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Can Gas Permeability of Fractured Shale Be Determined Accurately by Testing Core Plugs, Drill Cuttings, and Crushed Samples?


 Determining the nanodarcy gas permeability and other parameters of naturally and hydraulically induced fractured shale formations by testing the pressure transmission of core plugs, drill cuttings, and crushed samples is discussed. The author reviewed and modified the available methods for interpreting pressure tests with an emphasis on the differences between intrinsic and apparent permeability, and the generally overlooked temperature effects. It is significant to note that the temperature of gas varies during transport through porous rock samples and various dead-volumes when testing equipment used for permeability measurement is involved; this is because of unavoidable viscous dissipation and Joule-Thomson effects. Improved formulations and analysis methods that honor the relevant physics of gas transport and interactions with shale are presented, for both the generally assumed isothermal conditions and the realistic case of nonisothermal conditions. These improved formulations provide valuable insights when comparing and evaluating the currently available equations used for permeability calculations with the experimental data obtained by various testing methods. Better design and analysis of experiments for simultaneously determining several unknown parameters that impact the transport calculations, including deformation, adsorption, diffusion, viscous dissipation, Joule-Thomson effect, and deviation from Darcy flow, are described. It is recommended that the permeability and other parameters of shale samples be determined by simultaneous analysis of multiple pressure tests conducted under different conditions to accommodate temporally and spatially variable conditions by consideration of the temperature effect. The inherent limitations of the methods that rely on analytical solutions of the diffusivity equation on the basis of Darcy's law are also explained.