Fundamentals of Petrophysics by Shenglai Yang

Author:Shenglai Yang
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

6.2 Gas Permeability and Slippage Effect

6.2.1 Calculation Equations for Perm-Plug Method

When liquid is being used as the measuring fluid, the volume flow rate (Q) is a constant at all the cross-sections of the core, because that liquid can be regarded as incompressible. However, the situation is different for gas—as the volume of gas varies obviously with temperature and pressure. The pressure gradient is gradually decreasing pressure along with the direction of flow and causes expansion of gas volume and consequently increasing volume flow rate. Figure 6.3 shows this case. Given that the seepage flow of gas within the core is a steady flow which does not change with time, the mass flow rate at every cross-section is kept at constant along with its flowing through the core. If we consider that the flowing happens under isothermal conditions, the following equations can be obtained according to the Boyle–Mariotte’s Law:

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