Apparatus for the measurement of permeability is usually designed and 

 constructed by the operator, though commercial types are available. They all 

 consist of three basic parts : ( 1) a means of impressing air on the upstream 

 face of the plug; (2) a method of mounting the specimen to prevent by-passing 

 of the air; and (3) a method of measuring the air flow and pressure at the 

 downstream face of the specimen. 



The sample used in the porosity determination, if uncontaminated by this 

 test, or a fresh specimen cut parallel to the bedding plane and cleaned, is 

 mounted in such a way that the sides of the sample are sealed so that a pressure 

 differential can be applied across its length. Cylindrical samples can be inserted 

 in a soft rubber stopper which is forced into a tapered metal holder that com- 

 presses the rubber around the specimen; or a Hassler core holder, in which 

 the sample is slipped into a rubber tube and the sides sealed by air pressure 

 on the outside of the tube, can be used. Cubical or irregularly shaped samples 

 can be sealed with wax or plastic. After the sample is mounted, a pressure 

 differential is applied across its length with dry air; and the upstream pressure, 

 downstream pressure, and rate of flow are measured. 



The rate of flow can be measured in a number of ways. One of the simplest 

 is the collection of the evolved air in a graduated receiver by displacement of 

 water. Calibrated orifices or capillary tubes are most often used, and these can 

 be arranged so that combinations to cover any rate of flow are available. Other 

 methods utilize the rate of travel of a soap bubble in a vertical tube, or a drop 

 of mercury in a horizontal glass capillary. 



The rate of flow must be sufficiently low to avoid turbulent flow. This point 

 should be checked by making a series of determinations with various flow rates 

 and discarding those values that are obviously out of line. It is customary in 

 routine core analysis to maintain a rate of flow within certain predefined limits 

 by manipulation of the upstream pressure to insure viscous range. 



Klinkenberg (1941) discussed the phenomena of slip in the measurement 

 of the permeability constant by gases. He pointed out that the discrepancies 

 between air permeability and liquid permeability, where no interaction of rock 

 and fluid occur, were caused by the slippage of gas molecules and that a series 

 of measurements with gas at various average pressures extrapolated to infinite 

 pressure would result in an almost identical permeability constant. This has 

 subsequently been called the Klinkenberg effect. Though recognized, it is in 

 general, ignored by core analysis laboratories on the theory that permeabilities 

 are in a large measure relative and used principally for comparison; also, the 

 Klinkenberg effect is serious in the low permeability ranges only, and it is felt 

 that the results do not justify the additional work necessary to determine the 

 correction. 



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