84
The Chemistry and Technology of Petroleum
for the determination of in-place volumes and recovery-factor calculations as well as for technical
evaluation of reservoir development–depletion plans. Fluid characterization and distribution within
the reservoir help in defining reservoir continuity and communication among various zones.
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.
4.4.1 sAmPlIng metHods
The main objective of a successful sampling campaign is to obtain representative fluid samples for
determining properties and adequate volumes should be collected for analysis, geochemical analysis
for fluid-source identification and reservoir continuity, as well as crude assay for refinery processes
(Speight, 2001, 2002; API, 2003). The sampling program should focus on selecting an appropriate
sampling method and developing sound sampling, sample-transfer, and QC procedures. In addition,
sample character and specific sampling issues should be addressed in the form of a sample history
that details the acquisition, storage, and test carried out on the sample (Speight, 2001, 2002).
Proper management of production from a reservoir can maximize the recovery of the hydrocarbon fluids (gas and oil) originally in the reservoir. Developing proper management strategies
requires accurate knowledge of the characteristics of the reservoir fluid. Practices are recommended
herein for obtaining samples of the reservoir fluid, from which the pertinent properties can be determined by subsequent laboratory tests.
The objective of reservoir fluid sampling is to collect a sample that is representative of the fluid
present in the reservoir at the time of sampling. If the sampling procedure is incorrect or if samples
are collected from an improperly conditioned well, the resulting samples may not be representative of the reservoir fluid. A nonrepresentative sample may not exhibit the same properties as the
reservoir fluid. The use of fluid property data obtained from nonrepresentative samples, however
accurate the laboratory test methods, may result in errors in reservoir management. Poor planning
can also result in incomplete data being taken during the sampling program. Incomplete data can
make it difficult or impossible for laboratory personnel to perform and interpret tests that provide
accurate and meaningful fluid property information.
Reservoir fluids found in gas and oil fields around the world vary greatly in composition. In some
fields, the fluid is in the gaseous state and in others it is in the liquid state; frequently, gas and liquid
coexist in a given reservoir. The rocks that contain these reservoir fluids also vary considerably in
composition as well as in physical and flow properties. In certain cases, this can serve to complicate
the sampling procedure. Other factors, such as producing area, height of the column of hydrocarbon fluid, fracturing or faulting, and water production also serve to distinguish one reservoir from
another. The combination of all these factors affects the choice of sampling methods and preparations for sampling.
When a reservoir is relatively small, a properly taken sample from a single well can be representative of the fluid throughout the entire reservoir. For reservoirs that are large or complex,
samples from several wells and/or depths may be required. Significant variations in fluid composition often occur in very thick formations, in really large reservoirs, or in reservoirs subjected
to recent tectonic disturbances. Additional sampling during the later life of a reservoir is not
uncommon because production experience can show that the reservoir is more complex than
earlier information indicated.
Methods for sampling reservoir fluids fall into two general categories. They are referred to as
subsurface sampling or surface sampling and, as the names imply, each category reflects the location at which the sampling process occurs. Subsurface sampling may also be referred to as downhole or bottomhole sampling.
The Chemistry and Technology of Petroleum
for the determination of in-place volumes and recovery-factor calculations as well as for technical
evaluation of reservoir development–depletion plans. Fluid characterization and distribution within
the reservoir help in defining reservoir continuity and communication among various zones.
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.
4.4.1 sAmPlIng metHods
The main objective of a successful sampling campaign is to obtain representative fluid samples for
determining properties and adequate volumes should be collected for analysis, geochemical analysis
for fluid-source identification and reservoir continuity, as well as crude assay for refinery processes
(Speight, 2001, 2002; API, 2003). The sampling program should focus on selecting an appropriate
sampling method and developing sound sampling, sample-transfer, and QC procedures. In addition,
sample character and specific sampling issues should be addressed in the form of a sample history
that details the acquisition, storage, and test carried out on the sample (Speight, 2001, 2002).
Proper management of production from a reservoir can maximize the recovery of the hydrocarbon fluids (gas and oil) originally in the reservoir. Developing proper management strategies
requires accurate knowledge of the characteristics of the reservoir fluid. Practices are recommended
herein for obtaining samples of the reservoir fluid, from which the pertinent properties can be determined by subsequent laboratory tests.
The objective of reservoir fluid sampling is to collect a sample that is representative of the fluid
present in the reservoir at the time of sampling. If the sampling procedure is incorrect or if samples
are collected from an improperly conditioned well, the resulting samples may not be representative of the reservoir fluid. A nonrepresentative sample may not exhibit the same properties as the
reservoir fluid. The use of fluid property data obtained from nonrepresentative samples, however
accurate the laboratory test methods, may result in errors in reservoir management. Poor planning
can also result in incomplete data being taken during the sampling program. Incomplete data can
make it difficult or impossible for laboratory personnel to perform and interpret tests that provide
accurate and meaningful fluid property information.
Reservoir fluids found in gas and oil fields around the world vary greatly in composition. In some
fields, the fluid is in the gaseous state and in others it is in the liquid state; frequently, gas and liquid
coexist in a given reservoir. The rocks that contain these reservoir fluids also vary considerably in
composition as well as in physical and flow properties. In certain cases, this can serve to complicate
the sampling procedure. Other factors, such as producing area, height of the column of hydrocarbon fluid, fracturing or faulting, and water production also serve to distinguish one reservoir from
another. The combination of all these factors affects the choice of sampling methods and preparations for sampling.
When a reservoir is relatively small, a properly taken sample from a single well can be representative of the fluid throughout the entire reservoir. For reservoirs that are large or complex,
samples from several wells and/or depths may be required. Significant variations in fluid composition often occur in very thick formations, in really large reservoirs, or in reservoirs subjected
to recent tectonic disturbances. Additional sampling during the later life of a reservoir is not
uncommon because production experience can show that the reservoir is more complex than
earlier information indicated.
Methods for sampling reservoir fluids fall into two general categories. They are referred to as
subsurface sampling or surface sampling and, as the names imply, each category reflects the location at which the sampling process occurs. Subsurface sampling may also be referred to as downhole or bottomhole sampling.