Subsurface Laboratory Methods 299 



tion tests. Once this pressure is created, further evaporation ceases. The 

 practice of quick-freezing cores apparently has proved feasible. However, 

 the samples selected for saturation measurements should be thawed before 

 analysis because frosting occurs upon their exposure to air. The frosting 

 action picks up water from the atmosphere and alters the core saturation. 

 To avoid contamination from atmospheric water, frozen cores may be 

 thawed in airtight containers. It thus seems logical to place the cores in 

 airtight containers immediately upon sampling at the well site in order to 

 avoid undue water contamination from the atmosphere while thawing and 

 sampling for analysis. 



Types of Core Samples 



Normally the conventional and wire-line cores of diameter over 1^ 

 inches are the best type for reliable analysis. 



Cores obtained by a shoveling or scraping action or by percussion 

 bullets often prove of little value because the extent of compaction, frac- 

 turing, and abnormal contamination associated with taking samples of 

 these types cannot be adequately evaluated. Microscopic examinations of 

 many side-wall cores has shown varying degrees of mud contamination. 

 The basic fundamentals of core-data interpretation often prove valueless 

 for sidewall cores of small diameter. In general, residual-water satura- 

 tions of side-wall samples are higher than those for wire-line cores from 

 the same sand. Special techniques have been developed in order to mini- 

 mize the source of error due to the small size of the samples, but frac- 

 turing and compaction alter the permeabilities of small side-wall cores. 



Physical Characteristics of Core Samples 



Porosity 



Porosity is the available void or storage capacity of the reservoir 

 and may be expressed as a percentage or in barrels per acre foot. 



The effective porosity is of primary interest in the calculation of 

 reserves because it is the ratio of the interconnected pore spaces to the 

 total bulk volume. Porosity is a direct function of the grain size. 



There has been confusion in the industry between porosity and perme- 

 ability. A formation may have a high porosity but low permeability. 



Permeahility 



Whether the fluids will flow from the formation or remain locked in 

 the pores is dependent on the permeability. Conventional core-analysis 

 reports usually present the "specific" dry-gas permeability of cores, for 

 which the unit of measurement is the millidarcy. 



Experience has shown that the dry-gas permeability can be a very 

 poor index to the productivity of so-called bentonitic sands because of 

 the swelling action of fresh water on these sands.^^ Water-permeability 



" Caran, J. C, and Ca.an, R., Gas and Water Permeability Relationships of the Navarro Sand in the 

 South Texas Area: Mines Mag., vol. 38, no. 12, Dec. 1948. 



