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ICS uses the Steady State technique, which is the most preferred technique for measurement of gas permeability.

Drawbacks of the Unsteady State Technique

  • Commonly available unsteady state permeameters have their limitations above a permeability value more than 1000 md. The unsteady state values have to be cross checked by remeasuring the permeability of the sample using the Steady State technique.
  • The unsteady state technique involves flowing a fixed volume of gas through the sample and monitoring the pressure drop v/s time. Small errors in calibration of the system can lead to large errors in calculation of permeability, especially in tight samples.
  • The technique involves the introduction (or drawdown) of the gas to (or from) the sample at starting pressures above 200 psi. This sudden drawdown causes dislodging of the surface fines especially among chalky limestones causing the system to generate a value lower than the actual. Moreover, these samples may become useless for further SCAL tests as the fine particles may tend to block the pores.
  • In automated systems, where the samples are loaded in a carousel, the dust from the samples tend to deposit in the valves in the system causing leaks and hence generating errors in permeability measurements.



Steady State Technique
Permeability to gas

Permeability to gas can be measured using Nitrogen or Air.

Klinkenberg Permeability is derived by measuring the permeability to gas at 4 different mean pressures and extrapolation the permeability to an infinite mean pressure value.

KAIR = K ∞ ( 1+b / Pm )


Slip factor b can be calculated from the data generated. Low permeability values show a much larger correction than high permeability values.
Typical Values are as follows:

Kair
K ∞
Ratio Ka/K∞
0.17
0.13
1.31
1
0.72
1.39
10
7.9
1.27
100
89
1.12
1000
951
1.05


Forchhiemer’s Inertial Resistance &beta can be used to correct predictions of production where non darcy flow is encountered especially those in gas reservoirs. It calculated as follows:

&beta = 4.20 Kair -1.35


Turbulence Factor &alpha is used to calculate the pressure drop flow rate behavior in high pressure gas wells

&alpha = 3.238 X 10-9 &beta kinf


 
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