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The microstructure of coal has a significant influence on the permeability of the coal seam. To study the characteristics of microstructure changes in coal seam under temperature impact, we conducted temperature-impact experiments using a high–low temperature test system, and we studied the coal pores and fissure structure before and after the temperature impact using scanning electron microscopy, industrial micro-computed tomography, and mercury intrusion. Based on the digital image processing technology and thermal stress theory, we qualitatively and quantitatively analyzed the variation of crack width, specific surface area, and pore diameter, and deeply analyzed the failure mechanism of temperature impact on coal seam microstructure. The results showed that the temperature impact caused the macropores to interpenetrate and form macroscopic cracks in the coal sample, which resulted in a relatively small volume of macropores and increased the volume of mesopores and small pores. The maximum thermal stress generated during the temperature impact process was located in the tangential direction of the coal sample surface. The thermal stress generated by the temperature impact exceeded the tensile strength of the coal sample, which directly causes crack initiation, expansion, and mutual penetration. This study provided the technical support necessary for the efficient development of coalbed methane and the improvement of gas drainage rate in the coal seam. [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]

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]