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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]

The greenhouse effect has attracted increasing attention globally. The geologic sequestration of CO 2 is considered as an effective method for reducing the amount of CO 2 in the atmosphere. Carbon-dioxide enhanced coalbed methane (CO 2 -ECBM) recovery is extremely attractive owing to its ability to store CO 2 in coal seams while simultaneously enhancing the production of methane. However, the injection of CO 2 can induce a deformation of the coal matrix, which has a significant effect on the fluid flow and stability of the coal seams. In this study, the sorption kinetics experiments indicated the high anisotropy and heterogeneity of local sorption deformation in coal samples and a unique kinetic behavior of sorption deformation which was rarely reported and analyzed in detail in previous studies. This kinetic behavior demonstrated a rapid increase of sorption strain followed by a slight or significant falloff until the equilibrium state reached. The characteristics of component distribution in coal samples were investigated using CT scanning and micro-lithotype and EDS analysis. An extensive analysis of the experiment results indicated the following: 1) Carbon dioxide dissolved during the first sorption process induced a rearrangement of the coal structure, leading to a drastic variation of deformation behavior between the first and subsequent sorption process. 2) The unique kinetic behavior of sorption deformation could be induced by either the loss of moisture in coal or the interaction of different coal components. The mechanism of these two factors induced coal shrinkage was different and had a distinct effect on the global behavior of sorption deformation. 3) The quantitative relationship between compressional and dilative components directly verified the significant compression of inertinite-rich regions by the slow swelling of vitrinite-rich regions. 4) The kinetic behavior of sorption deformation was affected by the combination of the macroscopic bedding structure and the heterogeneous component distribution in coal. • CO 2 dissolved during the first sorption process induces a rearrangement of the coal structure. • Kinetic behavior of local sorption deformation strongly depends on coal heterogeneity. • The slow swelling of vitrinite compressed inertinite in coal during sorption process. • A quantitative relationship between vitrinite and inertinite sorption strain was investigated. [ABSTRACT FROM AUTHOR]