In a previous technical note by Walter Rose, evidence is offered in supportof the contention that "the possibility of describing oil recovery featuresin terms of capillary pressure phenomena has not been establishedentirely." It is not the purpose of this note to present further discussionof this possibility but rather to point out that the argument presented theredoes not appear to be generally valid.
In the cited note reference is made to the experiments of Welge wherein it wasshown that when water was displaced by oil from a core the pressure in thewater phase was less than in the oil phase but that when this oil was thenproduced by water drive the pressure in the water phase was greater than in theoil phase. Rose has concluded from these observations that conditions of staticequilibrium did not obtain since the fluid saturations were such that phasediscontinuities seemed unlikely. The argument from which this conclusion wasdrawn is based on the contention that "the pressure is always greatest inthe non-wetting phase at each static interstitial interface of contact with thewetting phase." This premise appears to have been reached from aconsideration of capillary phenomena in cylindrical tubes, although it may alsohold for other types of pore geometry, but as will be shown below it does nothold for all pore shapes and hence is not generally applicable.
Since the pore structure of porous media such as reservoir rocks is in generalso complex as to defy exact description, it is undoubtedly of great value toestablish a model in terms of pore shapes of known geometry, provided of coursethat the limitations of the model are recognized. That the limitations of thecapillary, or cylindrical tube model have not always been fully recognized willbe evident from the following discussion of a pore of different shape but onewhich may as logically be considered as the capillary tube.
T.N. 60
