Relative permeability, sometimes denoted by the symbol μr and often simply by μ, is the ratio of the absolute permeability to the permeability of free space μ0.
In geology, permeability is a measure of the ability of a material (typically, a rock) to transmit fluids. It is of great importance in determining the flow characteristics of hydrocarbons in oil and gas reservoirs and of water in aquifers. The usual unit for permeability is the darcy, or more commonly the milli-darcy or md (1 darcy = 1 x 10-12.m2). Permeability is used to calculate flow-rates using Darcy's law.
For a rock to be considered as an exploitable hydrocarbon reservoir, its permeability must be greater than approximately 100 md (depending on the nature of the hydrocarbon - gas reservoirs with lower permeabilities are still exploitable because of the lower viscosity of gas with respect to oil). Rocks with permeabilities significantly lower than 100 md can form efficient seals (see petroleum geology). Unconsolidated sands may have permeabilities of 5000+ md.