The concept of relative permeability is very simple. The measurement and interpretation of relative permeability versus saturation curves is not. For example, there is evidence that relative permeability may be a function of many more parameters than fluid saturation. Temperature, flow velocity, saturation history, wettability changes and the mechanical and chemical behaviour of the matrix material may all play roles in changing the functional dependence of the relative permeability on saturation. The best defined of these dependences is the variation of relative permeability with saturation history; relative permeability curves show hysteresis between drainage processes (wetting phase decreasing) and imbibition processes (wetting phase increasing).

There are two basic methods of obtaining relative permeability data: steady state and unsteady state. For the steady state method and a two fluid system, the two phases are injected at a certain volumetric ratio until both the pressure drop across the core and the composition of the effluent stabilize. The saturations of the two fluids in the core are then determined, typically by weighing the core or by performing a mass balance calculation for each phase. The relative permeability is calculated from the flow equations.

The unsteady state method is based on interpreting an immiscible displacement process. For a two phase system, a core, either in the native state (preserved) or restored to the saturation conditions that exist in the reservoir, is flooded with one of the phases. Typically, the flood phase is water or gas since in the reservoir one or the other of these phases displaces oil.