Page 33 - PVT Property Correlations

14 PVT Property Correlations

[condensate gas ratio (CGR) of less than 67 STB/MMscf] and C7 1 of less
than 4%. Dry gases are those with initial producing GOR of more than
100,000 scf/STB (CGR of less than 10 STB/MMscf) and C7 1 mole % of
less than 0.7%. The distinction between gas condensate behavior and wet gas
behavior rests on the shape of two-phase z-factor. The behavior of the five
reservoir fluids is examined in a series of papers (McCain, 1993a,b, 1994a;
McCain and Bridges, 1994; McCain and Piper, 1994).
Whitson and Brule (2000) suggest a classification of five fluid types:
(1) dry gases, (2) wet gases, (3) gas condensates, (4) volatile oils, and
(5) black oils. This study used C7 1 mole % to distinguish between fluid
types according to the following criteria: gas condensates have C7 1 mole
% less than 12.5%, volatile oils from 12.5% to 17.5%, and black oils greater
than 17.5%. GOR for gas condensates is from 3000 to 150,000 scf/STB
(which is equivalent to oil-gas ratios from about 350 to 5 STB/MMscf). The
API of stock-tank oil for gas condensate fluids typically ranges between 40

and 60 API. It is noted in the study that although the color of stock-tank oil
lightens from volatile oils to gas condensates, color is not a reliable criterion
to distinguish reservoir-fluid types.
Pederson et al. (2015) also classify reservoir fluids into five categories:
(1) natural gas mixtures, (2) gas condensate mixtures, (3) near-critical mix-
tures or volatile oils, (4) black oils, and (5) heavy oils. Near-critical mixtures
are defined as fluids with reservoir temperature close to the critical tempera-
ture of the fluid. They may be volatile oils or gas condensates, depending on
the proximity of reservoir temperature to critical temperature. For near-
critical fluids, the composition and properties of the gas and liquid inside the
two-phase region are similar. The change in composition and properties is
rapid with the decline in pressure below saturation pressure.
Ahmed (2016) discusses additional categories in the classification of
petroleum reservoir fluids. This classification depends on composition of
fluid, location of reservoir temperature relative to critical fluid temperature,
initial reservoir pressure and temperature, and pressure and temperature of
surface production. Four types of oils can be recognized: (1) ordinary black
oil, (2) low-shrinkage crude oil, (3) high-shrinkage (volatile) crude oil, and
(4) near-critical crude oil. Ahmed identifies initial GOR as the most impor-
tant distinguishing criterion of reservoir-fluid type. Color of stock-tank oil is
not a defining criterion. Gas reservoirs are divided into four categories as fol-
lows: (1) retrograde gas condensates, (2) near-critical gas condensate reser-
voirs, (3) wet gas reservoirs, and (4) dry gas reservoirs. Ahmed (2016)
provided GOR limits for each category.
The criteria of McCain (1994b), offering practical and quantified guide-
lines to define the fluid type early in the life of the reservoir, seem to be the
most widely applied for selection of reservoir-fluid type. Some modifications
of these criteria are suggested here for practical purposes.