METHODS OF ANALYSIS The most important function of a reservoir 



rock is its storage capacity, or porosity. This 

 Porosity factor determines the quantity of fluid that a 



rock can hold, and though permeability or 

 fluid movement may be induced by various means, the storage capacity of a 

 reservoir is fixed and unvarying even though the distribution of the fluids therein 

 may be alterable. An acre-foot contains 7758 barrels; thus each porosity 

 percent has storage capacity for 77.58 barrels of fluid under reservoir pressure 

 and temperature conditions. 



All rocks have porosity. The pore space may be extremely small in dense 

 rock and unfavorable for production; it may consist of interstices between sand 

 grains that will range as high as 40 percent of the bulk volume; it may be vugs 

 and cavities ranging from pin point to thumb size; it may be cracks and fractures 

 that form an inter-communicating network extending for long distances; it may 

 be any, or a combination of these. 



There are two types of porosity — absolute total and effective. A rock may 

 have pore space that has been filled with fluid and subsequently sealed from 

 other pores by deposition of secondary minerals; in which instance, some of 

 the pores do not communicate. Yet this is pore space that can be measured by 

 reducing all the bulk to individual grains, breaking down and rupturing the 

 walls of the sealed portions. Porosity thus determined is called absolute or total 

 porosity. The oil industry, however, is interested in producible porosity — that 

 porosity in which the pores communicate, regardless of how tortuous the path of 

 communication might be. This type of porosity is called effective, and may range 

 from as low as 10 percent to 100 percent of the absolute. 



Table 12-1 was prepared from an average of approximately 400 individual 

 specimens. These specimens had been analyzed over a period of years for both 

 types of porosities. Both limestones and sandstones are represented, and the 

 steadily increasing ratio of effective to total porosity is clearly illustrated. 



The pore space in a rock may be calculated by the relationship of its 

 bulk volume to either its grain volume or its pore volume, as follows: 



„ . , _ _ bulk volume — grain volume 



% porosity = 100 X t- t , E. 



1 r J bulk volume 



„ . ,_ rt pore volume 



% porosity = 100 X f-^ = 



r bulk volume 



The actual laboratory measurement of porosity is performed by determin- 

 ing: (1) the bulk volume of the specimen; and (2) either the grain volume 

 or pore volume of the same specimen. The particular method to be adopted 



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