--> Gas Permeability Measurements on Australian Subbituminous Coals: Fluid Dynamic and Poroelastic Aspects

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Gas Permeability Measurements on Australian Subbituminous Coals: Fluid Dynamic and Poroelastic Aspects

Abstract

The interaction and the coupling of slip-flow, a fluid dynamic phenomenon, and the cleat volume compressibility which is a poroelastic phenomenon has been investigated on two samples from the Taroom coal measure, Surat Basin, Queensland Australia (Gensterblum et al., 2014). Measurements were performed using inert (helium and argon) and sorbing gases (nitrogen, methane and carbon dioxide) at controlled effective stress. Apparent permeability coefficients decreased in the order helium >> argon = nitrogen > methane > carbon dioxide. Even after slip-flow correction different permeability coefficients were obtained for the same sample and identical stress conditions when different gases were used as permeating fluids. These observations are inconsistent with the concept of “intrinsic permeability” which, as a material property, should be independent of the permeating fluid. Obviously the sequence of the “intrinsic” permeability is identical to the sequence of increasing non-ideality. Therefore, it should be considered that the classical Darcy equation, which is derived using the ideal gas law has a reduced validity for non-ideal gases like N2, CH4 and especially CO2. The cleat volume compressibility cf was evaluated using the “matchstick approach” (Robertson and Christiansen, 2008). The cleat volume compressibility coefficients cf are almost identical for the two samples taken from the same well. However, for one sample a strong dependence of the cf with the mean pore pressure was observed. This is attributed to a strong slip-flow effect caused by a narrow cleat system as compared to the sister sample. The cleat volume compressibility coefficient cf is almost the same for non-sorbing and sorbing gases. The obvious strong coupling of slip-flow and poro-elastic properties is due to the generally more compressibility of coals in comparison to sandstones.