Correctly predicting sandstone permeability from variables such as porosity, composition, and texture is one of the major problems of petroleum geology and hydrology. Using data sets comprising composition, texture, porosity, and permeability for sandstone samples from the Norwegian continental shelf and the Fontainebleau Sandstone, we have tested several previously suggested methods for permeability prediction, and also suggest a new method for sandstones containing significant volumes of clay.
Our results indicate that permeability is successfully predicted from porosity and mean grain size by the Kozeny equation for most sandstones with clay contents less than 3% when porosities exceed 6–14%, with the lowest threshold values of porosity tending to occur in the sandstones containing least clay. The Kozeny equation also seems to enable calculation of the probable maximum possible permeabilities for sandstones with given porosities and grain sizes. For clay contents above 3%, the Kozeny equation typically overpredicts permeability.
Including a percolation threshold in the Kozeny equation, using estimated macroporosity rather than total porosity, or applying a form of the equation where internal surface area is used as input instead of grain size does not provide generally acceptable permeability estimates. However, a modified Kozeny equation which includes a parameter reflecting the type of pore system often provides useful permeability estimates even for clay-rich samples. This parameter can often be regarded as a constant for a specific formation or sandstone unit when porosity variations are not excessive. Mercury-injection data suggest that its value is controlled by the width and connectivity of the dominant flow paths through the system, which in turn are probably determined by factors such as volume and distribution of clay.