Miscellaneous Subsurface Methods 699 



of grain size with distance results, and core-analysis data may make 

 possible determination of the constants in the following equation: 



k = Cr + b (12) 



Equation (8), integrated for the isothermal inward radial flow of a homo- 

 o^eneous liquid from a circular boundary through a zone in which perme- 

 ability varies according to equation (12), becomes 



Q _ 2TTbt{pe- Pu) 



(13) 



^ 1 



re{Cr^ + b) 



r,,.{Cre + b) 

 Measurement of Permeability 



The basis for the measurement of permeability from core samples 

 has been provided in the preceding discussion. The rate of flow of a 

 fluid of definite viscosity in response to an observed pressure gradient 

 through a sample of known or easily determined dimensions provide the 

 essential data.^^ Practically, a core holder ^^ of convenient form, which 

 can be combined with reliable and accurate auxiliary devices for obser- 

 vation of rate of flow and pressure gradients, is necessary, the assemblage 

 of equipment constituting a permeameter (fig. 377). Air usually is the 

 fluid used in a permeameter because of availability and convenience from 

 every point of view. Liquids such as oil-field salt water, or synthetic 

 brine may be preferred in the testing of samples containing clay and 

 other hydratable minerals. American Petroleum Institute Code 27 pro- 

 vides a convenient and complete guide and reference for the measure- 

 ment of permeability, and the details of procedure in measurement need 

 not be repeated. Samples for testing should be cut or shaped into con- 

 venient and simple geometrical forms so that the permeability can be 

 measured either perpendicular or parallel to the planes of bedding. 



Relative Permeability 



The concept of permeability was formalized, the unit defined, and 

 its measurement standardized on the basis that the permeability of a 

 rock or porous body constituting a reservoir for fluids was constant in 

 magnitude and uniquely determined by the geometry and mineral com- 

 position of the rock when the pores contained only a simple, homogeneous 

 fluid. Seldom, however, does an oil-bearing rock contain only a homo- 

 geneous fluid because of the presence of connate water. Usually two or 

 three fluids (oil, gas, water) compete for space. Under these practical 

 conditions of multifluid saturation, the movement of a particular fluid, 

 although obviously dependent in a qualitative sense upon permeability, is 

 impeded by the other fluids, the degree of interference depending upon 

 their distribution and the relative amounts present. In other words, the 



^^ Clough, K. H., Oil Weekly, vol. 83, no. 3, pp. 33-34; no. 4, pp. 27-34; no. 5, pp. 54-58, no. 6, 

 pp. 46-54; no. 7, pp. 42-50; no. 8, pp. 39-44. 1936. 



^^ Fancher, G. H., Lewis, J. A., and Barnes, K. B., op. cit. 



