What is Permeability?
Permeability is a measure of the ability of fluid to flow through the pores or interstices of a material. Permeability explains the capacity of materials to store and transmit fluids. An important application is in the study of the flow characteristics of hydrocarbons in gas and petroleum reservoirs. Permeability depends on density, viscosity of the fluid and also on the properties of the medium. The property described here is strictly the intrinsic permeability which only depends on the properties of the medium. Hydraulic conductivity can also be expressed as permeability.
The SI unit for permeability is the square metre (m2), although the darcy, the c.g.s. unit named after Henry Darcy, is also widely used.
Permeability of Rock and Soil
Permeability measures are different for different rock materials. The main factor which affects the permeability is the size of the grain. The permeability value is high for rocks which have more porous sandstone and whose grain size is coarse. Rocks which have small grain size have small permeability, for example, crystalline limestone, clay and metamorphic rocks. Rocks with a permeability greater than 100mD (millidarcies - a unit of permeability widely used in petroleum engineering) can change into exploitable hydrocarbon reservoirs whereas rocks with a permeability lower than 100mD tend to form seals.
Soil permeability is influenced by many factors including void ratio, temperature, the size and shape of the soil particles, and the degree of saturation.
Measuring or Calculating Permeability
Darcy's law, which was derived in the 19th century by French engineer Henry Darcy, can be applied in finding the permeability of different materials. It is a result of applying the Navier-Stokes equations, although it was originally deduced by experimentation and the theoretical analysis was arrived at later. Darcy's law gives the flow rate in terms of a pressure difference, fluid viscosity, length and area. The permeability is the constant of proportionality in Darcy's law, and so by measurement of the other factors the permeability for a material can be derived.
Permeability can also be computed, or approximated, by characterization of the pores within the material as a series of equivalent tubes. This is a common technique in obtaining a value for permeability in chemical engineering where various packing materials are used in vessels to promote gas/liquid contact. The permeability can be defined in terms of the product of the square of the effective pore diameter and a dimensionless constant which represents the shape and structure of the flow paths.