What is permeability?
Permeability is the ability of the rock, sediment, or soil for transmitting a fluid. It is a measure of the relative ease of fluid flow under unequal pressure and is a function only of the medium. This property of the rock is due to the interconnected voids present in the rock or sediments (i.e., porosity). The unit of measurement is the millidarcy.
Absolute permeability
The ability of a rock to conduct a fluid, e.g. gas, at 100% saturation with that fluid.
Effective permeability
The ability of a rock to conduct one fluid, e.g. gas, in the presence of other fluids, e.g. oil or water.
Relative permeability
The ratio between the effective permeability to a given fluid at a partial saturation and the permeability at 100% saturation (the absolute permeability). It ranges from zero at a low saturation to 1.0 at a saturation of 100% (Levorsen, 1967, p. 110).
Rocks with high porosity are expected to be more permeable too. Of course, this is not always true. For example:
- Shales are highly porous but less permeable because their intergranular space is too narrow to allow fluid to pass through the rock.
- Similarly, some vesicular basalts also are highly porous but less permeable, because the vesicles in them are not interconnected (i.e., the effective porosity is less).
Since groundwater has to move through minute pores or narrow fractures, its flow will naturally permeability to several centimeters per second in the large passages of some caves. The flow rates of ground water vary from a few centimeters per year in rocks of low.
Yes, that is correct.