Miscellaneous Subsurface Methods 695 



to the hydraulic gradient and inversely proportional to the viscosity of 

 the fluid. Conseqeuently, 



which, if ^ be a coefficient of proportionality, becomes 



and for rectalinear flow of a simple fluid (figure 374), becomes upon in- 

 tegration, 



OA 



L 



(9) 



in which 



Q is the volume of a fluid in cubic centimeters. 



9 is the time required for Q, seconds. 



A is the cross-sectional area perpendicular to the rate of flow, square 

 centimeters. 



/i, is the absolute viscosity of the fluid, centipoises. 



Lp is the pressure difference, atmospheres. 



L is the length of the path of flow, centimeters. 



k is the coefficient of proportionality or the permeability of the porous 

 medium darcys. 



Clearly by this definition, the permeability of a porous rock to a 

 simple, homogeneous fluid is dependent entirely on the geometry of the 

 porous system; the permeability, i.e. the quantity k, has the dimensions 

 of an area {Lr) . The qualitative relation of permeability to porosity 

 likewise is clear, a rock being permeable by virtue of porosity. More- 

 over, a rock may be porous but impermeable, but cannot be both perme- 

 able and nonporous. The exact quantitative relation between the por- 

 osity and permeability has not been determined because of the complexity 

 of the problem. More specifically, the nature of the constant, k, has 

 occupied the attention of many investigators whose findings and experience 

 have been reported extensively in the literature. Although much of value 

 has been learned as a result of these efforts, a general solution to the 

 problem has not been obtained, despite the fact that all investigators 

 agree that the coefficient, k, is an expression of those characteristics of a 

 porous body which require that the relations implied by equation (8) be 

 satisfied and, consequently, that k is substantially independent of the 

 nature of the fluid and, therefore, is determined solely by the geometry 

 of the system. 



A rock has a permeability of 1.0 darcy, if the permeability be such 

 as to allow a flow of 1.0 cubic centimeter per second per square centi- 

 meter of cross-sectional area perpendicular to the path of flow of a fluid 



