Effective and Relative Permeability

When there is only one type of fluid flowing through porous media, the permeability for this case is called “absolute permeability.” However, when there is more than one type of fluids present in a rock, a permeability of each fluid to flow is decreased because another fluid will be moving in the rock as well.  A new term of permeability called “effective permeability” is a permeability of a rock to a particular fluid when more than one type of fluid is in a rock.

Reservoir consists of three fluids (gas, oil, and water) so these are commonly used abbreviations for effective permeability for each fluid.

kg = effective permeability to gas

ko = effective permeability to oil

kw = effective permeability to water

Normally, it is common to state effective permeability as a function of a rock’s absolute permeability. Relative permeability is defined as a ration of effective permeability to an absolute permeability of rock. The relative permeability is widely used in reservoir engineering. These functions below are the relative permeability of gas, oil, and water.

Relative permeability to gas – krg = kg÷k

Relative permeability to oil – kro = ko÷k

Relative permeability to water – krw = kw÷k


k = absolute permeability

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Absolute Permeability

Absolute permeability is an ability to flow fluid through a permeable rock when only one type of fluid is in the rock pore spaces.  The absolute permeability is used to determine relative permeability of fluids flowing simultaneously in a reservoir.


Darcy’s equation is widely used in an oil field to measure flow in porous media and the Darcy’s linear flow equation is shown below;

Darcy's equation Continue reading