596 



russell-dickey. POROSITY AND PERMEABILITY 



[Ch. 32 



Theoretical study of the problem indicates that at some capillary pressure 

 difference the residual water saturation should approach zero provided com- 

 plete equilibrium is attained. Experimental work on soils at several thousand 

 atmospheres of capillary pressure has shown that near zero water saturation 

 can be produced when the displacing phase is a gas. In practice, laboratory 

 experiments have indicated that, within the accuracy of saturation measure- 

 ments, a minimum saturation does exist within the range of pressures en- 

 countered in reservoirs having less than 1,000 feet of closure. 



70,000 



_ 60,000 



S> ~ 50,000 



40,000 



\ I 



Permeability 17.5 darcys 

 Porosity 39 percent 

 "(From experimental data) 



S w 



30,000 



20,000 



10,000 



Note:-l Ib./sq. in.= 

 69,000 dynes/cm: 



J L 



20 



40 60 

 Percent 



80 100 



Fig. 7. Equilibrium distribution of air and water in uniform unconsolidated sand, 

 having a permeability of 17.5 darcys. (After Leverett, 1941, p. 163.) 



In the discussion above, thus far only the drainage type of capillary 

 pressure vs. saturation curve has been discussed. Actually the capil- 

 lary pressure-saturation curve is different if the data are obtained by 

 imbibition. Leverett (1941) studied this hysteresis zone for uncon- 

 solidated sand. Figure 7 gives the results of this study. It is quite 

 possible that the conditions under which hydrocarbons accumulate in 

 and are produced from the earth correspond to the imbibition equilib- 

 rium. In this regard, Muskat (1948) points out 



. . . that the application of capillary pressure curves obtained by the drain- 

 age or desaturation processes to the calculation of the fluid distribution in 

 interphase transition zones involves a number of difficulties, namely: (1) the 

 development of very low non-wetting phase saturations appears to be in con- 

 tradiction to the lack of mobility of such distributions indicated by permeabil- 

 ity-saturation curves, (2) dispersed non- wetting phases are thermodynamically 

 unstable, (3) discontinuous phases should not be subject to hydrostatic re- 

 quirements. 



