
water produced on surface comes from the reservoir water phase.
For most calculations, the oil PVT properties required to describe phase
behavior and PVT properties changes for the black-oil model include solu-
tion gas oil ratio, oil formation volume factor, and oil viscosity. The gas
PVT properties include gas z-factor or gas formation volume factor and gas
viscosity. Partitioning of the produced gas between reservoir gas phase and
reservoir oil phase is accomplished through the solution gas oil ratio func-
tion (R s ). All properties are defined in the respective chapters. The techni-
ques to obtain these properties are also reviewed in different chapters.
Variations of the traditional black-oil model can be considered in certain
cases to account for gas solubility in water and water vaporization in gas.
These variations of the traditional black-oil model usually have limited
applications.
The black-oil model is adequate in modeling phase behavior and PVT
properties for low gas oil ratio oil, black-oil, dry gas, and wet gas fluids.
The black-oil model is sufficient for most reservoir and production engineer-
ing applications (e.g., material balance, reserves estimation, conventional
and advanced decline curve analysis, pressure transient analysis, reservoir