Permeability of crystalline rocks

In contrast to most other petrophysical parameters, intrinsic permeability for any single rock type varies by the decade rather than by the factor (see, for example, Bear 1972). Both the type of measurement and its characteristic scale (that is, the rock volume over which an individual permeability value is integrated) are responsible for this. Brace 1980, 1984 compiled permeabilities of sedimentary (porous) and crystalline (fractured) rocks and pointed out that both types of rock exhibit a “scale effect”: the larger the experiment's scale, or characteristic volume, the greater the permeability. One other transport parameter, dispersion length or dispersivity, which is closely related to permeability, behaves similarly. There are both data compilations for dispersivity documenting this scale effect Beims , 1983; Gelhar et al ., 1985 and theoretical studies (such as Neumann 1990) discussing possible scaling rules. An interesting conclusion by Neumann 1990, which is in good agreement with Brace's results, is that "porous and fractured media appear to follow the same idealized scaling rule for both flow and transport, raising a question about the validity of many distinctions commonly drawn between such media."