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Abstract
The mechanism of gas flow through tight gas siltstone formations is yet to be fully understood due to complex distribution of pore-throats over micro to nano pore-scales. The nano-pore structure causes gas flow to deviate from Darcy flow. Depending on gas properties, pore-throat size distribution and P-T conditions, a non-Darcy flow mechanism composed of slip flow, transition flow and free molecular flow may simultaneously undergo. It is essential to quantify these deviations from Darcy’s law and make correction to the laboratory measured gas permeability. This task requires a thorough characterization of the pore structure of tight gas siltstone formations and measuring its absolute/apparent matrix permeability.
In this study, we present experimental results of geological and petrophysical characterizations that delineate the mechanisms of gas flow through tight gas siltstone formations and their impact on formation permeability. In experiments, we used rock samples from the Permian Upper Rutherford Formation of Eastern Australia. The XRD and SEM tests on these samples define the Rutherford Formation as a clay-rich tight gas siltstone. The permeabilities of the core plugs, measured under an effective stress of 1,000 psi using pressure fall-off and pulse-decay techniques, show a range of 0.082-0.97 microdarcy. The measured helium porosities vary from 2.5 - 9.8%. The pore throat size obtained from a high pressure (60,000 psi) mercury intrusion test varies from 1.7 to 122 nanometers.
The Rutherford rock samples correction factor for the Knudsen apparent permeability to absolute permeability based on the modified form of the Beskok and Karniadakis (1999) model lies between 1.1 and 7.8. Knudsen’s correction is noted to optimally correct apparent permeability for an extremely tight sample. This correction is most pronounced when the Knudsen number exceeds 0.001, indicating that gas flow occurs under slip and transition flow regimes. Compared with Knudsen’s correction factor model, Klinkenberg’s model underestimates the permeability correction by roughly 25%, especially when gas flows at a high Knudsen number. This clearly suggests that a special attention must be given to the apparent permeability determination for tight gas siltstone and shale.