Application Relative Permeability

Relative permeability is one of the most important petrophysical parameters assessed in special core analysis.  Traditionally, relative permeability is measured via one of two methods: steady state and unsteady state.  In both measurements, relative permeability is calculated from measurements of fluid saturations, pressure differences and fluid flow rates.  NMR can be used to determine relative permeability through measuring saturation directly in the rock during an unsteady state experiment.

For steady-state relative permeability measurement methods, two fluids are injected simultaneously into the porous medium at a fixed ratio until the inflows equal the outflows and a constant pressure drop have been reached. It may take 2 to 40 hours or even longer to reach the steady-state conditions.

The injection ratio is then changed, until a new steady flow is established to calculate the relative permeability corresponding to this saturation. Different approaches may be employed to eliminate the capillary end effects and try to ensure uniform saturation distribution in the whole sample. The steady-state measurements are very time consuming. In addition, the conditions of steady-state and uniform saturation distribution are very rarely reached, and errors are introduced therefrom.

Because the steady state method is so time consuming, the unsteady state method is often preferred for determining relative permeability.   In the unsteady state measurement, the core plug is saturated with one immiscible fluid.  The other fluid is then injected into the core and displaces the first fluid.  The pressure drop across the core and the average saturation by material balance (via collection of effluent volumes at the outlet) in the core are monitored as a function of time.  For example, if the core is initially saturated with oil and brine is used to displace it, the amount of oil ejected as a function of time would be recorded.  The monitoring of the amount of oil ejected can either be done by eye or with the assistance of optical/acoustic instruments which monitor the position of the oil/water boundary as a function of time.