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Water seepage in rocks, in geotechnical engineering such as the hydrofracturing of hard rocks, excavation of underground chambers, and prevention of mine water disasters, is a common problem. According to rock mechanics theory, the deformation and stress of rocks influence seepage behavior. In this study, a modified permeability model of argillaceous sandstone under coupled hydromechanical conditions was established to reveal the relationship between permeability and effective stress, including external stress and internal water pressure. The modeling results indicate a negative exponential relationship between the argillaceous sandstone permeability and the effective stress. The proposed effective stress-sensitive permeability model was validated by conducting two sets of seepage experiments based on controlling the water pressure and external stress, with the results obtained considered satisfactory. Based on the proposed permeability model, a fully coupled multifield model of the water seepage and rock deformation was developed. Fully coupled scenario-based numerical simulations were conducted in a finite element environment to investigate water seepage evolution and rock deformation. The experimental and numerical results show that the trends in the evolution of the entire compressive stress–strain and permeability curves are reversed, and the maximum value of the permeability was not consistent with the failure of argillaceous sandstone. This model and corresponding numerical simulations can provide insights for water seepage research and serve as a reliable theoretical basis for evaluating roof water injection and hydraulic fracturing in rock and mining engineering. Article Highlights: Effective stress-sensitive permeability model was developed and validated experimentally Established fully coupled model of water seepage and rock deformation Numerical simulation revealed evolutions of the rock permeability and deformation [ABSTRACT FROM AUTHOR]

Reasonable use of different types of low-frequency vibration can better improve the production of coalbed methane (CBM). To understand the gas transport properties under low-frequency vibration in coal, a new permeability model of CBM reservoirs was proposed in this study. This model takes into account the deformation of coal matrix and fractures caused by vibration stress, gas adsorption and effective stress. In addition, an experimental system of low-frequency vibration was built to explore the influences of low-frequency vibration on permeability of coal samples. The experimental results were compared with theoretical permeability values to verify the reliability of proposed permeability model. Finally, a series of sensitivity analyses were conducted to identify key vibration parameters to the permeability of CBM reservoir. The results showed that the vibration time, vibration force and vibration frequency have significant effects on permeability of CBM reservoirs. The longer vibration time and the higher vibration force caused the greater damage of coal, resulting in the increase of permeability. However, with the increase of vibration frequency, coal permeability growth rate of coal first increases and then decreases, and a better permeability enhancement is observed near the resonant frequency. The experimental results indicate that the permeability growth rate of bituminous coal and anthracite coal is the highest at 25 Hz, reaching 371.8% and 268.5%, respectively. These results are expected to provide theoretical guidance for employing the low-frequency vibration to improve the gas extraction rate. [ABSTRACT FROM AUTHOR]

In the process of coalbed methane (CBM) exploitation, permeability is a key controlling parameter for gas transport in the CBM reservoirs. The CBM reservoir contains a large number of micropores and microfractures with complex structures. In order to accurately predict the gas permeability of the micropores and microfractures in CBM reservoirs, a fractal permeability model was developed in this work. This model considers the comprehensive effects of real gas, stress dependence, multiple gas flow mechanisms (e.g., slip flow, Knudsen diffusion and surface diffusion) and fractal characteristics (e.g., pore size distribution and flow path tortuosity) of micropores and microfractures on the gas permeability. Then, this fractal permeability model was verified by the reliable experimental data and other theoretical models. Finally, the sensitivity analysis is conducted to identify key factors to the permeability of CBM reservoir. The results showed that the fractal characteristics of the micropores and microfractures have significant effects on the permeability of CBM reservoir. Higher fractal dimension of micropores diameter and microfractures aperture represents the larger number of micropores and microfractures, resulting in a higher permeability. Higher tortuosity fractal dimension of micropores and microfractures means higher gas flow resistance, leading to the lower permeability. The multiple gas transport mechanisms coexist in micropores and microfractures of CBM reservoir. The permeability of slip flow and Knudsen diffusion both increases with the decrease in pore pressure. Surface diffusion is an important gas transport mechanisms in micropores, but it can be ignored in the microfractures. Knudsen diffusion plays a more obvious role in the lower pore pressure, which controls the gas transport in microfractures. And microfractures are beneficial to improve gas transport capacity of coalbed methane reservoir. [ABSTRACT FROM AUTHOR]

Production data of coalbed methane have shown that coalbed may be wet for a long time after the completion of water flow and water-gas two-phase flow stages. In this period, water flows out in moisture vapor, but the water in matrix does not change so much. The moisture loss is mainly from the water film in fracture network. Experiments also observed that such a moisture loss has a profound impact on the storage and transport of coalbed methane. However, this impact has not been investigated so far. This study investigates this impact through following works: firstly, a new conceptual permeability model is proposed based on water film adhered to the surface of fractures in a dual-porosity porous medium. The effect of water film is further described in gas flow equation by a non-Darcy law with threshold pressure gradient. Thirdly, a coupled multi-physical model is established to consider the interactions among coal deformation, gas flow, gas sorption and moisture loss. This model is validated by the gas production data of a coal seam in the Fruitland formation of San Juan basin. Finally, four scenarios are computed to comprehensively study the impact of moisture loss. These simulations show that the proposed model can well fit the history of gas production data. Non-Darcy flow has different velocity profile from Darcy flow. For the non-Darcy flow, the gas flow velocity increases quickly, then slowly, and finally decreases once gas starts to flow at a point. Moisture evaporation with gas flow mainly occurs in the zone near wellbore. This loss has a delay to the gas flow velocity. It also reveals that this moisture loss in coal seams can significantly improve coal permeability and thus enhance gas production. Therefore, the change of water film has significant impacts on gas production. [ABSTRACT FROM AUTHOR]