Pore structure is a common term employed to characterize the

porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...

, pore size, pore size distribution, and pore morphology (such as pore shape, surface roughness, and

tortuosity Tortuosity is widely used as a critical parameter to predict transport properties of porous media, such as rocks and soils. But unlike other standard microstructural properties, the concept of tortuosity is vague with multiple definitions and vari ...

of pore channels) of a

porous medium A porous medium or a porous material is a material containing pores (voids). The skeletal portion of the material is often called the "matrix" or "frame". The pores are typically filled with a fluid (liquid or gas). The skeletal material is us ...

. Pores are the openings in the surfaces impermeable porous matrix which gases, liquids, or even foreign microscopic particles can inhabit them. The pore structure and

fluid flow In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including '' aerodynamics'' (the study of air and other gases in motion) ...

in porous media are intimately related. With micronanoscale pore radii, complex connectivity, and significant heterogeneity, the complexity of the pore structure affects the

hydraulic conductivity Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on ...

and retention capacity of these fluids. The intrinsic permeability is the attribute primarily influenced by the pore structure, and the fundamental physical factors governing fluid flow and distribution are the grain

surface-to-volume ratio The surface-area-to-volume ratio, also called the surface-to-volume ratio and variously denoted sa/vol or SA:V, is the amount of surface area per unit volume of an object or collection of objects. SA:V is an important concept in science and engin ...

and grain shape. The idea that the pore space is made up of a network of channels through which

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shea ...

can flow is particularly helpful. Pore openings are the comparatively thin sections that divide the relatively large portions known as pore bodies. Other anatomical analogies include "belly" or "waist" for the broad region of a pore and "neck" or "throat" for the constrictive part. Pore bodies are the intergranular gaps with dimensions that are generally significantly smaller than those of the surrounding particles in a medium where textural pore space predominates, such as sand. On the other hand, a

wormhole A wormhole ( Einstein-Rosen bridge) is a hypothetical structure connecting disparate points in spacetime, and is based on a special solution of the Einstein field equations. A wormhole can be visualized as a tunnel with two ends at separate p ...

can be regarded as a single pore if its diameter is practically constant over its length. Such pores can have one of three types of boundaries: (1) constriction, which is a plane across the locally narrowest part of the pore space; (2) interface with another pore (such as a wormhole or crack); or (3) interface with solid.

Porosity

The proportion of empty space in a porous media is called

. It is determined by dividing the volume of the pores or voids by the overall volume. It is expressed as a percentage or as a decimal fraction between 0 and 1. Porosity for the majority of rocks ranges from less than 1% to 40%. Porosity influences fluid storage in geothermal systems,

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturate ...

and

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

fields, and

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteri ...

s, making it evident that it plays a significant role in

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

. Fluid movement and transport across geological formations, as well as the link between the bulk properties of the rock and the characteristics of particular minerals, are controlled by the size and

connectivity Connectivity may refer to: Computing and technology * Connectivity (media), the ability of the social media to accumulate economic capital from the users connections and activities * Internet connectivity, the means by which individual terminals ...

of the porous structure.

Measuring porosity

The samples' total volume and pore space volume were measured in order to calculate the porosities. Measuring pore space volume A helium pyrometer was used to calculate the volume of the pores and relied on

Boyle's Law Boyle's law, also referred to as the Boyle–Mariotte law, or Mariotte's law (especially in France), is an experimental gas law that describes the relationship between pressure and volume of a confined gas. Boyle's law has been stated as: Th ...

. (P₁V₁=P₂V₂) and helium gas, which easily passes through tiny holes and is inert, to identify the solid

fraction A fraction (from la, fractus, "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight ...

of a sample. A sample chamber with a known volume is where the core is put. Pressure is applied to a reference chamber with a known volume. The helium gas may now go from the reference chamber to the sample chamber thanks to the connection between the two rooms. The volume of the sample solid is calculated using the ratio between the starting and final pressures. The pore volume, as calculated by the helium pycnometer, is the difference between the total volume and the solid volume.

Pore size and pore size distribution

Pore size

Typically, the effective radius of the pore body or neck is used to define the size of pores. The position, shape, and connection of pores in solids are only a few of their numerous attributes and the most straightforward aspect of a pore to visualize is likely its size, or its extent in a single spatial dimension. In comparison to other factors like pore shape, it is arguable that pore size has the biggest or broadest impact on the characteristics of solids. Therefore, using pore size or pore size distribution to describe and contrast various porous substances is definitely convenient and valuable. The three main pore size ranges (The current classification of pore size recommended by the International Union of Pure and Applied Chemistry) are as following:

Pore size distribution

The relative abundance of each pore size in a typical volume of soil is represented by the pore size distribution. It is represented by the function f(r), whose value is proportional to the total volume of all pores whose effective radius is within an infinitesimal range centered on r. And f(r) can be thought to have textural and structural components.

Measuring pore size distribution

Mercury intrusion porosimetry Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Mercury ...

and gas adsorption are common techniques for determining the pore size distribution of materials and power sources. When studying the pore size distribution using the gas

adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...

technique utilizing the nitrogen or argon adsorption isotherm at their boiling temperatures, it is possible to determine the pore size from the molecular level to a few hundred nm. The

pressure sensor A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually a ...

's precise constraints and the coolant's temperature stability result in a maximum observed pore size of just a little bit more than 100 nm in a realistic environment. Mercury porosimetry determines the pore size distribution and quantifies the associated incursion amount by applying pressure to the non-wetting mercury. The pore size may be readily estimated using this method and ranges from a few nm to 1000 m. The material must be robust enough to withstand the pressure since mercury intrusion requires 140 MPa of pressure for pores smaller than 10 nm. Additionally, it utilizes the idea to determine the pore size of the inkbottle neck.

The relation of pore size to pore size distribution

The relationship between pore size and pore size distribution in a randomly constructed porous system, is expected to be monotone: bigger pores are connected to larger particles. The relationship between pore size and particle size is complicated by the nonrandom nature of most soils. Big pores may be found in both large and tiny particles, including clays, which promote aggregation and therefore the development of large interaggregate pores. Subdivisions of a pore size distribution in randomly structured media can express more specific characteristics of soils with more complex conceptualizations, such as the hysteresis of soil water retention.

Pore morphology

The pore morphology is the shape, surface roughness, and tortuosity of pore channels representing the liquid and gaseous phases.

Tortuosity of pore channels

Tortuosity Tortuosity is widely used as a critical parameter to predict transport properties of porous media, such as rocks and soils. But unlike other standard microstructural properties, the concept of tortuosity is vague with multiple definitions and vari ...

of pore channels is a unique geometric quantity that is utilized not only to measure the transport characteristics of porous system, but also to express the sinuosity and complexity of internal

percolation Percolation (from Latin ''percolare'', "to filter" or "trickle through"), in physics, chemistry and materials science, refers to the movement and filtering of fluids through porous materials. It is described by Darcy's law. Broader applicatio ...

routes. Toruosity is intimately connected to the transport behavior of

electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

, fluid permeation, molecular diffusion, and

heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction ...

geoscience Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spher ...

, impacting petrophysical parameters such as permeability, effective

diffusivity Diffusivity is a rate of diffusion, a measure of the rate at which particles or heat or fluids can spread. It is measured differently for different mediums. Diffusivity may refer to: *Thermal diffusivity, diffusivity of heat *Diffusivity of mass: ...

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

, and formation resistivity factor.

Surface roughness

The standard definition of surface roughness for porous medium is based on the average measured vertical coordinate value in comparison to a relative surface height, such as root-mean-square roughness or arithmetic roughness. However, the lack of fractal topology consideration led to the relative surface height definition being deemed inadequate in reality. The ratio of "real surface area" to "geometric smooth-surface area" was used as the second definition of surface roughness. This definition has been applied in several research to alter flow equations or measure the fluid-fluid interfacial area. The fundamental idea of fractal geometry is where the third definition of surface roughness comes from, in which either modifies the pore surfaces (two-dimensional) or the whole porous medium (three-dimensional) using

fractal dimension In mathematics, more specifically in fractal geometry, a fractal dimension is a ratio providing a statistical index of complexity comparing how detail in a pattern (strictly speaking, a fractal pattern) changes with the scale at which it is meas ...

adjustments, resulting in larger surface dimensions or reduced media dimensions. The hurst roughness exponent, a similar definition, is occasionally used. This quantity, which spans from 0 to 1, is connected to the fractal dimension.

References

External links

Geology Buzz: Porosity

Defining Permeability

Tailoring porous media to control permeability

Permeability of Porous Media

* ttp://perminc.com/resources/fundamentals-of-fluid-flow-in-porous-media/ Fundamentals of Fluid Flow in Porous Media {{Earth science Hydrology Soil mechanics Soil science Porous media In situ geotechnical investigations