Permeability Converter
Convert permeability units between darcies, millidarcies, square meters, square centimeters, and square micrometers.
Permeability Converter
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Formula: Converted = Input x (From Unit in m2) / (To Unit in m2)
Worked example โ 0.5 D = 500 mD = 4.9346e-13 m2
Formula
Converted = Input x (From Unit in m2) / (To Unit in m2)
Each permeability unit maps to a value in square meters. One darcy = 9.869233e-13 m2. One millidarcy = 9.869233e-16 m2. Convert by multiplying the input by the source unit factor, then dividing by the target unit factor.
Worked Examples
Example 1: Converting Darcies to Millidarcies
Problem:A rock sample has a permeability of 0.5 darcies. Express this in millidarcies and square meters.
Solution:Millidarcies = 0.5 D x 1000 = 500 mD Square meters = 0.5 x 9.869233e-13 = 4.9346e-13 m2
Result:0.5 D = 500 mD = 4.9346e-13 m2
Example 2: Reservoir Rock Permeability
Problem:A reservoir has 250 millidarcies permeability. Convert to darcies and square micrometers.
Solution:Darcies = 250 / 1000 = 0.25 D Square meters = 250 x 9.869233e-16 = 2.4673e-13 m2 Square micrometers = 2.4673e-13 / 1e-12 = 0.2467 um2
Result:250 mD = 0.25 D = 0.2467 um2
Frequently Asked Questions
What is permeability and how is it measured?
Permeability is a measure of how easily fluids can flow through a porous material such as rock, soil, or a filter. It is measured in units of area, with the darcy being the most common unit in petroleum engineering and geology. One darcy represents a relatively high permeability. Most reservoir rocks have permeabilities measured in millidarcies. The SI unit for permeability is square meters, but darcies are far more practical for real-world geological measurements.
What is the relationship between a darcy and square meters?
One darcy equals approximately 9.869 x 10^-13 square meters, or about one square micrometer. This conversion factor comes from the definition of the darcy based on Darcy's law for fluid flow through porous media. The darcy is defined as the permeability that allows a flow of 1 cm3/s of fluid with 1 cP viscosity through a 1 cm2 cross-section under a pressure gradient of 1 atm/cm. This practical definition makes it much more intuitive for engineers than using raw SI units.
What are typical permeability values for common materials?
Permeability varies enormously across different materials. Clean gravel has permeability of 10,000 to 100,000 darcies. Clean sand ranges from 1 to 1,000 darcies. Good oil reservoir rock typically measures 100 to 1,000 millidarcies. Tight sandstone falls between 0.01 and 1 millidarcy. Shale can be as low as 0.000001 millidarcies (nanodarcies). Granite and other unfractured crystalline rocks have permeabilities near zero, making them excellent barriers for groundwater containment.
How does permeability differ from porosity?
Porosity measures the fraction of void space in a material (expressed as a percentage), while permeability measures how interconnected those voids are and how easily fluid can flow through them. A material can have high porosity but low permeability if the pores are not well connected, such as pumice stone which has many air pockets but poor fluid transmission. Conversely, fractured rock may have low porosity but high permeability along the fractures. Both properties are essential for characterizing reservoir rocks in petroleum and groundwater engineering.
How does Darcy's law relate permeability to flow rate?
Darcy's law gives the volumetric flow rate as Q = k A dP / (mu L), where k is permeability in square metres, A is the cross-sectional area, dP is the pressure drop, mu is the fluid's dynamic viscosity and L is the flow length. Permeability is a property of the medium alone, while viscosity carries the fluid dependence. Doubling permeability doubles flow for the same pressure gradient, which is why a single darcy figure is so useful for comparing formations.
What is the difference between absolute, effective and relative permeability?
Absolute permeability is measured with a single fluid fully saturating the pore space. Effective permeability applies to one phase when several fluids share the pores, and it is always lower because the other phases occupy flow paths. Relative permeability is the dimensionless ratio of effective to absolute permeability, running from 0 to 1 and varying with saturation. Multiphase reservoir modelling needs relative permeability curves, not just an absolute darcy value.
How do I convert darcies to hydraulic conductivity in metres per second?
Hydraulic conductivity K equals k rho g / mu, where k is intrinsic permeability in square metres, rho is fluid density, g is gravitational acceleration and mu is dynamic viscosity. For fresh water near 20 degrees Celsius the group rho g / mu is roughly 9.8 x 10^6 per metre-second, so one darcy, or 9.869 x 10^-13 m2, corresponds to a conductivity near 1 x 10^-5 m/s. Because K embeds fluid properties it shifts with temperature, while k does not.
Why is permeability expressed in units of area?
Rearranging Darcy's law to solve for k leaves dimensions of length squared, because the viscosity and pressure-gradient terms cancel the flow dimensions. Physically the value scales with the square of a characteristic pore throat size, so it behaves like an effective cross-section for flow. The square metre is enormous on this scale, which is why the darcy, defined as 9.869233 x 10^-13 m2, and the millidarcy remain the practical working units.
What is the Klinkenberg effect in gas permeability measurements?
When permeability is measured with gas at low pore pressure, molecules slip along pore walls instead of sticking to them, so the apparent permeability exceeds the true liquid permeability. The effect grows as pressure falls and as pore size shrinks. Plotting measured permeability against the reciprocal of mean pore pressure and extrapolating to infinite pressure gives the Klinkenberg-corrected value, which is the figure comparable with liquid measurements and reservoir conditions.
When should I report millidarcies rather than darcies?
Use millidarcies whenever the value would otherwise be a small fraction, which covers most consolidated reservoir rock and nearly all tight formations. One darcy equals 1,000 millidarcies, and very low permeability shales are often quoted in microdarcies or nanodarcies. Unconsolidated sands and clean gravels are the cases where whole darcies read naturally. Matching the prefix to the material keeps decimal place errors out of the conversion.
References
Reviewed for accuracy by Manoj Kumar, Mathematics Educator ยท Editorial policy
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