Page 32 - PVT Property Correlations

Chapter 2

Reservoir-Fluid Classification

Petroleum reservoir fluids are hydrocarbons that can exist as gases (hydrocar-
bon vapor phase) or oils (hydrocarbon liquid phase). If the reservoir tempera-
ture is higher than the critical temperature of the fluid, the reservoir fluid is
gas. Otherwise, the fluid is oil. Although every fluid has a unique set of char-
acteristics, it is useful to distinguish fluids in terms of typical behavior groups.
Engineers are interested in classifying reservoir-fluid types for practical
reasons: reservoir-fluid classification can affect decisions as to fluid sam-
pling, production, and laboratory PVT experiments. Categorizing fluids into
distinct groups according to similarity in behavior facilitates the use of PVT
models to describe phase behavior and predict fluid properties. More impor-
tantly, fluid types affect the choice of model to apply for reservoir behavior
prediction (e.g., material balance or reservoir simulation models). Several
qualitative (but relatively few quantitative) classifications of reservoir fluids
have been suggested.

REVIEW OF RESERVOIR-FLUID CLASSIFICATIONS

Moses (1986) classified oil fluids into ordinary oils (also called black oils)
and near-critical or volatile-oils. Black oils are characterized by gas oil ratio
(GOR) up to 2000 scf/STB, stock-tank oil gravities up to 45 API, and oil-

formation volume factor of less than 2.0 rbbl/STB; volatile oils usually have
GOR of 2000 3000 scf/STB, stock-tank oil gravity is usually 40 API or

higher, and formation volume factor is above 2.0 rbbl/STB. C7 1 for vola-
tile oils is between 12.5% and 20%. Noting the lack of sharp division
between black oils and volatile oils, Moses highlighted the difficulty of clas-
sifying the oil as black oil or volatile oil outside of a laboratory setting.
Retrograde condensate systems are characterized by GOR of 3000 to

150,000 scf/STB and stock-tank oil gravities between 40 and 60 API. The
color of stock-tank fluid cannot be used to distinguish fluid type.
McCain (1994b) classified reservoir fluids into five categories, relying
primarily on the criteria of Moses (1986) to distinguish black oils, volatile
oils, and gas condensates, and using C7 1 mole % and initial producing
GOR for wet gases and dry gases. Other criteria (e.g., stock-tank oil API
gravity and color of stock-tank fluid) are not decisive in classifying the fluid
type. Wet gases are those which produce GOR higher than 15,000 scf/STB

PVT Property Correlations. DOI: https://doi.org/10.1016/B978-0-12-812572-4.00002-3
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