Fundamentals of Fluid Flow in Porous Media


Chapter 2

Relative Permeability

Experimental Consideration

Aside from the actual testing of the cores, there are numerous other experimental considerations that must be accounted for in order to minimize the error associated with measuring core data.

Capillary End Effects

The most common source of error associated with laboratory measurements of relative permeability is capillary end effects. This problem comes as a result of the saturation of the wetting phase being much higher at the inlet and outlet of the core. This is caused by the tendency of the wetting phase to remain in the capillaries, rather than enter a non-capillary space. There have been numerous methods developed to negate this problem. One of the most common is the Hassler technique

[1], in which porous plates of similar wettability to the core sample are placed at the inlet and outlet of the core. The wetting phase is passed through these plates, while the non-wetting phase is introduced just to the core. The Penn-State method is quite similar to the Hassler method, the major difference being that both the wetting and non-wetting fluid are passed through the porous plates.

Sampling Considerations

The accuracy of the laboratory measurements depends greatly on whether or not the core sample has been damaged in any way. In order to get a representative sample of the reservoir, it is important to keep the core as close to reservoir conditions as possible. An example of this would be to use a drilling fluid that does not adversely affect the core sample. As well, taking larger diameter cores (although more expensive) can potentially minimize flushing concerns. Weathering is a definite problem, so once the cores have been brought to surface they should be sealed and covered as quickly as possible. Once the core gets to the lab, all handling and cleaning should also be kept to a minimum.  It is recognized that it is completely unrealistic to expect that a core will not be introduced to non-reservoir conditions from the ground to the lab. However, these damaging effects should be kept to a minimum if accurate results are desired.

Steady or Unsteady State?

The majority of literature acknowledges that steady state techniques provide more accurate values than unsteady state techniques. However, the main advantage to using unsteady state techniques is that they are much faster and more economical to run. Therefore, if time and budget constraints are not a deciding factor in the test, it is recommended that the steady state technique be performed in order to obtain the most reliable relative permeability data.


[1] Honarpour, M., Mahmood, S.M (1988)


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