# Permeability

A porous material's insitu permeability tensor can take one of several forms. It can be constant, spatially varying, or depend on porosity.

## Constant

The simplest case where the permeability tensor is a constant value.

To implement a heterogeneous permeability distribution, PorousFlow also provides a constant permeability formulation where the elements of the permeability tensor can be `AuxVariables`

, enabling permeability to vary spatially. This is implemented in `PorousFlowPermeabilityConstFromVar`

## Exponential

`PorousFlowPermeabilityExponential`

A simple porosity-permeability model where (1) where is the porosity, and is a user-defined constant.

## Kozeny-Carman

`PorousFlowPermeabilityKozenyCarman`

Permeability is calculated from porosity using the Kozeny-Carman relationship (Oelkers, 1996) (2) where and are user-defined constants.

## Permeability with a solid phase

A solid phase (from chemical precipitation, for instance) can be included in the framework described herein simply by setting its relative permeability to zero. However, in this case, the absolute permeability of the porous material should be (3) where is the solid-phase saturation.

## References

- E. H. Oelkers.
Physical and chemical properties of rocks and fluids for chemical mass transport calculations.
*Reviews in Mineralogy and Geochemistry*, 34:131–191, 1996.[BibTeX]