What's the difference between permeability and porosity?

Porosity is the fraction of void space in a material (dimensionless, 0-1). Permeability is how easily fluid flows through those voids (in m² or darcy). A material can have high porosity but low permeability if pores aren't well-connected (like pumice stone).

Why was the darcy unit created?

SI permeability values are extremely small (10⁻¹² m² or less), making them impractical. The darcy, defined through Darcy's law with convenient units (cP, cm, atm), gives manageable numbers. 1 darcy ≈ 10⁻¹² m².

What factors affect permeability?

Permeability depends on: pore size (larger = higher k), pore connectivity, grain size and sorting, fractures, and cementation. Clay content typically reduces permeability. Temperature affects fluid viscosity but not the permeability itself.

What is hydraulic conductivity?

Hydraulic conductivity (K) combines permeability with fluid properties: K = k × ρg / μ, where ρ is density, g is gravity, μ is viscosity. Units are velocity (m/s). It's permeability normalized for a specific fluid—usually water in groundwater applications.

النفاذية Converter

About Permeability Conversion

Permeability measures how easily fluids flow through porous materials like reservoir rock, soil, sand, or filters—a fundamental property governing groundwater movement, oil and gas production, and filtration processes. It's distinctly different from porosity (the fraction of void space): a material can be highly porous yet have low permeability if its pores aren't well-connected, like a sponge with isolated bubbles. Permeability depends on pore throat size and connectivity, not just total void volume.

The SI unit is square meters (m²), but the darcy (D) is the universal practical standard in petroleum engineering, hydrogeology, and soil science. One darcy represents high permeability; most conventional oil reservoirs measure in millidarcies (mD), and tight formations in microdarcies. The darcy was defined to give convenient values with common fluid properties and pressure gradients according to Darcy's law, which describes viscous flow through porous media.

Our converter handles all standard permeability units for petroleum engineering, groundwater modeling, geotechnical analysis, and filtration system design.

Common Permeability Conversions

From	To	Multiply By
Darcy (D)	m²	9.87 × 10⁻¹³
m²	Darcy	1.013 × 10¹²
Darcy	millidarcy (mD)	1,000
millidarcy	Darcy	0.001
Darcy	cm²	9.87 × 10⁻⁹
m²	cm²	10,000
Darcy	μm²	0.987
μm²	Darcy	1.013
mD	μm²	0.000987

Permeability Unit Reference

Square meter (m²) – The SI unit for permeability, representing the area through which flow occurs in Darcy's law formulation. Values for real materials are extremely small—typically 10⁻¹² to 10⁻¹⁸ m² for geological formations. This makes the unit impractical for everyday use, explaining why the darcy became standard. SI purists can use m² in calculations, but reporting in darcies is universal in practice.

Darcy (D) – The practical standard unit, named after Henry Darcy who established the governing law for flow through porous media in 1856. 1 D ≈ 9.87 × 10⁻¹³ m². By definition, a medium with 1 D permeability flows 1 cm³/s of fluid with 1 cP viscosity through 1 cm² area under 1 atm/cm pressure gradient. One darcy is actually quite high permeability—clean sand and gravel are in the darcy range.

Millidarcy (mD) – One thousandth of a darcy, the standard unit for petroleum reservoir characterization. Most conventional oil and gas reservoirs have permeabilities between 1 and 1000 mD. Good reservoir rock is typically 10-500 mD; below 1 mD is considered tight. Well test reports, core analyses, and reservoir simulation universally use mD.

Microdarcy (μD) – One millionth of a darcy, used for tight formations, shales, and very low-permeability rocks. Shale gas reservoirs typically have permeabilities of 0.001-0.1 μD (nanodarcies to microdarcies). Fracturing is required to produce from such tight rocks economically.