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During deep underground coal mining, coal is typically fractured under high in situ and mining-related stresses, thus increasing the risk for dynamic disasters. However, knowledge regarding the mechanical and permeability characteristics of initially fractured coal remains limited. In this study, laboratory investigations were performed to examine the anisotropic strength, failure modes, and permeability of initially fractured coal subjected to true triaxial stresses. The results indicated that initially fractured coal can be classified into semi-brittle Class I failure modes. A new failure plane can be formed for initially fractured coal samples after failure, which was different from the intact coal samples' failure mode. The peak strength of initially fractured coal was typically observed when the maximum principal stress was subjected perpendicular to the bedding planes, while minimum values were obtained when the maximum principal stress was subjected parallel to the butt cleats. The brittleness of initially fractured coal was increased with intermediate stress. The permeability evolution curves for initially fractured coal exhibited an L-shaped trend. The pulverized coal grains in initially fractured coal can block the gas flow channel contributing to the continuous decrease in permeability. Intermediate stress exerts a more significant effect on permeability reduction when deviatoric stresses are applied parallel to cleats in coal. [ABSTRACT FROM AUTHOR]

To study the fracture law of different overlying strata during mining of the lower protective seam under a close distance coal seam at a depth of 1000 m, roof bed‐separation and fissure imaging monitoring were carried out in the inlet roadway ahead of the working face of the JI15‐31040 lower protective seam in the Pingdingshan XII coal mine. The results show that with the overlying strata of the lower protective seam being affected by mining, the displacement of the overlying strata in the roof of 0–4 m was relatively large, and the fissure development was very significant. The displacement of the overlying strata in the roof of 4–12 m was relatively small, the fissures in this range were mostly randomly closed fissures, and this area was stable. Bed separation occurred in the roof of 12–16 m, and the bed‐separating fissures were mostly located at the coal–rock junction. During the mining process, the fractal dimension of the roof gradually increased, and the fissure network developed continuously. The changing process of the fractal dimension of the roof can be roughly divided into three typical change areas: less influence area (distance from the working face > 70 m), rapid increase area (15 m < distance from the working face < 70 m), and stable area (distance from the working face < 15 m), and the area within 70 m in front of the working face was the key supporting area. Under the influence of mining, the roof fissures showed an overall change trend of "from zero to some, from short to long, from small to large, and from narrow to wide." This study can provide a reference for mining of the protective seam in deep coal seams. [ABSTRACT FROM AUTHOR]

Due to the insufficient understanding of the outburst mechanism, the coal and gas outburst disasters in China are more serious. Gas expansion energy is the main source of energy that causes outburst. In order to explore the distribution law of gas expansion energy in outburst coal seams, a gas-solid coupling equation of outburst coal seams was established. The distribution law of coal stress field, deformation field, gas flow field, and gas expansion energy were simulated and analyzed by using COMSOL Multiphysics. The results showed that from the excavation face to the deep part of coal seam, the stress presented unloading zone, stress concentration zone, and original stress zone. The volumetric strain and permeability reached the minimum, while the gas pressure reached the maximum at the peak value of vertical stress. As time goes on, the gas pressure in the fracture near the working face gradually decreased and was less than the pressure in coal matrix. The total gas expansion energy consists of free gas and desorption gas expansion energy. Affected by the excavation, free gas expansion energy maintained a constant value in the original coal seam and gradually decreased in the area close to the working face. The expansion energy provided by desorption gas was zero in the original coal seam. And it first increased and then decreased rapidly near the working face. Compared with stress and coal seam thickness, gas pressure and initial diffusion coefficient had significant influence on gas expansion energy of coal seam. When the diffusion coefficient was greater than 1e-9 m2/s, the gas expansion energy of the coal seam near the working face was significantly higher than that of the original coal seam, which had the risk of inducing outburst. [ABSTRACT FROM AUTHOR]

In underground coal mining, coal failure generally occurs due to the relatively weak strength of the coal and the high applied mining-induced stresses. The outer complex geological conditions (i.e., tectonic structure and water intrusion) and internal structural anisotropy of the coal introduce uncertainty in predicting its mechanical properties and failure behavior. In this study, laboratory investigations of the mechanical properties and failure behavior of dry and water-saturated anisotropic coal samples subjected to different true-triaxial loading stresses were conducted. The effects of water weakening, intermediate stress, and structural anisotropy on the mechanical properties and failure behavior of the coal were systematically studied. The results indicate that the presence of water significantly reduced the strength, elastic modulus, and strength anisotropy of the coal. The maximum stress at failure first increased and then decreased with increasing intermediate stress. The residual strength-to-peak strength ratios and failure plane angles of the coal showed a linear increase with increasing intermediate stress. When the coal samples were loaded in the bedding plane direction, the brittleness of the coal was higher than when they were loaded in the other two cleat plane directions. In addition, when the coal samples were loaded in the butt cleat plane direction, the brittleness of the coal decreased with increasing intermediate stress. Two typical failure modes of the dry and water-saturated coal samples were observed: shear and mixed splitting and shear failures. The dominant failure mode of the coal also varied with the loading direction relative to the weakness planes, which could be well recognized and predicted by the acoustic emission (AE) characteristic curves. To further reveal the fracture mechanism, the microcrack patterns of the coal were further identified based on the AE parameters. [ABSTRACT FROM AUTHOR]

Coal and rock mass constitute a type of porous medium. This study investigated the influence of temperature impact on the mechanical properties and acoustic emission (AE) characteristics of coal. A mechanism analysis was performed from the perspective of microstructure. The results show that the temperature impact causes the development of pores and cracks in the coal, which reduces the strength of coal. The elastic modulus of coal generally decreases with increasing temperature gradient. AE parameters increase with the increase in the load and reach the maximum value at the peak stress. AE parameters and cumulative parameters decrease with increasing temperature gradient. Not only does temperature impact change the fracture structure of the coal surface, but also the internal fracture structure of the coal is significantly affected. After temperature impact, the cracks expand and new cracks are initiated, and the fracture volume of the coal increases. Temperature impact causes the volume and specific surface area of small pores and meso-pores in coal to increase, and promotes the opening of the necking pores within the coal. The impact causes macro-pores to penetrate through to form cracks, which increases the transport of coal gas and significantly improves the permeability of coal. The thermal stress generated by coal under temperature impact is greater than its tensile strength, which promotes the cracking of coal, along with the initiation, widening, extension, and expansion of crack networks, which significantly change the fracture structure of coal. The research results lay a certain theoretical and experimental foundation for further study of mechanical properties of coal affected by liquid nitrogen. [ABSTRACT FROM AUTHOR]

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]

This study investigated the stress–strain–permeability relationship of a Chongqing coal under the stress path during the mining process. The abutment stress was first measured at the longwall (LW) face 3211 of Songzao Mine in Chongqing, China. The field monitoring results revealed that the concentration coefficient of the abutment stress was approximately 1.5–2.0 during protective layer mining. Then, triaxial compression tests for the gas-infiltrated coals were conducted under the above stress path and different gas pressures. These tests, with the simultaneous actions of unloading confining stress and loading axial stress, are called SUL tests. The triaxial compression tests revealed that the peak deviatoric stress and the corresponding strain of coal under SUL tests were lower than those under conventional triaxial compression (CTC) tests. Poisson's ratio was higher, but the elastic modulus was lower in SUL tests. The permeability evolution of coal under the SUL tests underwent four distinct stages: the increasing stage in the process of SUL, decreasing stage, slowly increasing stage beyond the yield point, and sharply increasing stage after the peak stress. With the increased gas pressure, the peak deviatoric stress and corresponding axial strain decreased, Poisson's ratio increased, and elastic modulus decreased. Further, the permeability of coal increased with increasing gas pressure in the complete deformation process. [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]

Because of periodic disturbance during coal seam excavation and support, the surrounding rock is subjected to cyclic unloading-reloading. To study the effect of pressure-relief on coal fields during gas drainage and mining, the permeability, acoustic emission (AE) and energy dissipation properties of coal under tiered cyclic unloading were experimentally investigated. Permeability enhancement rate (PER), AE signal change rate and a damage variable were defined to describe this process. Hysteresis, memory and Felicity effects are revealed. These results show that, the step-wise increase of strains and permeability display a hysteresis effect under tiered cyclic unloading. The axial residual strain increases exponentially, while the radial residual strain increases as a quadratic function, and the PER increases. The AE signals show Kaiser effect when the unloading capacity is small. Conversely, the Felicity effect is displayed when the unloading capacity is large. The cumulative AE signal and the axial strain exhibit good synchronisation and memory characteristics. The dissipated energy increases as a quadratic function, and the developed damage variable also increases, the critical damage variable of coal failure is estimated to be D c ≈ 0.8. • Seepage, AE and energy dissipation properties of coal and their intrinsic relationship are analysed. • Definition of PER and AE change rate for quantitative characterization of the effect of unloading confining pressure. • Hysteresis effect,Kaiser effect and Felicity effect under unloading confining pressure are revealed. • A new damage variable function based on dissipation energy is proposed under tiered cyclic unloading. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is one of the major serious natural disasters during underground coal, and the shock air flow produced by outburst has a huge threat on the mine safety. In order to study the two-phase flow of a mixture of pulverized coal and gas of a mixture of pulverized coal and gas migration properties and its shock effect during the process of coal and gas outburst, the coal samples of the outburst coal seam in Yuyang Coal Mine, Chongqing, China were selected as the experimental subjects. By using the self-developed coal and gas outburst simulation test device, we simulated the law of two-phase flow of a mixture of pulverized coal and gas in the roadway network where outburst happened. The results showed that the air in the roadway around the outburst port is disturbed by the shock wave, where the pressure and temperature are abruptly changed. For the initial gas pressure of 0.35 MPa, the air pressure in different locations of the roadway fluctuated and eventually remain stable, and the overpressure of the outburst shock wave was about 20~35 kPa. The overpressure in the main roadway and the distance from the outburst port showed a decreasing trend. The highest value of temperature in the roadway increased by 0.25 °C and the highest value of gas concentration reached 38.12% during the experiment. With the action of shock air flow, the pulverized coal transportation in the roadway could be roughly divided into three stages, which are the accelerated movement stage, decelerated movement stage and the particle settling stage respectively. Total of 180.7 kg pulverized coal of outburst in this experiment were erupted, and most of them were accumulated in the main roadway. Through the analysis of the law of outburst shock wave propagation, a shock wave propagation model considering gas desorption efficiency was established. The relationships of shock wave overpressure and outburst intensity, gas desorption rate, initial gas pressure, cross section and distance of the roadway were obtained, which can provide a reference for the protection of coal and gas outburst and control of catastrophic ventilation. [ABSTRACT FROM AUTHOR]

Coal belongs to sedimentary rock with obvious bedding planes. The existence of bedding weakens the continuity and integrity of coal. Deformation and permeability anisotropy of coals, to a certain extent, is attributed to the bedding planes. We performed experiments of deformation and permeability evolution of coals under true triaxial stress conditions. The experimental results indicate that the volumetric strain and bedding jointly affect permeability. An analytical model for stress-strain relationship of coals containing beddings is developed to quantify the bedding effects under true triaxial stress conditions. In the model, the deformation and permeability of the bedding and non-bedding are separated through bedding ratio and bedding elastic modulus. The bedding ratio is defined as the ratio of the total length of bedding to the total length of coal normal to bedding planes, which is a parameter closely related to the porosity. The elastic modulus is divided into four parts: a bedding elastic modulus and a non-bedding elastic modulus in the direction normal to bedding planes, and two elastic moduli in the direction parallel to bedding planes. The four elastic moduli are calculated by the analytical model. The bedding elastic modulus is relatively small, which shows that the bedding effect cannot be ignored. The permeability is highly sensitive to the bedding of coal with low stress in the direction normal to the bedding. The bedding ratio always affects the volumetric strain. The permeability of the coal is divided into bedding and non-bedding permeability. The ratio of the bedding permeability to the total permeability is different due to different stresses in the direction normal to bedding planes, which is linking with the compression of bedding. A permeability model which can reflect the different effects of beddings and non-beddings to the total permeability is proposed and verified. [ABSTRACT FROM AUTHOR]

Investigations of the directional permeability evolution of intact and fractured coal are conducted under different simulated geological conditions. The effects of fracture geometry, water adsorption and stress conditions on the permeability evolution of coal as a function of stress are systematically studied. The results indicate that permeability anisotropy is more pronounced in fractured coal than in intact coal. The permeability order, i.e., the k fa > k bu > k be relationship, is maintained following the introduction of macrofractures into coal. The fracture compressibility in the butt cleat plane flow direction is higher than that in the other two flow directions for both intact and fractured coal. The presence of water in coal can reduce the permeability by up to one order of magnitude, and a more significant permeability decrease is observed in coal specimens containing rough macrofractures. Permeability hysteresis for both intact and fractured coal is somewhat dependent on the stress condition. The hysteresis effect of coal is more significant under triaxial stress conditions and less pronounced under true-triaxial stress conditions. [ABSTRACT FROM AUTHOR]

In deep underground coal mining, engineering activities are performed within anisotropic in-situ stress fields due to engineering disturbances and tectonic stress. Many such activities involve the development of excavations in soft rock and anisotropic coal. Accordingly, studying the mechanical properties of soft rocks is important for the stability of deep underground excavations. In this study, the deformation, strength, and failure characteristics of soft sandstone and raw coal under two different true triaxial loading paths were investigated using a self-developed true triaxial test apparatus. The results indicated that the inelastic strain in the pre-peak stage of sandstone and coal gradually increased with increasing intermediate principal stress. Also, the strength-drop in the post-peak stage increased. The crack initiation stress, crack damage stress, and peak strength of sandstone and coal first increased and then decreased with increasing intermediate principal stress for a given σ 3. Moreover, with increasing intermediate principal stress, the failure mode of sandstone and coal changed from shear to tensile shear, and from brittle to semi-brittle. The linear Mogi criteria as found to characterize the true triaxial strength of coal well, while the modified Lade criteria was more applicable to soft sandstone. Owning to the symmetry assumption, the linear Mogi criteria predicted low strength when the intermediate principal stress coefficient exceeded to 0.5. In addition, the peak strength curve of rock on the π plane and the influence of weak structures on the failure mode of anisotropic coal was discussed. Weak structures have an important influence on the failure mode of coal, which depends on the strength difference between the structural plane and the coal rock mass. The strength envelope on the π plane had a significant stress Lode angle effect, which gradually decreased as the mean stress increased. • Influence of the intermediate principal stress on deformation, strength, and failure mode of sandstone and coal. • Different Mogi-type strength criteria were discussed and compared. • The interval effect of intermediate principal stress on coal rock strength was discussed. • The envelope of true triaxial strength of rocks on the π plane was discussed. • The influence of weak structures on the failure mode of coal was discussed. [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]