Porosity is a measure of the void spaces in a material, and is measured as a fraction, between 0–1, or as a percentage between 0–100%. In Mathematics, a percentage is a way of expressing a number as a Fraction of 100 ( per cent meaning "per hundred" 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 the fraction of void space in the material, where the void may contain, for example, air or water. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit Hydrogeology ( hydro- meaning water and -geology meaning the study of the Earth) is the area of Geology that deals with the distribution and Soil science is the study of Soil as a Natural resource on the surface of the Earth including soil formation, classification and mapping Building science is the collection of scientific knowledge that focuses on the analysis and control of the physical phenomena affecting buildings A porous medium or a porous Material is a Solid (often called frame or matrix permeated by an interconnected network of pores (voids filled with a In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of It is defined by the ratio:

where V_V is the volume of void-space (such as fluids) and V_T is the total or bulk volume of material, including the solid and void components. A ratio is an expression which compares quantities relative to each other Both the mathematical symbols 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. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. 5 for peat and clay, although it may also be represented in percent terms by multiplying the fraction by 100. Peat is an accumulation of partially Decayed Vegetation matter. Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and

The porosity of a rock, or sedimentary layer, is an important consideration when attempting to evaluate the potential volume of water or hydrocarbons it may contain. Water is a common Chemical substance that is essential for the survival of all known forms of Life. In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. Sedimentary porosities are a complex function of many factors, including but not limited to: rate of burial, depth of burial, the nature of the connate fluids, the nature of overlying sediments (which may impede fluid expulsion). 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 alternatively be calculated from the bulk density ρ_bulk and particle density ρ_particle:

Normal particle density is assumed to be approximately 2. Bulk density is a property of powders granules and other "divided" solids especially used in reference to Soil. The particle density or true density of a Particulate solid or powder is the density of the particles that make up the powder in contrast to the Bulk density 65 g/cm³, although a better estimation can be obtained by examining the lithology of the particles. In Geology, petrology (from Greek πέτρα petra, rock and λόγος logos, knowledge is the study of rocks and the conditions on which

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. Hydraulic conductivity, symbolically represented as K is a property of vascular plants soil or rock that describes the ease with which water can move through pore spaces Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles 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. Clay minerals are hydrous Aluminium phyllosilicates, sometimes with variable amounts of Iron, Magnesium, Alkali metals Alkaline

Sorting and porosity

Effects of sorting on alluvial 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). Alluvium (from the Latin, alluvius, from alluere, "to wash against" is Soil or Sediments deposited by a river or other running 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. Hydrogeology ( hydro- meaning water and -geology meaning the study of the Earth) is the area of Geology that deals with the distribution and

Porosity of rocks

Consolidated rocks (e. g. sandstone, shale, granite or limestone) potentially have more complex "dual" porosities, as compared with alluvial sediment. Sandstone is a Sedimentary rock composed mainly of Sand -size Mineral or rock grains. Shale (also called mudstone) is a fine-grained Sedimentary rock whose original constituents were Clay minerals or Muds It is characterized by Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 The rock itself may have a certain (low) porosity, and the fractures (cracks and joints), or dissolution features may create a second (higher) porosity. The interaction of these porosities is complex and often makes simple models highly inaccurate.

Porosity of soil

Porosity of surface soil typically decreases as particle size increases. This is due to soil aggregate formation in finer textured surface soils when subject to soil biological processes. Soil life or soil biota is a collective term for all the organisms living within the soil Aggregation involves particulate adhesion and higher resistance to compaction. Typical bulk density of sandy soil is between 1. 5 and 1. 7 g/cm³. This calculates to a porosity between 0. 43 and 0. 36. Typical bulk density of clay soil is between 1. 1 and 1. 3 g/cm³. This calculates to a porosity between 0. 58 and 0. 51. This seems counterintuitive because clay soils are termed heavy, implying lower porosity. Heavy apparently refers to a gravitational moisture content effect in combination with terminology that harkens back to the relative force required to pull a tillage implement through the clayey soil at field moisture content as compared to sand. Tillage is the agricultural preparation of the Soil by Ploughing ripping or turning it

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. In Oceanography and Limnology, bioturbation is the displacement and mixing of Sediment particles by benthic Fauna (animals or Porosity in finer material below the aggregating influence of pedogenesis can be expected to approximate this value. Pedogenesis or soil evolution (formation is the process by which Soil is created

Soil porosity is complex. Traditional models regard porosity as continuous. This fails to account for anomalous features and produces only approximate results. Furthermore it cannot help model the influence of environmental factors which affect pore geometry. A number of more complex models have been proposed, including fractals, bubble theory, cracking theory, Boolean grain process, packed sphere, and numerous other models. A fractal is generally "a rough or fragmented geometric shape that can be split into parts each of which is (at least approximately a reduced-size copy of the whole" See also Characterisation of pore space in soil. Soil is essential to most life on the Earth. It is a relatively thin crust where an even smaller portion contains much of the Biological activity.

Types of geologic porosities

Primary porosity

The main or original porosity system in a rock or unconfined alluvial deposit. In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere Alluvium (from the Latin, alluvius, from alluere, "to wash against" is Soil or Sediments deposited by a river or other running

Secondary porosity

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

This 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

This is secondary porosity generated by dissolution of large features (such as macrofossils) in carbonate rocks leaving large holes, vugs, or even caves. Vugs are small to medium-sized cavities inside rock that may be formed through a variety of processes A cave is a natural underground void large enough for a human to enter

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). The term effective porosity lacks a single or straightforward definition This is very important for groundwater and petroleum flow, as well as for 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).

Macro porosity

Refers to pores greater than 50 nm in diameter. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a Flow through macropores is described by bulk diffusion.

Meso porosity

Refers to pores greater than 2 nm and less than 50 nm in diameter. Flow through mesopores is described by knudsen diffusion.

Micro porosity

Refers to pores smaller than 2 nm in diameter. Movement in micropores is by activiated diffusion.

Measuring porosity

Several methods can be employed to measure porosity, including the volume/density method (pore volume = total volume - material volume), water saturation method (pore volume = total volume of water - unsaturated water), water evaporation method (pore volume in cubic centimeters = weight of saturated sample in grams - weight of dried sample in grams, mercury intrusion porosimetry (several non-mercury intrusion techniques have been developed due to toxicological concerns), and nitrogen gas adsorption (nitrogen gas condensation in pores is measured either by volume or weight).

See also

• Petroleum geology
• Poromechanics
• Bulk density
• Particle density

References

• Glasbey, C. Petroleum geology refers to the specific set of geological disciplines that are applied to the search for Hydrocarbons ( Oil exploration) Poromechanics is a branch of Physics and specifically Continuum mechanics and Acoustics that studies the behaviour of fluid-saturated porous media Bulk density is a property of powders granules and other "divided" solids especially used in reference to Soil. The particle density or true density of a Particulate solid or powder is the density of the particles that make up the powder in contrast to the Bulk density A. ; G. W. Horgan and J. F. Darbyshire (September 1991). "Image analysis and three-dimensional modelling of pores in soil aggregates". Journal of Soil Science 42 (3): 479–486. doi:10.1111/j.1365-2389.1991.tb00424.x. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
• Horgan, G. W. ; B. C. Ball (1994). "Simulating diffusion in a Boolean model of soil pores". European Journal of Soil Science 45 (4): 483-491. doi:10.1111/j.1365-2389.1994.tb00534.x. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
• Horgan, Graham W. (1996-10-01). "A review of soil pore models".  
• Horgan, G. W. (June 1998). "Mathematical morphology for soil image analysis". European Journal of Soil Science 49 (2): 161-173. doi:10.1046/j.1365-2389.1998.00160.x. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
• Horgan, G. W. (February 1999). "An investigation of the geometric influences on pore space diffusion". Geoderma 88 (1-2): 55-71. doi:10.1016/S0016-7061(98)00075-5. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
• Nelson, J. Roy (January 2000). "Physics of impregnation". Microscopy Today 8 (1).