Basics differences between porosity and permeability

Porosity and permeability are two basic petrological properties of rock.

Porosity is basically, void space within rock mateiral.(pores, channels, vugs or fractures or anything which is not rock grains)

So, we can imagine porosity as, 'storage capisity of rock.'

If you have closed container filled with sand and you spray it with water. Then it will absorb some amount of water into it. Thats the porosity. 

Mathematically porosity is volume fraction of void space with respect to bulk volume.

Porosity is described as percentages.

Many people has mis conception that porosity depends upon grain size. Which is wrong. Porosity is independent of grain size. Porosity depends on grain sorting and not on grain size.

For types of  porosity : Click here.

And

Absolute Permeability is ability of rock (not fluid) to pass fluid through it.

Remember, it is ability of rock and not fluid. 

Permeability of rock shows us, how easily rock can pass fluid through it. If Permeability of rock is more, than it can easily pass fluid through it.

Mathematically,Permeability is determined by Darcy's law.

For all fluids, absolute Permeability of a rock is constant.

So,  permeability refers to 'connectivity of pores'.

Unit of absolute Permeability is darcy (length square). 

Permeability depends on grain size as well as grain sorting.

Also remember that there is difference between absolute permeability and relative permeability.

  

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