Page 173 - PVT Property Correlations

Chapter 7

Black Oils

Black oils constitute the majority of the oils we produce. They are present in
nearly every basin. The majority of black-oil reservoirs are initially discov-
ered as undersaturated reservoirs (initial pressure is higher than the bubble-
point pressure). Black-oil fluids consist of a large number of hydrocarbon
components from methane to sometimes very high carbon number mole-
cules. They may also include impurities of variable amounts of
nonhydrocarbons.
A typical phase diagram of black oils is shown in Fig. 7.1. In black-oil
phase diagrams, the reservoir temperature is usually far lower than the criti-
cal temperature. The iso-volume lines are spaced approximately evenly
around the reservoir temperature. In addition, the gas that is liberated in the
reservoir at pressures below the bubble point can usually be considered dry
gas for engineering purposes.
According to McCain, oils that produce initially with gas oil ratio
(GOR) less than 1750 scf/STB or have more than 20% C7 1 mole percent
are considered black oils. Oils that produce initially with GOR higher than
that level, or contain less C7 1 than 20%, are considered volatile oils. The
distinction between black oils and volatile oils is not very sharp. For practi-
cal purposes, we divided black oils into two groups (low GOR oils and mod-
erate GOR oils) in Chapter 2, Reservoir-Fluid Classification. The low GOR
oils have distinct characteristics that distinguish them from black oils with
higher GOR.

PVT PROPERTIES FOR BLACK OILS

PVT properties for black oils are required in almost all reservoir, production
and surface facilities calculations to manage and forecast oil reservoir behav-
ior. They also affect calculations that are sensitive to the economic value of
oil reservoirs (e.g., reserves and production forecast).
The engineering of black-oil fluids requires knowledge of multiple PVT
properties including bubble-point pressure, oil formation volume factor, solu-
tion GOR, oil density, oil viscosity, and isothermal compressibility of oil.
The following sections define these properties and comment on their
expected range of values.

PVT Property Correlations. DOI: https://doi.org/10.1016/B978-0-12-812572-4.00007-2
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