Miscellaneous Subsurface Methods 



693 



The sand-grain volume likewise can be obtained by displacement of 

 a fluid. The Washburn-Bunting porosimeter ^"^ (fig. 373) is used widely in 

 obtaining the pore volume by direct observation. 



The porosity of a zone or stratum composing part or all of a petro- 

 leum reservoir must be ascertained from laboratory data obtained from 

 small core samples coming from wells. A single average quantity is 



vy 



-Groduatcd Stem 



/-"^. 



-Ground G\aiS Joint 



-Bottle 



Somple *\ \ 



Figure 372. Russell volumeter. 



desired for many quantitative uses. The accuracy of the figure employed 

 depends not only upon the accuracy of measurement but also upon the 

 adequacy of sampling. The distribution of porosity horizontally and 

 vertically within a stratum is not uniform. The porosity of a reservoir, 

 then, is a statistical quantity, the numerical value of which depends not 

 only upon the porosity of the individual sample of rock but also upon 

 the thickness and areal extent to which that porosity is assigned. 



Average porosity, therefore, may be expressed by the relationship 



Jav 





(6) 



'■"Washburn, E. W., and Bunting, J., op. cit. 



