Ch. 32] POROSITY 587 



measurement or by observing the volume of liquid displaced when the 

 sample is totally submerged in the liquid. A dry weight of the sample 

 is taken, and the interstices of the rock are filled with a liquid of 

 known density. The weight of the rock plus the liquid is thus obtained. 

 The pore volume can then be calculated by subtracting the dry weight 

 of the rock from the weight of the rock plus liquid and dividing this 

 difference in weight by the density of the liquid. The pore volume 

 divided by the bulk volume is the fractional porosity, which when 

 multiplied by 100 expresses the porosity in percent. When such a 

 determination is examined critically, a number of specific points can 

 be noted. For example, if there were extractable solids in the rock 

 which were not part of the fluid phases in the interstices but actually 

 a part of the matrix of the porous medium, the determined value for 

 the porosity would be higher than the actual porosity. In some in- 

 stances, it is possible for the rock to contain swelling clay particles in 

 the interstices. In an aqueous phase, this swelling is a function of the 

 ionic strength of the dissolved salts. The swelling is greater in fresh 

 distilled water than in brine. Hence it is possible that a different pore 

 volume would be obtained for each type of fluid used for saturating the 

 interstices, although this effect is small. If the sample of rock is so 

 small that experimental errors arise in the determination of bulk 

 volume, such inaccuracies would be carried over in the calculation of 

 the final porosity values. 



There are several other methods of determining porosity. One 

 method is to take a sample of clean, extracted, dry rock, measure its 

 bulk volume, and obtain a dry weight. This dry weight divided by 

 the rock grain density, which for silica is about 2.65 grams per cubic 

 centimeter, gives the grain volume. The pore volume is the grain 

 volume subtracted from the bulk volume. The percent porosity would 

 then be 



V B - V G 



— X 100 (1) 



Vb 



where V B is the bulk volume and V& the grain volume. This method 

 is fairly accurate for clean sandstones and limestones. 



A method often used in the oil industry for routine core analysis is 

 the Washburn-Bunting technique, which makes use of a mercury- 

 evacuation method to remove the air from the voids in the porous 

 medium. The volume of air corrected to the proper conditions is then 

 measured, and this is the pore volume. The porosity is computed as 

 shown above. Another technique is to force mercury into a clean core 



