A mechanical property of solids, a measure of their ability to allow the passage of a fluid. The narrow channels that make a material porous allow it to absorb fluid via capillarity, as a sponge absorbs water. Oil exists underground in porous rock; water drains through sand.
The term "porosity" is used in multiple fields including manufacturing, earth sciences and construction.
Porosity in earth sciences and construction
Used in geology, hydrogeology, soil science, and building science, the porosity of a porous medium (such as rock or sediment) describes how densely the material is packed. It is the proportion of the non-solid volume to the total volume of material, and is defined by the ratio:
where Vp is the non-solid volume (pores and liquid) and Vm is the total volume of material, including the solid and non-solid parts.
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%.
The porosity of a rock, or sedimentary layer, is an important consideration when attempting to evaluate the potential volume of hydrocarbons it may contain. One commonly used relationship between porosity and depth is given by the Athy (1930) equation:
where φ0 is the surface porosity, k is the compaction coefficient (m−1) and z is depth (m).
A value for porosity can be calculated from the bulk density and particle density.
Porosity and hydraulic conductivity
Porosity is indirectly related to hydraulic conductivity; for two similar sandy aquifers, the one with a higher porosity will typically have a higher hydraulic conductivity (more open area for the flow of water), but there are many complications to this relationship. Clays, which typically have very low hydraulic conductivity also have very high porosities (due to the structured nature of clay minerals), which means clays can hold a large volume of water per volume of bulk material, but they do not release water very quickly.
Sorting and porosity
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. For tables of common porosity values for earth materials, see the "further reading" section in the Hydrogeology article.
Porosity of rocks
Consolidated rocks (e.g. The rock itself may have a certain (low) porosity, and the fractures (cracks and joints), or dissolution features may create a second (higher) porosity.
Porosity of soil
Porosity of surface soil typically decreases as particle size increases. This seems counterintuitive because clay soils are termed heavy, implying lower porosity.
Porosity of subsurface soil is lower than in surface soil due to compaction by gravity. Porosity of 0.20 is considered normal for unsorted gravel size material at depths below the biomantle. Porosity in finer material below the aggregating influence of pedogenesis can be expected to approximate this value.
Soil porosity is complex.
Types of geologic porosities
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 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). Delayed yield, and leaky aquifer flow solutions are both mathematically similar solutions to that obtained for dual porosity;Porosity in manufacturing
In manufacturing of metal or plastic parts and assemblies, porosity in the raw material is a serious issue affecting the quality of the resulting products. Porosity may be caused by temperature control problems, material impurities, or other causes in the casting of metal or plastic parts. Porosity internal to cast parts may become external or surface pores when material is then removed from the raw part material by machining, grinding or other manufacturing operations. Detection of surface porosity requires the use of some form of 3-dimensional high-definition metrology, because pores of concern may be as small as 100 microns in diameter (roughly the diameter of an average human hair) and may occur anywhere on the surface of a part.
Measuring porosity
There are several ways to estimate the porosity of a given material or mixture of materials, which is called your material matrix.
The volume/density method is fast and surprisingly accurate (normally within 2 % of the actual porosity). To do this method you pour your material into a beaker, cylinder or some other container of a known volume. Then weigh your container full of this material, so you can subtract the weight of the container to know just the weight of just your material. The weight of your material divided by the density of your material gives you the volume that your material takes up, minus the pore volume. If you have a different material, you may look up its density) So, the pore volume is simply equal to the total volume minus the material volume, or more directly (pore volume) = (total volume) - (material volume). Again, take a known volume of your material and also a known volume of water. (Make sure the beaker or container is large enough to hold your material as well.) Slowly dump your material into the water and let it saturate as you pour it in. The total volume of the water originally in the beaker minus the amount of water not saturated is the volume of the pore space, or again more directly (pore volume) = (total volume of water) - (unsaturated water). Take a fully saturated, known volume of your material with no excess water on top. Since the density of water is 1 g/cm3, the difference of the weights of the saturated versus the dried sample is equal to the volume of the water removed from the sample (assuming you are measuring in grams), which is exactly the pore volume. So once again, (pore volume in cubic centimeters) = (weight of saturated sample in grams) - (weight of dried sample in grams). Nitrogen gas adsorption is used to determine fine porosity in materials such as charcoal.
User Comments Add a comment…