How do porosity and permeability differ? Give examples of substances with high porosity but low permeability.

Porosity is the measure of the void spaces in a material, usually expressed as a percentage of total material volume occupied by the voids. It determines the ability of a material to store fluids or how much fluid can be contained within a porous medium.

Permeability, on the other hand, is the measure of how easily fluids can pass through a material. It is determined by the degree of interconnectivity between the pores in the porous material and the size of those pores, typically measured in units called darcies or millidarcies (md). Permeability depends on factors like grain size, grain shape, compaction, and cementation.

While porosity is about the open spaces within a material, permeability tells us how well these spaces are connected to one another and how easily fluids can move through them. In other words, porosity represents the capacity of a material to hold fluids, while permeability describes the ease with which those fluids can flow through it.

There are some substances with high porosity, meaning they can store a lot of fluids due to a large number of void spaces. However, their pores may not be well connected, resulting in low permeability and hindering the flow of fluids between the pores. Examples of such substances include: 1. **Clays:** Clays generally have high porosity, but their pores are not well connected, and the very small pore size hampers fluid movement, resulting in low permeability. 2. **Shales:** These are fine-grained sedimentary rocks, typically with high porosity. However, their pores are poorly connected and often closer to horizontal, which makes fluid flow difficult and results in low permeability. 3. **Unfractured granites:** Granites can have high porosity, particularly if they contain large crystals or natural voids. However, if there are no fractures or other pathways for fluid flow, the permeability will be low despite the material's high porosity.