696 



Subsurface Geologic Methods 



which has a viscosity of 1.0 centipoise at the temperature of flow in re- 

 sponse to a pressure gradient of 1.0 atmosphere per centimeter.^^ A 

 millidarcy is one-thousandth of a darcy. Accordingly, a rock with a 

 permeability of two darcys is twice as permeable as one with a perme- 

 ability of one darcy : i.e., twice as much of a given fluid will pass through 

 the first rock as through the second under equal pressure gradients, or 

 just as much of a given fluid will pass through the first as through the 

 second under half the pressure gradient, or under equal pressure gradients 

 just as much fluid will pass through the first rock as through the second 

 if the viscosity of the fluid in the first rock is twice that of the fluid in the 

 second. 



Geologists and engineers in the oil industry are interested in perme- 

 ability chiefly from the standpoint of measurement of permeabiliy and 

 use of the data thereby obtained for practical purposes. Accurate measure- 

 ment of permeability and precise use depend basically upon the validity 



of Darcy's law and analysis of the conditions to which it can be applied 

 with reasonable accuracy. Fortunately, research has established rather 

 clearly the limitations as well as the scope of Darcy's law. 



For example, an obvious limitation is that the porous medium must 

 be saturated with a homegeneous fluid for equations (8) or (9) to hold. 

 Implicit in this limitation, however, is the implication that the coefficient 

 of equation (8) is established solely by the geometry of the system and 

 is independent of the nature or kind of fluid. This implication can be 

 rationalized and put in accord with the fact that some fluids "react" with 

 some porous media through hydration, dehydration, or some other phe- 

 nomenon by means of the concept of a new medium or a change in 

 geometry of the medium resulting from these eff^ects. 



Another restriction is the requirement for linear relation of pressure 

 gradient and rate of flow in equation (8). Obviously, k, the coefficient of 

 proportionality (permeability) can be constant only if this linear rela- 

 tion obtains. By analogy to the flow of fluids in pipes, and not in a 



^^ American Petroleum Institute, Standard Procedure Jor Determining Permeability of Porous Media, 

 API Code No. 27, 2d ed., April, 1942. 



