58
3 PARTICLES, PORES, AND PERMEABILITY
The porosity of rocks ranges from effectively zero in unfractured cherts to, theoretically, 100% if the "sample" is taken in a cave. Typically porosities in sediments range
between 5 and 25 %, and porosities of 25-35 % are regarded as excellent if found in an
aquifer or oil reservoir.
An important distinction must be made between the total porosity of a rock and its effective porosity. Effective porosity is the amount of mutually interconnected pore space
present in a rock. It is, of course, the effective porosity that is generally economically
important, and it is effective porosity that is determined by many, but not all, methods
of porosity measurement. The presence of effective porosity gives a rock the property
of permeability. Permeability is the ability of a fluid to flow through a porous solid. Permeability is controlled by many variables. These include the effective porosity of the
rock, the geometry of the pores, including their tortuosity, and the size of the throats
between pores, the capillary force between the rock and the invading fluid, its viscosity,
and pressure gradient.
Permeability is conventionally determined from Darcy's law using the equation:
Q
KAA
/z.L'
where Q is the rate of flow in cubic centimeters per second, A is the pressure gradient,
A is the cross-sectional area,/z is the fluid viscosity in centipoises, L is the length, and
K is the permeability. Figure 3.12 illustrates the system for measuring the permeability
of a rock sample.
This relationship was originally discovered by H. Darcy in 1856 following a study of
the springs of Dijon, France. Permeability is usually expressed in darcy units, a term
proposed and defined by Wycoff et al. in 1934. One darcy, is the permeability which
allows a fluid of one centipoise viscosity to flow at one centimeter per second, given a
P1
Fluid of
viscosity,
Cross~
area, A
P2
~Rate of
flow, O
Fig. 3.12. Illustration showing how permeability is measured for a rock specimen. A fluid of viscosity ~ is
passed through a sample of known cross-sectional area A and length L. The rate of flow is measured, together
with the pressure differential recorded on gauges at either end of the sample. Permeability is then calculated
according to Darcy's law, as described in the text.
3 PARTICLES, PORES, AND PERMEABILITY
The porosity of rocks ranges from effectively zero in unfractured cherts to, theoretically, 100% if the "sample" is taken in a cave. Typically porosities in sediments range
between 5 and 25 %, and porosities of 25-35 % are regarded as excellent if found in an
aquifer or oil reservoir.
An important distinction must be made between the total porosity of a rock and its effective porosity. Effective porosity is the amount of mutually interconnected pore space
present in a rock. It is, of course, the effective porosity that is generally economically
important, and it is effective porosity that is determined by many, but not all, methods
of porosity measurement. The presence of effective porosity gives a rock the property
of permeability. Permeability is the ability of a fluid to flow through a porous solid. Permeability is controlled by many variables. These include the effective porosity of the
rock, the geometry of the pores, including their tortuosity, and the size of the throats
between pores, the capillary force between the rock and the invading fluid, its viscosity,
and pressure gradient.
Permeability is conventionally determined from Darcy's law using the equation:
Q
KAA
/z.L'
where Q is the rate of flow in cubic centimeters per second, A is the pressure gradient,
A is the cross-sectional area,/z is the fluid viscosity in centipoises, L is the length, and
K is the permeability. Figure 3.12 illustrates the system for measuring the permeability
of a rock sample.
This relationship was originally discovered by H. Darcy in 1856 following a study of
the springs of Dijon, France. Permeability is usually expressed in darcy units, a term
proposed and defined by Wycoff et al. in 1934. One darcy, is the permeability which
allows a fluid of one centipoise viscosity to flow at one centimeter per second, given a
P1
Fluid of
viscosity,
Cross~
area, A
P2
~Rate of
flow, O
Fig. 3.12. Illustration showing how permeability is measured for a rock specimen. A fluid of viscosity ~ is
passed through a sample of known cross-sectional area A and length L. The rate of flow is measured, together
with the pressure differential recorded on gauges at either end of the sample. Permeability is then calculated
according to Darcy's law, as described in the text.