Ch. 32] CAPILLARY PROPERTIES 597 



Muskat states that an assumption that the capillary pressure curve 

 has an initial horizontal segment might eliminate these objections, but 

 it seems more likely that the imbibition curve should be applied to 

 the lower part of the water-oil transition zone. It appears that this 

 is a very controversial subject, which is by no means settled at this 

 time. In fact, tied in with the above is the concept of wettability 

 which appears to govern the mechanism of displacement. Depending 

 on how the fluids wet the rock, the displacement process may follow 

 different mechanisms. If a wetting fluid is displaced by a non-wetting 

 fluid, the wetting fluid is progressively removed so as to occupy always 

 the smallest pores in the medium. Finally, the wetting fluid becomes 

 discontinuous and is separated into pendular rings around the rock 

 grains at this irreducible minimum saturation. The larger pores are 

 thus left free for the passage of the non-wetting fluid through the 

 porous medium. On the other hand, if a non-wetting fluid is displaced 

 by a wetting fluid, the wetting-displacing phase invades preferentially 

 the smaller pore openings and causes a partial trapping of the non- 

 wetting fluid to be displaced. This trapping occurs progressively, 

 isolating portions of the non-wetting fluid that remain in the larger 

 pore openings, until the non-wetting phase becomes discontinuous but 

 not in pendular rings as above. In fact, the smaller pores only are 

 thus left free for the passage of more wetting fluid through the porous 

 medium. It is possible that the actual mechanism of displacement 

 might be a complex combination of the above two mechanisms, as in a 

 sand that is partially water-wet and partially oil-wet. 



Hassler, Brunner, and Deahl (1943) give this subject of wettability 

 some thought. These authors present data to show that some wet- 

 tability information can be obtained by determining capillary pressure- 

 saturation curves on the same porous medium with different combina- 

 tions of fluids. This again is further complicated by possible trap- 

 ping of the displaced phase, which gives a false picture of capillary 

 equilibrium. 



A clear view of the situation requires a sharp distinction between viscous 

 pressure differences, which always occur as gradients proportional to the rate 

 of flow of a particular fluid and which do rise to high values in an oil field, and 

 the capillary pressure differences, which always occur as sharp discontinuities 

 in pressure across phase boundaries; which are not associated with viscous 

 flow, and which never exceed the pressure caused by the most highly curved 

 phase boundary in the rock. The capillary pressure is everywhere the same 

 in a rock in equilibrium, although trapped bubbles, which do not have any 

 effect on pressure distribution other than to act as plugs to viscous flow of 

 liquids, may have many curvatures (for a few days). Threshold pressure, be- 



