81
Reservoirs and Reservoir Fluids
The relative proportion of the different kinds of openings varies with the rock type, but pores
usually account for the bulk of the storage space. The effective porosity for oil storage results
from continuously connected openings, which also provide the permeability but, although a rock
must be porous to be permeable there is no simple quantitative relationship between the porosity
and permeability.
Reservoir rocks tend to show far greater variations in permeability than in porosity, and in addition, these two properties, as measured on core samples from reservoir rocks, are not always identical with the values indicated for the rock in bulk underground. The differences arise from the
nonrepresentative nature of cores, especially when there are wide variations in the sizes of the
openings in the rocks and irregularities in their distribution. Porosity is generally in the range of
5%–30%, while permeability is commonly between 0.005 and several Darcys (5 mD to several
thousand milliDarcys) as measured in small samples. It should be noted that pores may be, at best,
only a millimeter or so in width, whereas fossil and solution cavities may sometimes be 30–50 times
wider. Many joints and fractures are probably only a millimeter across, although they may extend
for considerable distances.
However, for the purposes of reservoir definition and understanding reservoir behavior as it
relates to heavy oil recovery, the process involves using date data from a variety of length scales
(pore, core, and reservoir) and sources (laboratory and field) to improve understanding of reservoirs
and their petrophysical properties (Kovscek, 2002).
4.2.2 HeterogeneIty
In addition to understanding the petrophysics of the reservoir, oil recovery requires an understanding of displacement and flow through porous media (Dawe, 2004). However, flow through
porous media is complicated. Inside a reservoir there can be displacements and miscible and/or
immiscible flow, with one two or sometimes three mobile phases (oil, gas, and water) (Grattoni and
Dawe, 2003). Understanding the physics of displacement is important for the correct interpretation of laboratory core data, for the assessment of quantities and position of residual oil, and for
reservoir simulation.
Thus, heterogeneity in the form of layers, lenses, cross-beds, and quadrants can have a profound effect on fluid displacement patterns. Even modest changes in rock permeability give rise
to distortions in displacement profiles, disperse the streamlines, and lower sweep efficiencies and
recovery.
Physically, hydrocarbon reservoirs themselves are complicated geological heterogeneous
bodies. They are not the homogeneous porous media that is often envisaged on paper and used
in calculations. Heterogeneity means that a specific property of interest will vary vertically
and longitudinally within the reservoir (Dawe, 2004) much like the coal in a seam that varies
in composition from one part of the seam to another (Speight, 1994). For example, well log and
core analysis reports show that all reservoirs are heterogeneous with rock properties (such as
porosity and saturation) varying within the reservoir. However, permeability heterogeneities
cause variations in the fluid movements compared to the equivalent homogeneous system. Often,
the effects of heterogeneities are generally not well accounted for at the planning stage of an
operation and only become evident when it may be too late and water has started to be produced
before the predicted time (Dawe, 2004). An understanding of the movement of fluids within such
heterogeneous porous media is therefore fundamental to petroleum production and its efficient
management.
For reservoir studies the heterogeneities that are needed to be studied are those that interfere
with the flow of fluids. Geological variations themselves that do not create flow pattern changes are
not important for flow. The heterogeneities can be permeability or wettability variations (Caruana
and Dawe, 1996a,b; Dawe, 2004). The effects of layer thickness, permeability contrast, angle of
layer to flow direction, mobility ratio, wettability, and flood rate have been examined. Each of these
Reservoirs and Reservoir Fluids
The relative proportion of the different kinds of openings varies with the rock type, but pores
usually account for the bulk of the storage space. The effective porosity for oil storage results
from continuously connected openings, which also provide the permeability but, although a rock
must be porous to be permeable there is no simple quantitative relationship between the porosity
and permeability.
Reservoir rocks tend to show far greater variations in permeability than in porosity, and in addition, these two properties, as measured on core samples from reservoir rocks, are not always identical with the values indicated for the rock in bulk underground. The differences arise from the
nonrepresentative nature of cores, especially when there are wide variations in the sizes of the
openings in the rocks and irregularities in their distribution. Porosity is generally in the range of
5%–30%, while permeability is commonly between 0.005 and several Darcys (5 mD to several
thousand milliDarcys) as measured in small samples. It should be noted that pores may be, at best,
only a millimeter or so in width, whereas fossil and solution cavities may sometimes be 30–50 times
wider. Many joints and fractures are probably only a millimeter across, although they may extend
for considerable distances.
However, for the purposes of reservoir definition and understanding reservoir behavior as it
relates to heavy oil recovery, the process involves using date data from a variety of length scales
(pore, core, and reservoir) and sources (laboratory and field) to improve understanding of reservoirs
and their petrophysical properties (Kovscek, 2002).
4.2.2 HeterogeneIty
In addition to understanding the petrophysics of the reservoir, oil recovery requires an understanding of displacement and flow through porous media (Dawe, 2004). However, flow through
porous media is complicated. Inside a reservoir there can be displacements and miscible and/or
immiscible flow, with one two or sometimes three mobile phases (oil, gas, and water) (Grattoni and
Dawe, 2003). Understanding the physics of displacement is important for the correct interpretation of laboratory core data, for the assessment of quantities and position of residual oil, and for
reservoir simulation.
Thus, heterogeneity in the form of layers, lenses, cross-beds, and quadrants can have a profound effect on fluid displacement patterns. Even modest changes in rock permeability give rise
to distortions in displacement profiles, disperse the streamlines, and lower sweep efficiencies and
recovery.
Physically, hydrocarbon reservoirs themselves are complicated geological heterogeneous
bodies. They are not the homogeneous porous media that is often envisaged on paper and used
in calculations. Heterogeneity means that a specific property of interest will vary vertically
and longitudinally within the reservoir (Dawe, 2004) much like the coal in a seam that varies
in composition from one part of the seam to another (Speight, 1994). For example, well log and
core analysis reports show that all reservoirs are heterogeneous with rock properties (such as
porosity and saturation) varying within the reservoir. However, permeability heterogeneities
cause variations in the fluid movements compared to the equivalent homogeneous system. Often,
the effects of heterogeneities are generally not well accounted for at the planning stage of an
operation and only become evident when it may be too late and water has started to be produced
before the predicted time (Dawe, 2004). An understanding of the movement of fluids within such
heterogeneous porous media is therefore fundamental to petroleum production and its efficient
management.
For reservoir studies the heterogeneities that are needed to be studied are those that interfere
with the flow of fluids. Geological variations themselves that do not create flow pattern changes are
not important for flow. The heterogeneities can be permeability or wettability variations (Caruana
and Dawe, 1996a,b; Dawe, 2004). The effects of layer thickness, permeability contrast, angle of
layer to flow direction, mobility ratio, wettability, and flood rate have been examined. Each of these