Permeability is a factor that quantifies how hard or how easy it is for the fluid to flow
through the reservoir to the oil producing well; the greater the permeability, the easier the
fluid flows.
Permeability of a rock is a measure of the ability of the rock to transmit fluids through it.
It is of great importance in determining the flow characteristics of hydrocarbons in oil
and gas reservoirs, and of groundwater in aquifers. The usual unit for permeability is the
darcy, or more commonly the milli-darcy or md (1 darcy = 1 x 10−12m²).
Permeability is part of the proportionality constant in Darcy’s Law which relates
discharge (flow rate) and fluid physical properties (e.g viscosity), to a pressure gradient
applied to the porous media. The proportionallity constant specifically for the flow of
water through a porous media is the hydraulic conductivity. Permeability is a portion of
this, and is a property of the porous media only, not the fluid. In naturally occurring
materials, it ranges over many orders of magnitude .
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 . Unconsolidated sands may have permeabilities of 5000+ md.
Darcy’s Equation for linear incompressible fluid flowDarcy is a unit of permeability. It is not an SI unit, but it is widely used in petroleum
engineering and geology. The darcy has units of area.
Definition
Permeability measures the ability of fluids to flow through rock (or other porous media).
