Chapter 16, Problem 1RQ

To determine

The difference between porosity and permeability and the examples of substances with high porosity and low permeability.

Answer to Problem 1RQ

Porosity depends only in the presence of pores whereas permeability depends upon the interconnections between the pores. Both are expressed in terms of percentages. Some examples of the substances with high porosity and low permeability are mud-stones, shale etc. The substances with high porosity and low permeability are known as aquitards.

Explanation of Solution

Any open space present within the rock sediment or a given volume of rock is known as porosity. The porosity is expressed as a percentage. When the pore spaces are present within a rock during its formation, it is known as primary porosity. The pores spaces here are developed because grains do not grow to fill all the spaces during crystallization or open spaces are not completely filled with cement. When the new pore spaced is produced after the formation of a rock due to fracturing or chemical effect of some fluids passing through the cementing material, it is known as secondary porosity.

When a pore filled with water is surrounded by the solid rock mass, the water cannot flow out. The movement of water from pore to pore is possible only with the interconnections or conduits between them. For the ground water to flow, the conduits must present between the pores. The permeability of a material depends on many factors such as number of available conduits, size of the conduits and straightness of the conduits.