700 



Subsurface Geologic Methods 



permeability of the rock to a particular fluid phase becomes not only a 

 function of the geometry and mineral composition of the rock and the 

 pertinent physical properties of the respective fluid but also a function 

 of the fluid saturation. Nevertheless, with appropriate modification, the 

 concept of permeability may be extended to those porous systems con- 

 taining two or more fluid phases. The ability of a porous rock or body 



(a) 



(b) 



Figure 376. Idealized representation of distribution of wetting and nonwetting fluid 

 phase about intergrain contacts of spheres, (a) Pendular-ring distribution; (6) 

 funicular distribution. (After Muskat.) 



to allow passage of a particular fluid is designated effective permeability 

 when expressed in darcys or millidarcys or is termed relative perme- 

 ability when expressed as a ratio of the eff'ective permeability to the preme- 

 ability of the rock to a homogeneous fluid completely filling the pores. 

 Consequently, the numerical value of the effective permeability of a rock 

 will be between the limits of zero and the permeability, k, of the rock in 

 conventional units of darcys or millidarcys and for that of relative perme- 

 ability between the corresponding limits of zero and one. Between these 

 limits the numerical values of eff'ective and of relative permeability are 

 a function of fluid saturation. Standard terminology requires statement 

 of the fluid saturation and consequently 



A:o (65,20) =560 md (14) 



is read as the effective permeability of a particular rock to oil at a fluid 

 saturation of 65 percent oil, 20 percent water, and 15 percent gas (650 



