Gas transport in shale: A critical review of experimental studies on shale permeability at a mesoscopic scale

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Shale is a porous geological medium, hosting natural gas in its porous system. The pore network of shale is used for multiple purposes; including storage of greenhouse gases, nuclear waste. Quantifying gas transport in shale, particularly the parameter permeability, is necessary for assessing reservoir performance of shale, and implementing gas recovery and storage operations. Laboratory scale experiments were found to be a fruitful approach in characterizing gas transport through shale. There is significant diversity in terms of method used to measure shale permeability at laboratory, multiple findings have generated from laboratory scale permeability data. In this present work, we have reviewed and critically analyzed all aspect related to experimental research on shale permeability at laboratory scale excluding the microfluidic experiments.We have described the flow regimes of shale, and the parameters governing the transition between different flow regimes. We followed it up by describing the laboratory methods to determine shale permeability and also compared the advantages and disadvantages of each technique. We critically assessed the role of slippage on shale permeability and models that quantify gas slippage. We then focused on laboratory scale shale permeability data, and it was analyzed in two segments. The first segment was permeability data measured at constant confining pressure and the second at constant effective stress. We particularly focused on the methods to determine effective stress coefficient through laboratory scale experiments, and how permeability of shale modifies with stress as well as slippage. Finally, we have described the anisotropy effect on shale permeability, and focused on the reasons for very low number of studies reporting relative permeability of shale through laboratory experiments. We realized sensitivity of shale permeability to stress is well researched and the findings are quite matured. The major thrust area in future should be reporting relative permeability of shale, by designing innovative laboratory methods.