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6 Permeability

It is important to distinguish clearly between porosity and permeability. Porosity is a measure of how much water can be stored in a rock, whereas permeability is a measure of the properties of a rock which determine how easily water and other fluids can flow through it (see Section 4). Permeability depends on the extent to which pores are interconnected.

Table 1 gives the porosity and hydraulic conductivity of various geological materials. As hydraulic conductivity depends on permeability, the values in the last column could be said to represent relative permeabilities. However, hydraulic conductivities rather than permeabilities are given in the table, because these can be used to calculate flow rates using Darcy's law (Equation 1).

For unconsolidated sediments, the finer-grained silts and clays are less permeable than the coarser sands and gravels, even though they are more porous (Table 1). There are two major reasons for this: first, the smaller grain sizes in silt and clay result in a greater surface area of particles relative to volume, so water tends to be held in the pores by surface tension; and second, the platy and angular shape of clay particles means that they tend to interlock and isolate the spaces between them, which further inhibits the movement of water through the sediment.

  • Which three of the sediments and rocks depicted in Figure 14 should have the higher permeabilities and which should have the lowest permeability?

  • Rocks that are highly permeable must be porous and have interconnected pores; so the examples in Figures 14a, e and f, should have higher permeabilities (even though Figures 14e and f have a fairly low porosity). Rocks with a low permeability must have very few, or isolated pores, so the water cannot move through the rock, and the example in Figure 14d in which original pore space is filled by cement should have the lowest permeability.

Usually, consolidated and cemented sedimentary rocks (and igneous and metamorphic rocks) are not very permeable, but sometimes processes such as solution or fracturing create secondary permeability. The higher hydraulic conductivities of some of the igneous, metamorphic and consolidated sedimentary rocks in Table 1 are mainly due to secondary permeability caused by fracturing or for limestone, solution.

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