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Porosity and Permeability

The pore space of an aquifer is the spaces or voids between the solid material. The porosity of the aquifer is the volume of void space to the total volume, typically expressed as a percentage.

Effective porosity is the space available for fluid to flow, and is calculated as the volume of interconnected void space to the total volume, typically given as a percentage.

Porosity can be deemed either primary or secondary. Void spaces in primary porosity formed at the time the geologic material was created. Void spaces in secondary porosity formed after the rock was created. Examples of secondary porosity include fractures, solution-created channel ways, etc.

Porosity is highly dependent upon the arrangement, the shape, and the size of the material. However the diameter of the grain size does not affect porosity, since it is the ratio of open space volume to total volume. For example, a box full of soccer balls and a box full of tennis balls will have the same porosity, given they are packed the same way. To help illustrate this, see the following example:

As mentioned, packing of the grains also affects the porosity of a sample:

The arrangement of various grains is called the degree of sorting and is illustrated below. Well-sorted samples typically have higher portosiy than poorly sorted samples. Poorly sorted samples contain grains that tend to fill in the void spaces.

Angularity can also affect porosity. Samples with angular grains tend to have lower porosity than well rounded grains, especially of similar sizes:

The porosity of rocks and unconsolidated sediments vary. Some examples include (Source: Cheery and Freeze, 1979):

Rocks Porosity (%)
Fractured basalt 0.05 - 0.50
Karst limestone 0.05 - 0.50
Sandstone 0.05 - 0.30
Limestone, dolomite 0.00 - 0.20
Shale 0.00 - 0.10
Fractured crystalline rock 0.00 - 0.10
Dense crystalline rock 0.00 - 0.05

Unconsolidated Deposits
Gravel 0.25 - 0.40
Sand 0.25 - 0.50
Silt 0.35 - 0.50
Clay 0.40 - 0.70

Permeability is the ease in which water flows through the rock's pore structure. A rock may be highly porous, but if the pore spaces are not connected, it is not permeable. The following diagram represents hypothetical flow paths through a porous media.