87
Reservoirs and Reservoir Fluids
data accuracy. In addition, the C7+ components must be characterized accurately and rigorous modeling methods, such as energy minimization, and robust solution techniques are needed to model
near-critical fluids and processes.
Finally, ensuring high-quality data requires routine laboratory visits, evaluation of laboratory procedures and methods, and spot QC as data become available. The QA/QC methods can range from
simple graphical techniques to sophisticated material-balance calculations (Whitson and Brule, 2000).
In summary, reliable compositional-gradient models are needed to capture fluid-property variations in reservoirs with high relief and/or near-critical fluids.
4.5 PHYSICAL COMPOSITION AND MOLECULAR WEIGHT
The physical (bulk) composition of reservoir fluids is a part of the characterization and distribution
of the fluids within the reservoir, which helps in defining reservoir continuity and communication
among various zones. Interpretation of well-test data and the design of surface facilities and processing plants require accurate fluid information and its variation with time. In addition to initial
reservoir-fluid samples, periodic sampling is necessary for reservoir surveillance.
Reservoir fluid characterization consists of several key steps: (1) acquisition of representative samples, (2) identification of reliable service laboratories to perform PVT measurements, (3) implementation of QA/QC procedures to ensure data quality, and (4) development of mathematical models to
capture fluid-property changes accurately as functions of pressure, temperature, and composition. The
fluid type and production processes dictate the type and the volume of required fluid data. This chapter
outlines recommended sampling techniques, PVT-data-acquisition strategies, and modeling methods
and presents field examples covering a wide range of fluid types from heavy oils to lean gas condensates and production processes such as depletion, pressure maintenance, and miscible recovery.
The term physical composition (or bulk composition) refers to the composition of crude oil as
determined by various physical techniques. For example, the separation of petroleum using solvents
and adsorbents (Speight, 2007) into various bulk fractions determines the physical composition of
crude oil. These methods of separation are not always related to chemical properties and the terminology applied to the resulting fractions is often a terminology of convenience.
Proper management of production from a heavy oil reservoir can maximize the recovery of the
oil originally in the reservoir. Developing proper management strategies requires accurate knowledge of the characteristics of the reservoir fluid as long as fluid samples obtained from the reservoir
fluid reflect the pertinent properties of the fluid, as determined by subsequent laboratory tests.
4.5.1 AsPHAltene sePArAtIon
Heavy oil evaluation by separation into various fractions has been used successfully for several
decades. The knowledge of the bulk fractions of heavy oil (Figure 4.1) on a before recovery (core
sample analysis) and after recovery (well fluid analysis) basis, as well as variations over time, has
been a valuable aid to recovery process development.
The asphaltene fraction is that portion of heavy oil feedstock that is precipitated when a large
excess (40 volumes) of a low-boiling liquid hydrocarbon (e.g., n-pentane or n-heptane) is added to
the crude oil (1 volume) (Speight, 1994). n-Heptane is the preferred hydrocarbon with n-pentane still
being used although hexane is used on occasion (Speight, 1994).
Although, n-pentane and n-heptane are the solvents of choice in the laboratory other solvents can
be used (Speight, 1979) and cause the separation of the asphaltene fraction as a brown-to-black powdery solid material. In the refinery, supercritical low molecular weight hydrocarbons (e.g., liquid
propane, liquid butane, or mixtures of both) are the solvents of choice and the product is a semisolid
(tacky) to solid asphalt. The amount of asphalt that settles out of the paraffin/residuum mixture
depends on the size of the paraffin, the temperature, and the paraffin-to-feedstock ratio (Girdler,
1965; Mitchell and Speight, 1973; Speight et al., 1984).
Reservoirs and Reservoir Fluids
data accuracy. In addition, the C7+ components must be characterized accurately and rigorous modeling methods, such as energy minimization, and robust solution techniques are needed to model
near-critical fluids and processes.
Finally, ensuring high-quality data requires routine laboratory visits, evaluation of laboratory procedures and methods, and spot QC as data become available. The QA/QC methods can range from
simple graphical techniques to sophisticated material-balance calculations (Whitson and Brule, 2000).
In summary, reliable compositional-gradient models are needed to capture fluid-property variations in reservoirs with high relief and/or near-critical fluids.
4.5 PHYSICAL COMPOSITION AND MOLECULAR WEIGHT
The physical (bulk) composition of reservoir fluids is a part of the characterization and distribution
of the fluids within the reservoir, which helps in defining reservoir continuity and communication
among various zones. Interpretation of well-test data and the design of surface facilities and processing plants require accurate fluid information and its variation with time. In addition to initial
reservoir-fluid samples, periodic sampling is necessary for reservoir surveillance.
Reservoir fluid characterization consists of several key steps: (1) acquisition of representative samples, (2) identification of reliable service laboratories to perform PVT measurements, (3) implementation of QA/QC procedures to ensure data quality, and (4) development of mathematical models to
capture fluid-property changes accurately as functions of pressure, temperature, and composition. The
fluid type and production processes dictate the type and the volume of required fluid data. This chapter
outlines recommended sampling techniques, PVT-data-acquisition strategies, and modeling methods
and presents field examples covering a wide range of fluid types from heavy oils to lean gas condensates and production processes such as depletion, pressure maintenance, and miscible recovery.
The term physical composition (or bulk composition) refers to the composition of crude oil as
determined by various physical techniques. For example, the separation of petroleum using solvents
and adsorbents (Speight, 2007) into various bulk fractions determines the physical composition of
crude oil. These methods of separation are not always related to chemical properties and the terminology applied to the resulting fractions is often a terminology of convenience.
Proper management of production from a heavy oil reservoir can maximize the recovery of the
oil originally in the reservoir. Developing proper management strategies requires accurate knowledge of the characteristics of the reservoir fluid as long as fluid samples obtained from the reservoir
fluid reflect the pertinent properties of the fluid, as determined by subsequent laboratory tests.
4.5.1 AsPHAltene sePArAtIon
Heavy oil evaluation by separation into various fractions has been used successfully for several
decades. The knowledge of the bulk fractions of heavy oil (Figure 4.1) on a before recovery (core
sample analysis) and after recovery (well fluid analysis) basis, as well as variations over time, has
been a valuable aid to recovery process development.
The asphaltene fraction is that portion of heavy oil feedstock that is precipitated when a large
excess (40 volumes) of a low-boiling liquid hydrocarbon (e.g., n-pentane or n-heptane) is added to
the crude oil (1 volume) (Speight, 1994). n-Heptane is the preferred hydrocarbon with n-pentane still
being used although hexane is used on occasion (Speight, 1994).
Although, n-pentane and n-heptane are the solvents of choice in the laboratory other solvents can
be used (Speight, 1979) and cause the separation of the asphaltene fraction as a brown-to-black powdery solid material. In the refinery, supercritical low molecular weight hydrocarbons (e.g., liquid
propane, liquid butane, or mixtures of both) are the solvents of choice and the product is a semisolid
(tacky) to solid asphalt. The amount of asphalt that settles out of the paraffin/residuum mixture
depends on the size of the paraffin, the temperature, and the paraffin-to-feedstock ratio (Girdler,
1965; Mitchell and Speight, 1973; Speight et al., 1984).