85
Reservoirs and Reservoir Fluids
The two commonly used sampling methods are bottomhole sampling and surface sampling.
Bottomhole sampling attempts to capture samples close to reservoir conditions, while surface
sampling aims at capturing gas and oil samples from the separator under stable flow conditions.
Separator fluids then are recombined at a measured producing gas/oil ratio (GOR) to prepare representative reservoir fluid.
In bottomhole sampling operations, adequate cleaning of near-wellbore regions and controlled
drawdown are critical for obtaining uncontaminated representative samples (Witt and Crombie,
1999). Controlled drawdown helps avoid two-phase flow in the reservoir. Downhole fluid analyzers
are used to monitor sample contamination and ensure single-phase flow prior to sample capture.
In surface sampling operations, proper well conditioning with minimum drawdown is the key to
acquiring high-quality samples. Well conditioning requires that the well be flowed at an optimum
rate for an extended period of time with a stable producing gas/oil ratio but it must be recognized
that sample quality are separator efficiency introduce uncertainties in the quality of the fluids.
Modern open-hole wireline formation testers provide the means of recovering representative
samples before the effects of subsequent production take place. Selection of one particular method
over another is influenced by the type of reservoir fluid, the producing characteristics and mechanical condition of the well, the design and mechanical condition of the surface producing equipment,
the relative expense of the various methods, and the safety considerations. Detailed descriptions of
recommended sampling methods are presented in subsequent sections of this document. Factors
that should be considered in choosing a method are also discussed.
While bottomhole sampling has the advantage of capturing fluids at reservoir conditions, surface
sampling operation has a potential for obtaining cleaner samples as a result of large volumes of fluid
production before sampling.
The choice of either the surface or downhole sampling method cannot be considered a simple or
routine matter. Each reservoir usually presents certain constraints or circumstances peculiar to it.
For example, field operation requirements can impose restrictions on the preparation and execution
of a sampling program; sand production or downhole equipment in the well may limit the use of
some of the equipment normally used in the sampling operation. Wells that exhibit rapid variations
in production rate present special problems in making the necessary measurements with acceptable accuracy. Seasonal or daily weather changes can also influence the sampling operation. Thus,
the details of a given sampling procedure often require modification to circumvent local problems.
These modifications are usually made based upon on-the-spot judgments. Some of the modifications that can be made to accommodate special situations are presented herein.
Conditioning a well before sampling is almost always necessary. Initial well testing or normal
production operations often result in the fluid near the wellbore having a composition that has been
altered from that of the original reservoir fluid (for reasons described later). The objective of conditioning the well is to remove this altered (nonrepresentative) fluid. Well conditioning consists of
producing the well at a rate which will move the altered fluid into the wellbore and allow it to be
replaced by unaltered (representative) fluid flowing in from further out in the reservoir. Well conditioning is especially important when the reservoir fluid is at or near its saturation pressure at the
prevailing reservoir conditions because reduction in pressure near the wellbore, which inevitably
occurs from producing the well, will alter the composition of the fluid flowing into the wellbore.
4.4.2 dAtA ACQuIsItIon And QA/QC
The objective of the data-gathering phase is to obtain reliable high-quality data for reservoir evaluation and development. The data requirement depends on the fluid type and the expected development and production strategies (Whitson and Brule, 2000). In addition, some heavy oils require
customized PVT cells and experimental procedures to accelerate the time needed for attaining
equilibrium conditions because of the slow (sometimes nonexistent) gas liberation. On the other
hand, more-complex near-critical fluids and miscible-gas-injection processes need special PVT
Reservoirs and Reservoir Fluids
The two commonly used sampling methods are bottomhole sampling and surface sampling.
Bottomhole sampling attempts to capture samples close to reservoir conditions, while surface
sampling aims at capturing gas and oil samples from the separator under stable flow conditions.
Separator fluids then are recombined at a measured producing gas/oil ratio (GOR) to prepare representative reservoir fluid.
In bottomhole sampling operations, adequate cleaning of near-wellbore regions and controlled
drawdown are critical for obtaining uncontaminated representative samples (Witt and Crombie,
1999). Controlled drawdown helps avoid two-phase flow in the reservoir. Downhole fluid analyzers
are used to monitor sample contamination and ensure single-phase flow prior to sample capture.
In surface sampling operations, proper well conditioning with minimum drawdown is the key to
acquiring high-quality samples. Well conditioning requires that the well be flowed at an optimum
rate for an extended period of time with a stable producing gas/oil ratio but it must be recognized
that sample quality are separator efficiency introduce uncertainties in the quality of the fluids.
Modern open-hole wireline formation testers provide the means of recovering representative
samples before the effects of subsequent production take place. Selection of one particular method
over another is influenced by the type of reservoir fluid, the producing characteristics and mechanical condition of the well, the design and mechanical condition of the surface producing equipment,
the relative expense of the various methods, and the safety considerations. Detailed descriptions of
recommended sampling methods are presented in subsequent sections of this document. Factors
that should be considered in choosing a method are also discussed.
While bottomhole sampling has the advantage of capturing fluids at reservoir conditions, surface
sampling operation has a potential for obtaining cleaner samples as a result of large volumes of fluid
production before sampling.
The choice of either the surface or downhole sampling method cannot be considered a simple or
routine matter. Each reservoir usually presents certain constraints or circumstances peculiar to it.
For example, field operation requirements can impose restrictions on the preparation and execution
of a sampling program; sand production or downhole equipment in the well may limit the use of
some of the equipment normally used in the sampling operation. Wells that exhibit rapid variations
in production rate present special problems in making the necessary measurements with acceptable accuracy. Seasonal or daily weather changes can also influence the sampling operation. Thus,
the details of a given sampling procedure often require modification to circumvent local problems.
These modifications are usually made based upon on-the-spot judgments. Some of the modifications that can be made to accommodate special situations are presented herein.
Conditioning a well before sampling is almost always necessary. Initial well testing or normal
production operations often result in the fluid near the wellbore having a composition that has been
altered from that of the original reservoir fluid (for reasons described later). The objective of conditioning the well is to remove this altered (nonrepresentative) fluid. Well conditioning consists of
producing the well at a rate which will move the altered fluid into the wellbore and allow it to be
replaced by unaltered (representative) fluid flowing in from further out in the reservoir. Well conditioning is especially important when the reservoir fluid is at or near its saturation pressure at the
prevailing reservoir conditions because reduction in pressure near the wellbore, which inevitably
occurs from producing the well, will alter the composition of the fluid flowing into the wellbore.
4.4.2 dAtA ACQuIsItIon And QA/QC
The objective of the data-gathering phase is to obtain reliable high-quality data for reservoir evaluation and development. The data requirement depends on the fluid type and the expected development and production strategies (Whitson and Brule, 2000). In addition, some heavy oils require
customized PVT cells and experimental procedures to accelerate the time needed for attaining
equilibrium conditions because of the slow (sometimes nonexistent) gas liberation. On the other
hand, more-complex near-critical fluids and miscible-gas-injection processes need special PVT