# Darcy (unit) > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Darcy_(unit) > Markdown URL: https://mediated.wiki/source/Darcy_(unit).md > Source: https://en.wikipedia.org/wiki/Darcy_(unit) > Source revision: 1250122356 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) Unit of permeability darcy Unit of Permeability Symbol d Named after Henry Darcy Derivation cP⋅cm2/(s⋅atm) Conversions 1 d in ... ... is equal to ... SI 9.869233×10−13 m2 The **darcy** (or **darcy unit**) and **millidarcy** (md or mD) are [units](/source/Units_of_measurement) of [permeability](/source/Permeability_(earth_sciences)), named after [Henry Darcy](/source/Henry_Darcy). They are not [SI units](/source/SI), but they are widely used in [petroleum engineering](/source/Petroleum_engineering) and [geology](/source/Geology). The unit has also been used in biophysics and biomechanics, where the flow of fluids such as blood through capillary beds and cerebrospinal fluid through the brain interstitial space is being examined.[1] A darcy has [dimensions](/source/Dimensional_analysis) of [length](/source/Length)2. ## Definition Permeability measures the ability of [fluids](/source/Fluids) to [flow](/source/Fluid_flow_through_porous_media) through rock (or other porous media). The darcy is defined using [Darcy's law](/source/Darcy's_law), which can be written as: - Q = A k Δ P μ Δ x {\displaystyle Q={\frac {Ak\,\Delta P}{\mu \,\Delta x}}} where: - Q {\displaystyle Q\,} is the volumetric fluid flow rate through the medium A {\displaystyle A\,} is the area of the medium k {\displaystyle k\,} is the permeability of the medium μ {\displaystyle \mu \,} is the dynamic viscosity of the fluid Δ P {\displaystyle \Delta P\,} is the applied pressure difference Δ x {\displaystyle \Delta x\,} is the thickness of the medium The darcy is referenced to a mixture of unit systems. A medium with a permeability of 1 darcy permits a flow of 1 cm3/s of a fluid with [viscosity](/source/Viscosity) 1 [cP](/source/Poise_(unit)) (1 [mPa·s](/source/Pascal_second)) under a pressure gradient of 1 atm/cm acting across an area of 1 cm2. Typical values of [permeability](/source/Permeability_of_soils) range as high as 100,000 darcys for gravel, to less than 0.01 microdarcy for granite. Sand has a permeability of approximately 1 darcy.[2] Tissue permeability, whose measurement *in vivo* is still in its infancy, is somewhere in the range of 0.01 to 100 darcy.[1] ## Origin The darcy is named after [Henry Darcy](/source/Henry_Darcy).[1] Rock permeability is usually expressed in millidarcys (md) because rocks hosting hydrocarbon or water accumulations typically exhibit permeability ranging from 5 to 500 md. The odd combination of units comes from Darcy's original studies of water flow through columns of sand. [Water](/source/Water) has a viscosity of 1.0019 cP at about room temperature. The unit abbreviation "d" is not capitalized (contrary to industry use).[*[clarification needed](https://en.wikipedia.org/wiki/Wikipedia:Please_clarify)*] The American Association of Petroleum Geologists[3] uses the following unit abbreviations and grammar in their publications: - darcy (plural darcys, not darcies): d - millidarcy (plural millidarcys, not millidarcies): md ## Conversions Converted to [SI](/source/SI) units, 1 darcy is equivalent to 9.869233×10−13 [m2](/source/Square_metre) or 0.9869233 [μm2](/source/Micrometre).[4] This conversion is usually approximated as 1 μm2. This is the reciprocal of 1.013250—the conversion factor from atmospheres to [bars](/source/Bar_(unit)).[1] Specifically in the hydrology domain, permeability of soil or rock may also be defined as the [flux](/source/Flux) of water under [hydrostatic pressure](/source/Hydrostatics#Hydrostatic_pressure) (≈ 0.1 bar/m) at a temperature of 20 °C. In this specific setup, 1 darcy is equivalent to 0.831 m/day. [5] ## References 1. ^ [***a***](#cite_ref-drs_1-0) [***b***](#cite_ref-drs_1-1) [***c***](#cite_ref-drs_1-2) [***d***](#cite_ref-drs_1-3) Sowinski, Damian (January 2021). ["Poroelasticity as a Model of Soft Tissue Structure: Hydraulic Permeability Reconstruction for Magnetic Resonance Elastography in Silico"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9635531). *Frontiers in Physics*. **8**: 637. [arXiv](/source/ArXiv_(identifier)):[2012.03993](https://arxiv.org/abs/2012.03993). [Bibcode](/source/Bibcode_(identifier)):[2021FrP.....8..637S](https://ui.adsabs.harvard.edu/abs/2021FrP.....8..637S). [doi](/source/Doi_(identifier)):[10.3389/fphy.2020.617582](https://doi.org/10.3389%2Ffphy.2020.617582). [PMC](/source/PMC_(identifier)) [9635531](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9635531). [PMID](/source/PMID_(identifier)) [36340954](https://pubmed.ncbi.nlm.nih.gov/36340954). 1. **[^](#cite_ref-2)** Peter C. Lichtner, Carl I. Steefel, Eric H. Oelkers, *[Reactive Transport in Porous Media](https://books.google.com/books?ei=woosT4DHKYrXmAXjo531Dw&id=nMsPAQAAIAAJ)*, Mineralogical Society of America, 1996, [ISBN](/source/ISBN_(identifier)) [0-939950-42-1](https://en.wikipedia.org/wiki/Special:BookSources/0-939950-42-1), p. 5. 1. **[^](#cite_ref-3)** ["The American Association of Petroleum Geologist Style Guides"](https://web.archive.org/web/20110809042013/http://geoweb.princeton.edu/beijingconference/AAPG_StyleGuidelines.pdf) (PDF). Archived from [the original](http://geoweb.princeton.edu/beijingconference/AAPG_StyleGuidelines.pdf) (PDF) on 2011-08-09. 1. **[^](#cite_ref-spe_4-0)** [*The SI Metric System of Units and SPE Metric Standard*](http://www.spe.org/authors/docs/metric_standard.pdf) (PDF) (2nd ed.). [Society of Petroleum Engineers](/source/Society_of_Petroleum_Engineers). June 1984 [First published 1982]. 1. **[^](#cite_ref-5)** K. N. Duggal, J. P. Soni: Elements of Water Resources Engineering. Publisher New Age International, 1996, p. 270 - Richard Selley's "Elements of Petroleum Geology (2nd edition)," page 250. --- Adapted from the Wikipedia article [Darcy (unit)](https://en.wikipedia.org/wiki/Darcy_(unit)) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Darcy_(unit)?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.