586 russell-dickey. POROSITY AND PERMEABILITY [Ch. 32 



King (1898) published extensive experiments on the transmission of 

 water and air by porous media. At the same time Slichter (1898) 

 made a theoretical study of porosity and permeability. He showed 

 that spheres of equal size could be packed in various configurations, 

 and that the volume of the interstices would range between 26 and 

 47 percent of the bulk volume, depending on the packing. He at- 

 tempted to apply the laws of Poiseuille governing the flow of fluids 

 through pipes to the pores between the spheres, and he found that the 

 permeability should be a function of the porosity and the square of 

 the grain diameter. He also derived a formula for the radial flow of 

 water into a well. His attempts to apply the laws of fluid flow in 

 pipes to porous media were greatly oversimplified, for they did not take 

 into account the wide variation in pore size in natural reservoirs or 

 the mutual interconnections of the pores. 



In spite of the direct applicability of this work to the problems 

 of oil production, very little use of it was made by the oil industry. 

 In fact, porosity and permeability were generally confused by petro- 

 leum engineers and geologists for many years/ even though Slichter 

 distinguished them clearly and showed that they were partly in- 

 dependent of each other. About 1920 the introduction of gas and water 

 into the depleted oil pools of Pennsylvania began, and research on the 

 behavior of fluids in underground reservoirs was sponsored by the oil 

 producers of that area. Interest spread quickly in other regions, and 

 fundamental studies have continued, in an expanding manner, to the 

 present time. 



Porosity 



The porosity of a porous medium has been defined in soil mechanics 

 and in the petroleum industry as: "The property possessed by a rock 

 of containing interstices, without regard to size, shape, interconnection 

 or arrangement of openings. It is expressed as the percentage of total 

 (bulk) volume occupied by the interstices" (A.P.I. , 1941; Tolman, 

 1937). It is also possible to classify porosity as to type. For example, 

 effective porosity is that property possessed by a rock of containing 

 intercommunicating interstices. Similarly, isolated porosity is that 

 property of a porous medium of containing non-communicating inter- 

 stices. In making such a fine distinction in porosity, it is at once ap- 

 parent that the quantitative determination of these values is a func- 

 tion of the laboratory method used. 



One of the most common methods used for determining porosity is 

 to take a sample of rock, extract the fluids and soluble materials 

 present, and obtain the bulk volume of the sample either by direct 



