permeability (geology)

In geology, permeability is a measure of the ability of a material (typically, a rock or unconsolidated alluvial material) to transmit fluids through it. It is of great importance in determining the flow characteristics of hydrocarbons in oil and gas reservoirs, and of groundwater in aquifers. The usual unit for permeability is the darcy, or more commonly the milli-darcy or md (1 darcy = 1 x 10^-12.m²). Permeability is the proportionallity constant in Darcy's law which relates discharge (flow rate) to a pressure gradient applied to the porous media. In naturally occuring materials, it ranges over many orders of magnitude (see table below for an example of this range). For a rock to be considered as an exploitable hydrocarbon reservoir, its permeability must be greater than approximately 100 md (depending on the nature of the hydrocarbon - gas reservoirs with lower permeabilities are still exploitable because of the lower viscosity of gas with respect to oil). Rocks with permeabilities significantly lower than 100 md can form efficient seals (see petroleum geology). Unconsolidated sands may have permeabilities of 5000+ md.

Ranges of intrinsic permeability (κ) values found in nature

colspan="4" align="center" | Pervious

colspan="4" align="center" | Semi-Pervious

colspan="5" align="center" | Impervious

bgcolor="#FAEBD7" | Unconsolidated Sand & Gravel

colspan="2" align="center" | Well Sorted Gravel

colspan="3" align="center" | Well Sorted Sand or Sand & Gravel

colspan="4" align="center" | Very Fine Sand, Silt, Loess, Loam

colspan="4" |

bgcolor="#FAEBD7" | Unconsolidated Clay & Organic

colspan="4" |

colspan="2" align="center" | Peat

colspan="3" align="center" | Layered Clay

colspan="4" align="center" | Fat / Unweathered Clay

bgcolor="#FAEBD7" | Consolidated Rocks

colspan="4" align="center" | Highly Fractured Rocks

colspan="3" align="center" | Oil Reservoir Rocks

colspan="2" align="center" | Fresh Sandstone

colspan="2" align="center" | Fresh Limestone, Dolomite

colspan="2" align="center" | Fresh Granite

bgcolor="#FAEBD7" | κ (cm²)

0.001

0.0001

10⁻⁵

10⁻⁶

10⁻⁷

10⁻⁸

10⁻⁹

10⁻¹⁰

10⁻¹¹

10⁻¹²

10⁻¹³

10⁻¹⁴

10⁻¹⁵

bgcolor="#FAEBD7" | κ (milliDarcies)

10⁺⁸

10⁺⁷

10⁺⁶

10⁺⁵

10,000

1,000

100

0.1

0.01

0.001

0.0001

Source: modified from Bear, 1972

Reference

Bear, Jacob, 1972. Dynamics of Fluids in Porous Media, Dover. — ISBN 0486656756

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Permeability (Geology)

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