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Permeability is a crucial parameter for characterizing gas transport and exchange in coal seams and is fundamental for designing and optimizing gas extraction and control methods. However, the permeability of coal seams is mainly determined through laboratory tests, with few studies using in-situ tests. To address the limitations of laboratory tests, such as their small scale and poor ability to provide engineering guidance, this study proposes a novel in-situ permeability test (IPT) system to investigate the fluid-flow aspects of geological formations. The system comprises the water-injection sealing and gas-injection testing subsystems. By considering the coal seam surrounding a gas injection borehole interval as a filter, the equation for calculating the in-situ coal seam permeability was derived based on steady-state gas-seepage theory. The IPT system was then assembled and used to test the in-situ permeability of the JI 15 coal seam at the Pingdingshan XII coal mine. The study estimated the representative elementary volume (REV) of the tested coal seam permeability, analyzed the applicability of Darcy's law during the tests, and obtained the anisotropic permeability of the coal seam by drilling test boreholes in different directions within the coal seam. Finally, the reliability of the new test method was verified by comparing its results with laboratory and well test data. The results demonstrated the effectiveness and reliability of the novel IPT system and encouraged further investigations into efficient gas extraction and control in deep coal seams. • A novel in-situ permeability test system based on steady-state flow theory is proposed. • The anisotropic permeability of in-situ coal seams is tested at engineering scale. • Τhe tested in-situ permeability are compared with the laboratory and well test results. [ABSTRACT FROM AUTHOR]