594 



russell-dickey. POROSITY AND PERMEABILITY 



[Ch. 32 



In this type of experiment, it can be observed that considerable pres- 

 sure is built up before the non-wetting phase enters the core to dis- 

 place the wetting phase. Thus for each type of porous medium there 

 is a threshold pressure. This is observed in Fig. 6 as the vertical line 

 at 100 percent saturation of the wetting phase. However, another 



Inlet for displacing phase under pressure 



Outlet for 

 drain valve 



Z_ 



Cellophane-membrane type 



7^> Semi-permeable membrane 

 Porcelain-barrier type 



Fig. 5. Single-core displacement cell. (After Rose and Bruce, 1949, p. 129.) 



vertical portion of the curve is reached at much lower saturations of 

 the wetting phase. For the Weber sand this saturation is about 30 

 percent. At this point only small and decreasing amounts of brine are 

 forced out by large increases in pressure. This is called the region of 

 minimum residual brine saturation. The wetting phase has become dis- 

 continuous and exists in the form of pendular rings around the grains 

 making up the porous medium. A more complete discussion of this 

 irreducible minimum is given by Rose and Bruce (1949), Leverett 

 (1941), Bruce and Welge (1947), and earlier workers. 



The capillary pressure experiment, as described above, has its count- 

 erpart in nature. When the sediments were first laid down in geological 

 time, they were sands, rather loosely consolidated, but filled with salt 



