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Outbursts comprise the predominant natural hazards in Chinese underground coal mines. Extracting high content gas within coal seams is one of the most effective ways to reduce the risk of triggering outbursts. However, with the depth increase of coal mining production, conventional gas drainage might be failed due to the low permeability of coal seams. Additional permeability enhancement methods should be adopted to improve the gas drainage efficiency. In this study, we proposed the application of liquid carbon dioxide fracturing for enhancing gas drainage in a underground coal mine in China with low permeability coal seams. By drilling cross-seam boreholes, liquid carbon dioxide fracturing was simultaneously performed in the two drilled fracturing boreholes. The effective damage radius of liquid carbon dioxide fracturing was further identified by continuously monitoring pure gas flow rates in different observation boreholes. After liquid carbon dioxide fracturing, there was no significant decrease in pure gas flow rate of observation boreholes for a period of 2 weeks. A conceptual model of liquid carbon dioxide fracturing was proposed for illustrating the mechanisms of improving gas drainage efficiency. Some methods for further optimizing gas drainage with the operation of liquid carbon dioxide fracturing under various thickness of coal seams and geological conditions were discussed. These results suggest that the application of liquid carbon dioxide fracturing are effective for gas control and hazards prevention in underground coal mines. [ABSTRACT FROM AUTHOR]

The extraction of coalbed methane can supply clean energy to society and improve the safety of subsurface coal mining. The low and ultra-low permeability of coal seams has resulted in the significantly low efficiency of methane production in surface and subsurface extraction systems. Fluid stimulation has been extensively applied for the enhancement of formation permeability. However, the response of coal seams to fluid stimulation may be different from other formations due to their unique structure and mechanical properties. Moreover, the presence of shear-fracturing stimulation and its effect on permeability enhancement is more significant in coal seams during fluid stimulation. In this study, a fluid injection-induced hydrofracturing treatment and shear-fracturing stimulation were separately investigated through laboratory experiments. The permeability of both intact and fractured specimens was used to directly evaluate the efficiency of these two types of fluid stimulations in different types of reservoirs. The experimental results indicated that the closure of hydraulic (i.e., tensile) fractures significantly inhibited the permeability enhancement of the hydrofracturing treatment in coal seams, and resulted in the permeability of fractured coal specimens were considerably lower than that of hard rocks (e.g., shale and sandstone). The results also demonstrated that fluid injection-induced shear stimulation in the coal specimens resulted in dilation behavior primarily associated with permeability enhancement. This was attributed to the fact that the exfoliated particles and masses prop shear fractures and increase their stiffness. It was found that, because of the weak mechanical and discontinuous properties of coal seams, an increase in the deviator stress improved the feasibility of shear-fracturing stimulation. Finally, fluid stimulation can be achieved using low viscosity fluids, such as L/Sc-CO 2 , as the fracturing fluid tended to form a shear dilation zone with high permeability in the coal seams. • The closure of hydraulic fractures inhibit the efficiency of hydrofracturing in coal seams. • The shear-dilation zone induced by fluid stimulation significantly enhance coal permeability. • The mechanical properties of coal and low viscosity fluid advance the feasibility of shear stimulation. [ABSTRACT FROM AUTHOR]

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]

Fracking to approach permeability enhancement is indispensable to enhance coalbed methane recovery, which can significantly reduce greenhouse gas emissions and produce substantial clean energy. To access efficient fracking, samples of Permian low permeable coal reservoir were cracked into different sizes or scales of blocks and particles with liquid nitrogen cryogenic freeze fracturing (FF) by maximizing the underlying heterogeneity of coal. To investigate the crack efficacy, this study systematically examined the permeability evolution and mesoscopic cracking behaviors of coal with different water contents and cleat–fracture systems in the context of cryogenic FF. Results showed that the permeability enhancement and microcracking tended to occur with increasing water content of coal sample; and the efficacy of cryogenic FF on the tighter coal sample was more remarkable. However, the permeability does not strictly increase with the cycles of cryogenic FF and has a close relationship with water content, structural plane direction, efficacy of cryogenic FF, and porosity compaction. The mesoscopic cracking behaviors indicate that numerous smaller pores are iteratively, not strictly sequentially, cracked and become connective in this process. The permeability evolution of coal sample is identified as significantly associated with the mesoscopic cracking behaviors. Notably, the first cycle of cryogenic FF acted on the detected several scales of pores and micro fissures, and partially caused these structures to be opened and interconnected to be permeable. Two main changes were observed in the microcracking of the coal samples in this study: 1) nonuniform shrinkage deformation and micro fissure expansion; and 2) pores opening mostly in the macropore and mesopore scale. It is reasoned that thermal cracking and intermittent opening of seepage pores due to the phase transition of free water in pores or micro fissures ultimately contribute to the permeability enhancement in low permeable and heterogeneous coal. [ABSTRACT FROM AUTHOR]