Summary Comments 



The above methods are all approved procedures, and other types of appar- 

 atus are usually variations or combinations of these. It is recommended that 

 porosity and permeability measurements be made on the same specimen; thus 

 the cube or cylindrical plug is usually prepared and the bulk volume determined 

 by calipering as the L/A factor is immediately available. Effective porosity is 

 almost always determined, there being little or no demand for absolute porosity. 

 Criticism can be leveled against all current methods of pore-space determination, 

 and none of these methods can satisfy rigid requirements for accuracy in all 

 ranges of porosity, but they are the methods that have survived the test of time 

 and judged within the tolerances of good porosity determination. 



Permeability 



Permeability is a measure of the capacity of a porous medium to transmit 

 fluids. The unit of permeability is the darcy, which is defined as follows: 

 A porous medium having a permeability of one darcy will, under conditions of 

 viscous flow, transmit 1 milliliter per second of a fluid of 1 centipoise viscosity 

 through 1 square centimeter cross-section when a pressure gradient or 1 atmos- 

 phere is applied. The commonly used unit is the millidarcy (0.001 darcy) (md) . 



The fundamental assumption of permeability is that as long as the flow 

 is viscous, the permeability of a porous medium is a property of the medium 

 and independent of the fluid used in its determination, and the same numerical 

 value should be obtained regardless of fluid used. Liquids, however, usually 

 interact with the rock material, and the numerical value of permeability by 

 liquid measurement is almost invariably lower than the value as measured with 

 a gas. It is also obvious that the presence of any liquid in the rock will have 

 an adverse effect on the measurement of permeability by any other fluid. Thus, 

 permeability measured with a dry gas on a clean and dried specimen of rock 

 is the maximum that can be obtained and is called absolute permeability and 

 designated by K. 



When the saturation of a particular fluid in the porous material is less 

 than 100 percent of the pore space, the permeability of the material to that 

 fluid is known as the effective permeability and is written with a subscript as 

 Ko, Kg, or Kw. The ratio of the effective permeability at a definite saturation 

 to the permeability at 100 percent saturation is termed relative permeability, 

 and designated as Kg/k, Kw/k, or Kg/Ko. Effective and relative permeability 

 will vary from a value of zero at some low saturation of the fluid to a value of 

 1.0 at 100 percent saturation of that fluid. 



Effective and relative permeabilities are concepts of interest to the reservoir 

 engineer. The permeability determined as a routine factor in core analysis work 



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