Exercise 3 Solution

Porosity is the ratio of pore void volume divided by total volume of a rock and is always less than 1. The term effective porosity refers to the ratio of interconnected void volume to total rock volume and is always smaller than total porosity. In general, the larger the porosity and effective porosity, there is the potential of greater water flow through a rock. Exceptions can occur as in a pure clay mineral, where the sheet minerals of clay have slight electric charge and the slightly charged molecules of water stay trapped within the clay mineral sheets and so the porosity created by the space between mineral sheets actually traps water.

Permeability also called intrinsic permeability (k) is related to the hydraulic conductivity (K) but is a function of only the properties of the porous medium and not the fluid properties.

\displaystyle k=K\frac{\mu }{\rho g}


k = permeability, m2
K = hydraulic conductivity, m/s
μ = dynamic viscosity of the fluid, Pascal·s, that is, kg/(ms)
ρ = density of the fluid, kg/m3
g = acceleration due to gravity, m/s2

Hydraulic conductivity is usually calculated from aquifer tests or permeameter tests and based on Darcy’s law. It is not uncommon for people use the term permeability when they mean hydraulic conductivity because both relate to how easy a fluid can move through porous media, but it is important not to confuse people and to correctly use the terms.

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Introduction to Karst Aquifers Copyright © 2022 by Eve L. Kuniansky, Charles J. Taylor, and Frederick Paillet. All Rights Reserved.