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4
Reservoirs and Reservoir Fluids
4.1 INTRODUCTION
Reservoir fluids are the fluids (including gases and solids) that exist in a reservoir (Dawe, 2003). The
fluid type must be determined very early in the life of a reservoir (often before sampling or initial
production) because fluid type is the critical factor in many of the decisions that must be made about
producing the fluid from the reservoir.
Reservoir-fluid properties play a key role in the design and optimization of injection/production
strategies and surface facilities for efficient reservoir management. Inaccurate fluid characterization
often leads to high uncertainties in in-place-volume estimates and recovery predictions, and hence
affects asset value. Indeed, evaluating reservoir fluids is an important aspect of prospect evaluation,
development planning and reservoir management. Although many different types of fluid exist, the
composition of the reservoir fluids provides important information that can influence the recovery
process.
The reservoir fluid data are an important complement to the pressure data and have the advantage of providing a direct measure of reservoir fluid continuity. Many different measurements of
the reservoir fluid are available, both at reservoir and atmospheric conditions, but it has become
increasingly common to apply some type of detailed oil fingerprinting for this purpose. Typically,
the analysis of reservoir pressures and the oil fingerprinting data will give similar results. In some
situations, however, this is not the case and can be a source of confusion in determining the true
nature of the reservoir architecture. When these situations occur, it is often because the processes
that control the reservoir pressures and the timing of their development are different from those controlling the reservoir fluid filling history. In this chapter, we will show several case studies illustrating the complementary nature of reservoir pressure and fluid composition data. We will also discuss
geological settings where these two types of data give divergent views of reservoir continuity and
offer an explanation for this behavior.
Reservoir fluids, including heavy oil, vary greatly in composition. In some fields, the fluid is
in the gaseous state and in others it is in the liquid state but gas and liquid frequently coexist in a
reservoir. The rocks that contain these reservoir fluids also vary considerably in composition and in
physical and flow properties, and 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.
4.2 RESERVOIRS
A reservoir is a subsurface porous permeable rock body or formation that has the capability to
store and transmit fluids: oil, water, and gas. A reservoir is characteristically large and extensive in
volume and capacity and is created by the sequential steps of deposition, conversion, migration, and
entrapment.
Deposition occurs when organic-rich sediment occurring in areas of high primary productivity is deposited and settles due to low energy currents. Such environments include rivers, deltas,
estuaries, and continental shelves. Further deposition of sediment results in build-up of layers of
sediment such as silt or clay on top of the previously organic-rich sediment so as to reduce oxygen
4
Reservoirs and Reservoir Fluids
4.1 INTRODUCTION
Reservoir fluids are the fluids (including gases and solids) that exist in a reservoir (Dawe, 2003). The
fluid type must be determined very early in the life of a reservoir (often before sampling or initial
production) because fluid type is the critical factor in many of the decisions that must be made about
producing the fluid from the reservoir.
Reservoir-fluid properties play a key role in the design and optimization of injection/production
strategies and surface facilities for efficient reservoir management. Inaccurate fluid characterization
often leads to high uncertainties in in-place-volume estimates and recovery predictions, and hence
affects asset value. Indeed, evaluating reservoir fluids is an important aspect of prospect evaluation,
development planning and reservoir management. Although many different types of fluid exist, the
composition of the reservoir fluids provides important information that can influence the recovery
process.
The reservoir fluid data are an important complement to the pressure data and have the advantage of providing a direct measure of reservoir fluid continuity. Many different measurements of
the reservoir fluid are available, both at reservoir and atmospheric conditions, but it has become
increasingly common to apply some type of detailed oil fingerprinting for this purpose. Typically,
the analysis of reservoir pressures and the oil fingerprinting data will give similar results. In some
situations, however, this is not the case and can be a source of confusion in determining the true
nature of the reservoir architecture. When these situations occur, it is often because the processes
that control the reservoir pressures and the timing of their development are different from those controlling the reservoir fluid filling history. In this chapter, we will show several case studies illustrating the complementary nature of reservoir pressure and fluid composition data. We will also discuss
geological settings where these two types of data give divergent views of reservoir continuity and
offer an explanation for this behavior.
Reservoir fluids, including heavy oil, vary greatly in composition. In some fields, the fluid is
in the gaseous state and in others it is in the liquid state but gas and liquid frequently coexist in a
reservoir. The rocks that contain these reservoir fluids also vary considerably in composition and in
physical and flow properties, and 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.
4.2 RESERVOIRS
A reservoir is a subsurface porous permeable rock body or formation that has the capability to
store and transmit fluids: oil, water, and gas. A reservoir is characteristically large and extensive in
volume and capacity and is created by the sequential steps of deposition, conversion, migration, and
entrapment.
Deposition occurs when organic-rich sediment occurring in areas of high primary productivity is deposited and settles due to low energy currents. Such environments include rivers, deltas,
estuaries, and continental shelves. Further deposition of sediment results in build-up of layers of
sediment such as silt or clay on top of the previously organic-rich sediment so as to reduce oxygen