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Glossary - P - porosity

Used in geology, building science and hydrogeology, the porosity of a porous medium (such as rock or sediment) is the proportion of the non-solid volume to the total volume of material, and is defined by the ratio:

where V_p is the non-solid volume (pores and liquid) and V_m is the total volume of material, including the solid and non-solid parts. Both φ and n are used to denote porosity.

Porosity is a fraction between 0 and 1, typically ranging from less than 0.01 for solid granite to more than 0.5 for peat and clay, although it may also be represented in percent terms by multiplying the fraction by 100%.

Well sorted (grains of approximately all one size) materials have higher porosity than similarly sized poorly sorted materials (where smaller particles fill the gaps between larger particles). The graphic illustrates how some smaller grains can effectively fill the pores (where all water flow takes place), drastically reducing porosity and hydraulic conductivity, while only being a small fraction of the total volume of the material.

Types of porosity

• Primary porosity is the main or original porosity system in a rock or unconfined alluvial deposit.
• Secondary porosity is a subsequent or separate porosity system in a rock, often enhancing overall porosity of a rock. This can be a result of chemical leeching of minerals or the generation of a fracture system. This can replace the primary porosity or coexist with it (see dual porosity below).
• Fracture porosity is porosity associated with a fracture system or faulting. This can create secondary porosity in rocks that otherwise would not be reservoirs for hydrocarbons due to their primary porosity being destroyed (for example due to depth of burial) or of a rock type not normally considered a reservoir (for example igneous intrusions or metasediments).
• Vuggy porosity is secondary porosity generated by dissolution of large features (such as macrofossils) in carbonate rocks leaving large holes, vugs, or even caves.
• Effective porosity (also called open porosity) refers to the fraction of the total volume in which fluid flow is effectively taking place (this excludes dead-end pores or non-connected cavities). This is very important in solute transport.
• Dual porosity refers to the conceptual idea that there are two overlapping reservoirs which interact. In fractured rock aquifers, the rock mass and fractures are often simulated as being two overlapping but distinct bodies. Delayed yield, and leaky aquifer flow solutions are both mathematically similar solutions to that obtained for dual porosity; in all three cases water comes from two mathematically different reservoirs (whether or not they are physically different).

Grain properties

Both porosity and permeability depend on grain properties:

A	B
C	D

• A - interlocking grains - ingeous rocks, metamorphics rocks, chemical cementation - very low porosity
• B - well sorted coarse grains - high porosity
• C - poorly sorted grains - lower porosity
• D - well sorted but packed plate shaped grains

Pore sizes depend on grain sizes:

For more detailed information - see Wikipedia