Porosity Notes

Porosity

Courtesy J. H. Wittke & T. E. Bunch, 2004-2008

Definition of Porosity
Vv ≡ void volume
VT ≡ total volume
VV
φ≡
≡ Total Porosity
VT

Properties of Real Rocks
That Influence Porosity
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•
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•
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Sorting
Shape
Clay minerals
Cementation
Bound water
Fracturing
Diagenesis
Solution

•

Particle size (only weakly)

S
Pore

Grain
Rg

Courtesy Jon Burger, UH and Chevron

Geological Classifications of Porosity
• Matrix (intergranular)
Between the grains. Think marbles.
• Fracture
Natural and induced

• Vugular

Think of a “vug” as a very small cavern.

• Fenestral

Disconnected small holes in carbonates, often due to biological action. “windows”

• Intragranular
• Intracrystaline
Think of a flaw in a diamond

Courtesy Jon Burger, UH and Chevron

Courtesy Jon Burger, UH and Chevron

Courtesy Jon Burger, UH and Chevron

Courtesy Jon Burger, UH and Chevron

How is porosity formed?
• Sediment grains are transported by water or air • Ensembles of packed sediment grains contain porosity • As sediments are compacted into rocks, the porosity can persist

Engineering Classification of Porosity
• Total Porosity

VV φ≡ VT
• Effective Porosity
Connected porosity that allows fluid flow through the rock

Looks like it might be disconnected and not contribute to “effective” porosity.
Be careful.

Looks like it might be connected and contribute to “effective” porosity.
Be careful.

Courtesy Jon Burger, UH and Chevron

Simple Model of Matrix Porosity
Cubic Packing of Identical Spheres

Unit Cell

Calculate the porosity. φ≡ S
Pore

Grain

Vv
VT

Rg

Vv = VT − Vg where Vg ≡ volume of grains

2D picture to aid 3D calculation

Note: You must define the unit cell in such a way that the entire rock can be constructed from contiguous copies of the unit cell.
However, the unit cell can be moved for computational convenience.

Simple Model of