Ch. 32] 



CAPILLARY PROPERTIES 



595 



water. This salt water was trapped in the interstices of the sand 

 and existed in this state while other strata were forming on top of the 

 sand, causing consolidation of the porous medium. At some time 

 during geological history, oil migrated under pressure into this brine- 

 filled formation, displacing the brine in the same manner as the wet- 



10 20 30 40 50 60 70 80 90 100 

 Connate water, percent pore volume 



Fig. 6. Restored-state curves. 



ting phase was displaced by the non-wetting phase in the above experi- 

 ment. The pressure difference between the oil and the water, which 

 enables the oil to displace water from the pores of the rock, is brought 

 about by their different densities. In a dipping stratum whose pores 

 are jointly occupied by oil and water, the difference in pressure between 

 the two phases depends on the vertical height of the oil column. The 

 pressure difference A p is given by the equation 



A p = hg(p w - p ) 



where h is the height above a free water surface of the portion of the 

 reservoir sand under examination, g the acceleration of gravity, and p w 

 and po are the densities of the brine and oil phases, respectively. 

 Equilibrium exists when this A p equals the P c for the formation. The 

 capillary pressure will be lowest, and the water saturation highest, 

 near the base of the oil column, the "water level." It will be the 

 highest, and the water saturation least, at the highest part of the 

 reservoir. 



Bruce and Welge (1947) considered whether a minimum residual 

 water saturation really exists. 



