Porosities

Porosity is a property of a material that describes the fraction of its volume occupied by voids or pore spaces. It is dimensionless and usually expressed as a percentage. Porosity governs a material's ability to store fluids such as water, oil, or gas and influences mechanical behavior through the presence of voids.

There are several porosity concepts used in different disciplines. Total porosity includes all void spaces, regardless of connectivity. Effective or connected porosity excludes isolated pores and represents the portion that can contribute to fluid flow. In geology and petrophysics, porosity is also described as primary (formed during deposition) and secondary (created by dissolution or fracturing). Pore-size classification into macro-, meso-, and micro-porosity is common for materials like rocks and polymers.

Porosity is measured by several methods. Bulk porosity can be determined using Archimedes principle with saturated and dry masses. Skeletal porosity is measured by helium pycnometry. Pore-size distribution and connected porosity are assessed by mercury intrusion porosimetry, gas adsorption, or imaging techniques such as X-ray computed tomography and NMR porosimetry.

Typical values vary widely. Soils commonly range around 0.3–0.5, while sedimentary rocks span roughly 0.05–0.25; highly porous rocks may exceed 0.3. Importantly, porosity alone does not determine fluid flow; permeability depends on pore connectivity and the tortuosity of the pore network.

Porosities are central to hydrogeology, petroleum engineering, and materials science. They affect groundwater storage and movement, reservoir performance, filtration, thermal or acoustic insulation, and lightweight, porous ceramics and polymers.