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The coal-rock projectiles induced by gas explosions in coal mines have a strong destructive effect on mine facilities and equipment, which are mostly made of steel. The LS-DYNA is used to simulate the process of coal projectile striking the steel plate. The results show that during the failure process, the morphological change of the projectile consists of three stages including plastic deformation, partial crushing, and complete crushing, and the steel plate pits at the impact point. The duration of the stress peak value of the steel plate increases as the impact velocity increases. The farther away from the center point, the smaller the stress peak value is, and the shorter the duration is. When the impact velocity is 100 m/s and 300 m/s, the axial velocity curve at the impact point of the projectile is pulsating. When the impact velocity reaches 500 m/s, the axial velocity of the projectile rapidly increases and then tends to be stable. As the impact velocity increases, the energy absorbed by the steel plate increases, and the rate of increase also increases. However, the proportion between the absorbed energy of the steel plate and the initial kinetic energy of the projectile decreases. The projectile conversion energy of the 1 mm steel plate is slightly larger than that of 1.5 mm and 2 mm steel plates, and the energy absorbed by the steel plate decreases with the increase in thickness of the steel plate. When the thickness of the steel plate is more than 2 mm, the thickness of the steel plate has little effect. The results of the study have a direct sense for the antiknock design of coal mine facilities and equipment.

At present, gas drainage technology from boreholes is an effective means of preventing gas disasters in deep mines and realise better utilisation of gas. To study the spatio-temporal evolution of gas flow fields around boreholes for gas drainage, it was postulated that the permeability of a coal mass is a non-free vector relating to spatial position based on anisotropic characteristics of the coal mass. The permeabilities of anisotropic coal mass in different directions were represented based on the principle of equivalent displacement and a model for coupling multi-physical fields of coal seams containing gas with structural anisotropy was established. Moreover, the spatio-temporal evolution of gas pressures and permeabilities of fractures in coal masses around boreholes were analysed. The results demonstrate that, differing from the conclusions of traditional mathematical models, gas pressures and permeabilities around boreholes for gas drainage in coal seams are anisotropic. The distributions of gas pressures and permeabilities present the vertical and horizontal figure of eight (8) shapes, separately. Over time, the vertical 8-shaped distribution of gas pressures gradually shrinks inwards, while the horizontal 8-shaped distribution of permeabilities expands outwards. With increasing distance from drainage boreholes, the trends therein were mutually contrary. The residual gas concentration in the vertical bedding direction of the borehole is about 1.4 times that of the parallel bedding direction under a same time period after gas drainage. The decrease of gas pressure can shrink the coal matrix and increase the effective stress on the coal mass. In the decrease of gas pressure caused by drainage over time, matrix shrinkage affects the permeability, while increasing effective stress on the coal mass plays a leading role in affecting the permeability in decreasing gas pressure resulting from spatial distance. Finally, the arrangement of boreholes in a coal mass and drainage affected zones under isotropic and anisotropic conditions were discussed. In the gas drainage project of underground coal seam, the layout of the borehole should consider the coal seam bedding direction.

Mining causes stress redistribution and stratum movement. In this paper, a numerical model was built according to the geological conditions in the 12th coal mine in Pingdingshan city to study the strata movement and the evolution of stress when mining two overlapping longwall panels, named panels #14 and #15. The strata close to the mined panel move directly towards the gob, while the strata that are farther away swing back and forth during the mining process. The directed movement and swinging can break the transverse boreholes for gas extraction; a surface borehole should not be within the range of directional movement. The stress evolution suggested that the mining of the lower panel #15 after the upper panel #14 would further increase the de-stressed range, while the stress concentration around the mined panel would be increased. Hard strata usually carry a greater stress than adjacent rocks and soft coal seams. The stress in a hard stratum increases greatly, and the stress decreases greatly in the coal seams below the hard stratum. This study supplies a reference for similar coal mines and is useful for determining the de-stressed range and transverse borehole arrangement for gas extraction.

Laboratory experiments and field application of high-voltage electrical pulse (HVEP) technology, a good method for coalbed methane (CBM) enhancement, have achieved great progress. However, previous studies mainly concentrated on the crushing effect of a single pulse on coal, ignoring the influence of multiple cyclic electrical pulses on the pore structure. Besides, the current in electrical breakdown was rarely studied. In this study, the effect of cyclic number on pore variation was investigated, and the mechanism of electrical breakdown was discussed by measuring the current waveform. Moreover, the micro fissure and pore variations of coal before and after electrical breakdown were measured by adopting scanning electron microscopy (SEM) and low temperature liquid nitrogen adsorption (LT-N2A). Under the action of cyclic HVEP, both Linhua (LH) anthracite coal and Hongliu (HL) bituminous coal can be crushed into small pieces. SEM results indicate that the number of cracks increase with the number of cycle. LT-N2A analyses show that the cumulative pore volumes and surface areas of LH and HL coals both increase after electrical breakdown, so does the pore structure fractal dimension, which provides a smoother channel for CBM desorption. Additionally, the great energy instantly injected into the coal could produce huge expansion stress and immediately break the coal from inside. Current waveforms during the breakdown demonstrate that the peak current increase with the number of cycle, whereas the breakdown time decrease with it. This indicates the alteration of electrical properties of coal during the breakdown, which affects the next discharge.

Unsteady creep of coal seams and the resultant abnormal flow of coal seam gas are the main reasons for dynamic disasters in deep mines. In this study, for investigating the evolution of gas depletion in deep mines, the flow field evolutions during gas adsorption and desorption were simulated first based a multi-field (coal deformation field, diffusion field and seepage field) coupling model considering the creep deformation. Firstly, a laboratory experiment was performed to validate the mathematical model. Then, spatial and temporal evolutions of matrix and fracture gas pressures, coal permeability and equivalent stress during gas adsorption and desorption were analyzed. The results show that in the initial stage of adsorption, gas pressure in fractures increases rapidly under the action of pressure gradient. As the gas gradually diffuses into matrix pores, gas pressure in the matrix goes up, which results in swelling deformation of coal matrix and alters the fracture aperture. Accordingly, coal permeability and equivalent stress were affected. During the desorption process, gas pressure in fractures drops rapidly at first, and then it falls at a lower rate. The diffusion of gas into fractures gradually leads to matrix shrinkage and enhances coal permeability. With a decrease in fracture gas pressure, the equivalent stress of coal declines rapidly first, followed by a slow rise induced by the diffusion of gas in the matrix. Furthermore, changes in the amount of gas desorption was analyzed by fitting the experimental test data in a specific time, which verified rationality of the mathematical model. The study can provide theoretical guidance for prevention and control of dynamic disasters in coalmines and the gas pre-drainage through borehole in deep mines.

A deeper understanding of the mechanisms by which geological factors (depositional environment and tectonic evolution) control the occurrence of coalbed methane (CBM) is important for the utilization of CBM resources via surface-drilled wells and the elimination of coal-methane outbursts, the latter of which is a key issue for coal mine safety. Based on drill core data, high-pressure isothermal adsorption experiments, scanning electron microscopy experiments, mercury intrusion porosimetry, and X-ray diffraction experiments, the impact of the depositional environment and tectonic evolution on CBM occurrence of the II-1 coal seam of the Shanxi Formation in West Henan was analyzed. Results showed that the depositional environment led to the epigenetic erosion of tidal flat coal-accumulating structures by shallow-delta distributary channel strata. This resulted in the replacement of the original mudstone-sandy mudstone coal seam immediate roof with fine-to-medium grained sandstones, reducing methane storage capacity. Epigenetic erosion by the depositional environment also increased coal body ash content (from 6.9% to 21.4%) and mineral content, filling the cleat system and reducing porosity, reducing methane storage capacity. The maximum methane adsorption capacity of the coal body reduced from 35.7 cm3/g to 30.30 cm3/g, and Langmuir pressure decreased from 1.39 MPa to 0.909 MPa. Hence, the methane adsorption capacity of the coal body decreased while its capacity for methane desorption increased. Owing to the tectonic evolution of West Henan, tectonically deformed coal is common; as it evolves from primary cataclastic coal to granulitic coal, the angle of the diffraction peak increases, d002 decreases, and La, Lc, and Nc increase; these traits are generally consistent with dynamic metamorphism. This is accompanied by increases in the total pore volume and specific surface area of the coal body, further increasing the capacity for methane storage. Increases in micropore volume and specific surface area also increase the ability of the coal body to adsorb methane. Keywords: Coalbed methane, Depositional environment, Tectonic evolution, Epigenetic erosion, Tectonically-deformed coal

Coal and gas outburst is greatly affected by the stress distribution in front of the heading face when tunnelling in high gassy coal seam. In this paper, the FLAC3D software and Mohr-Coulomb failure criterion are used to study the stress differences in front of the heading face. A total of 31 homogeneous models are built to study the stress differences affected by the coal seam mechanical properties. And another 8 kinds of heterogeneous models are built to study the stress differences affected by the abruptly changes of coal seam mechanical properties. When tunnelling in homogeneous coal seam, the large elastic modulus of coal seam will increase the stress peak value in front of the heading face, but do not change the stress peak position. The small internal friction angle will push the stress peak far away from the heading face. Therefore, the range of stress-relief zone in front of the heading face could significantly increase. The small cohesion of the coal seam will also push the stress peak far away from the heading face, but will result in an increase in the stress peak value. The Poisson ratio and tensile strength of the coal seam have little influence on the value and position of stress peak. As the elastic modulus decreases, cohesion and internal friction angle can greatly increase the displacement of the coal in front of the heading face. Taking into account both the mechanical properties of coal seam and the corresponding stress peaks, gas outburst is not easy to occur in the homogeneous coal seam. When tunnelling in heterogeneous coal seam, the abrupt change of coal seam elastic modulus greatly affects the stress concentration peak. If the cohesion of the coal seams also changes greatly with the elastic modulus, the stress peak will be enhanced. The abrupt change of coal seam cohesion and Poisson ratio have a slight influence on the stress concentration, while the abrupt change of internal friction angle of coal seam have little effect on the stress concentration. An outburst accident is most likely triggered when the cohesion and elastic modulus of the coal seams change greatly at the same time. Outcomes of this study is very beneficial for the prediction and prevention of coal and gas outburst. Meanwhile, this study provides a reference for the setting of mechanical properties of the coal seam and rock layers in the numerical simulation of mining.

Currently, hydraulic slotted borehole technology for strengthening gas drainage is one of the main measures to control gas disasters in deep mines. In this paper, to analyze the evolution of gas flow field around the hydraulic slotted borehole, the borehole gas drainage was simulated based on the anisotropy of coal structure and the established multi-field (namely, coal deformation field, diffusion field and seepage field) coupling model. Then, the influences of characteristic parameters, including, initial diffusion coefficient, diffusion attenuation coefficient, initial gas pressure and initial permeability, on the gas production rate and reservoir parameters of hydraulic slotted borehole gas drainage were analyzed. Finally, the results were verified by comparing the model simulation with field tests in Yangliu Mine. The results show that the anisotropy of coal affects the effect of borehole gas drainage. The initial diffusion coefficient D0 and the diffusion attenuation coefficient λ have opposite effects on key parameters of coal. With a larger D0, it is easier for gas in matrix pores to diffuse into the fracture system, and accordingly the gas production rate and relative permeability grow. A larger λ makes it harder for gas in matrix pores to diffuse into fractures, thus decreasing the gas production rate and relative permeability. The initial gas pressure pm0 and the initial permeability kfx0 and kfy0 exert the same effect on the key parameters of coal. The larger their values are, the more easily the gas in fractures can be extracted under the same negative pressure of drainage, which contributes to the diffusion of gas from the matrix into the fracture system under the action of pressure difference. Through combining actual monitoring results of 3# and 4# drilling fields in Yangliu Mine and the results of numerical simulation, it is acquired that the gas production rate measured in situ coincides with the rate simulated by the model, verifying the feasibility and engineering applicability of the model.

Hydraulic slotting in a gas drainage borehole is an effective method of enhancing gas drainage performance. However, it frequently occurs that a large amount of slotting products (mainly the coal slurry and gas) intensely spurt out of the borehole during the slotting, which adversely affects the slotting efficiency. Despite extensive previous investigations on the mechanism and prevention-device design of the spurt during ordinary borehole drilling, a very few studies has focused on the spurt in the slotting process. The slotting spurt is mainly caused by two reasons: the coal and gas outburst in the borehole and the borehole deslagging blockage. This paper focuses on the second reason, and investigates the hydraulic deslagging flow patterns in the annular space between the drill pipe and borehole wall. Results show that there are six deslagging flow patterns when the drill pipe is still: pure slurry flow, pure gas flow, bubble flow, intermittent flow, layering flow and annular flow. When the drill pipe rotates, each of those six flow patterns changes due to the Taylor vortex effect. Outcomes of this study could help to better understand the slotting-spurt mechanism and provide guidance on the anti-spurt strategies through eliminating the borehole deslagging blockage. Keywords: Coal hydraulic slotting, Cross-measure borehole, Borehole spurt, Deslagging flow pattern

The gas outburst potential is typically high when roadways uncover the gassy coal seams associated with outburst hazards. This paper describes the complex stress and displacement evolution that occurs in roadway tunnelling through coal seams under different original stress conditions, which is helpful for better understanding how the stress leads to an outburst accident, and proposing some relevant control measures. The stress concentration is primarily resulted from the strength difference between coal and rock, and the abutment stress increases gradually as the forward residual rock thickness decreases when the roadway excavation enters a coal seam from rock stratum. When the abutment stress is sufficiently high, the forward residual rock will suddenly be broken after the next cycle, and the stress will decrease rapidly to release elastic energy and generate new fractures. Meanwhile, the coal and rock in front of the heading face will move rapidly towards the free space. Therefore, an outburst accident may occur immediately when the gas pressure in front of the heading face is high enough. Once the outburst accident is triggered, the gas expansion energy and elastic energy distant from the heading face will contribute to developing the accident. The stress perpendicular to the roadway is the primary factor causing such outbursts. The abutment stress concentration will be higher when the maximum original stress is perpendicular to the roadway, which will increase the outburst potential. Therefore, it is suggested that arranging roadways parallel to the maximum original stress, if possible, can also reduce the outburst threat. Outbursts are triggered by the elastic energy near the heading face and developed by the gas expansion energy and elastic energy distant from the heading face. In this study, the method of “strong-weak” coupling circle-layers was proposed to reduce the outburst hazard when roadways excavated through gassy coal seams. This method was proven to be valid by the comparative experiment. The weak circle can be realized by boreholes and hydraulic cutting. It could not only reduce the elastic energy near the heading face but also prevent the distant energy from contributing to the outburst accident. In addition, the strong circle can be made by injecting water slurry and applying anchors. It could help to reduce the strength difference between coal and rock, thereby decreasing the local stress concentration, resisting the residual outburst energy near the heading face, and preventing the outburst from being triggered. The “strong-weak” coupling structure was successfully applied in Bailongshan coal mine when uncovering the #7,8 coal seam. This structure can effectively promote gas drainage, reduce the coal and gas outburst risk. As a result, the mining efficiency improves a lot, and the uncovering time greatly decreases from the expected 2 years to 5 months.

The equation used to model the unidirectional flow of methane gas in coal seams is usually formulated as a nonlinear partial differential equation, which needs to be solved numerically with a computer program. Nevertheless, for people without access to the computer program, the conventional numerical method may be inconvenient. Thus, the objective here is to seek some method simpler than the conventional one for solving the flow problem. A commonly used model of the unidirectional methane gas flow is considered, where the methane adsorption is described by the Langmuir isotherm and the free gas is treated as real gas. By introducing the similarity solution, a simple method for solving the flow model is proposed, which can be done on a hand calculator. It is shown by two examples that the gas pressure profile obtained by the proposed method agrees well with the direct numerical solution of the flow model. Keywords: Coalbed methane, Migration, Unidirectional flow, Similarity solution, Simplified solving, Numerical method

Traditional ECBM techniques, which cannot extract methane adsorbed in coal pores effectively, is still difficult to greatly improve coalbed methane recovery rate until now. Waterflooding or CO2-ECBM can displace methane adsorbed in nanopores but increases the risk of coal and gas outbursts. We proposed a novel coalbed methane recovery technique, e.g. high-pressure water and gas alternating sequestration technique (H–P-WGAS), which can displace adsorbed methane in coal pores without introducing other gases with high adsorption capacity. Additionally, the injected H–P gas can eliminate the water lock and be easily extracted during methane drainage. Numerical simulation and field test were conducted to verify effectiveness of the H–P-WGAS technique. The numerical results show that the H–P-WGAS technique can enhance methane migration in coal seam by changing pore pressure distribution and increase influence radius to 70 m at 20 MPa in 48 h. For water and air co-injection, methane content can decrease by up to 37.6%, which is much more effective comparing with that under single water injection or gas injection. Meanwhile, field tests show that the reduction of methane content by percentage lies between 25.14% and 80.62% with an average of 53.17%. Meanwhile, the influence radius of field test reached ∼48 m. The average methane drainage flow rate of single borehole increased by 2.65 times. Average methane drainage concentration increased from 31.3% to 47.6%. The methane drainage flow rate and concentration of the whole coal mine increased by 50.2% and 32.4%, respectively. The above results indicate that the proposed H–P-WGAS has best effectiveness comparing with single water injection or air injection.

Hydraulic flushing (HF) is an efficient technique for improving coal mine gas extraction . Although much research has been conducted in recent decades, many problems on its mechanism of enhancing permeability still exist. For example, how does coal strength affect the enhanced permeability by HF? Can a larger borehole generate a higher gas flowrate ? Therefore, a coupled gas flow-geomechanics model was developed based on an equivalent fractured-coal model to solve these issues. Plastic volumetric strain was introduced in this model to quantify the coal structure, which can improve the numerical models for coal permeability, gas flow in fracture and diffusion in matrix, respectively. The model was used to describe gas flow in the virgin and disturbed coal seams . Results show that neglecting coal failure can cause erroneous stress and permeability distributions, and lead to an overestimation of gas pressure and an underestimation of gas extraction amount. Effects of coal strength and borehole diameter on the distributions of stress, plastic zone, permeability and gas pressure were systematically studied. It indicates that the stress relief, plastic and enhanced-permeability zones around the boreholes in soft coal are obviously larger than those in hard coal. The stress-concentration level and permeability reduction in soft coal are greater than those in hard coal. After 500 d extraction, gas pressure in soft coal has a higher decrease than that in hard coal, indicating that the enhanced permeability by HF of soft coal is larger than that of hard coal. Besides, the investigation on the effects of borehole diameter shows that there exists an optimal borehole diameter. Gas pressure between the boreholes decreases before the optimal diameter, and increases sharply after the optimal diameter due to the high stress concentration. For the Pingmei 8th mine, the optimal borehole diameter is about 1.0 m. Field tests indicate that the optimized HF can considerably eliminate the risk of coal and gas outburst.

Protective seam mining is one of the most commonly used technique to realize stress relief and permeability enhancement of reservoirs, so as to improve gas drainage efficiency. However, for coal reservoir with a hard roof, such as the tight sandstone, magmatic rocks, the distribution and evolution of destressed zone and gas field have not been fully understood, which leads to an inefficient gas control in the field. In this work, taking the geological background of Pingmei 12th Coal Mine as an example, we numerically investigated the evolution and distribution of stress, permeability and gas field in the hard-roof stope during protective seam mining. The results show that in the zone with a distance greater than 3.7 m, stress in the hard-roof stope is significantly higher than that in the conventional stope and the permeability enhancement ratio in the hard-roof stope is significantly lower than that in the conventional stope. In the overlying strata, the existence of the hard roof inhibits the upward extension of the fractured zone, and the range of the fractured zone in the hard-roof stope is obviously smaller than that in the conventional stope. Therefore, in the hard-roof stope, the relieved gas accumulates in the gob, instead of entering the overlying fractured zone, which may put a threat to the mining safety. To solve this problem, the presplitting blasting technique was applied to cut the hard roof. After the collapse of the hard roof, the overlying fractured zone is fully developed, and the gas accumulated in the gob enters the fractured zone driven by the concentration difference and buoyancy. This can not only prevent gas overrun, but improve gas drainage efficiency of the upward borehole. This practice could be used as a valuable example for other coal mines with similar geological conditions.

Roadways excavated through a coal seam can exert an adverse effect on roadway stability. To investigate the effects of in-situ stress on roadway stability, numerical models were built and high horizontal stresses at varying orientations were applied. The results indicate that stress concentrations, roadway deformation and failure increase in magnitude and extent as the excavation angle with respect to the maximum horizontal stress increases. In addition, the stress adjacent to the coal-rock interface sharply varies in space and evolves with time; coal is much more vulnerable to deformation and failure than rock. The results provide insights into the layout of roadways excavated through a coal seam. Roadways should be designed parallel or at a narrow angle to the maximum horizontal stress. The concentrated stress at the top corner of the face-end should be reduced in advance, and the coal seam should be reinforced immediately after excavation. Keywords: Roadway stability, Numerical simulation, In-situ stress, Stress concentration, Failure, Deformation

Realized by the conversion of the electromagnetic energy into thermal energy, microwave heating can be a sound method to enhance coalbed methane (CBM) recovery. Although the effects of microwave heating on the petrophysical characteristics of coal have been comprehensively investigated, few studies considered the complex coal-gas interactions under non-isothermal conditions. In this study, a fully coupled electromagnetic-thermal-mechanical model was developed to investigate the influence of coal compaction, thermal expansion, thermal-induced gas desorption, and sorption-induced coal deformation on CBM extraction. The results indicated that the sorption-induced coal deformation makes the predominant contribution to the porosity evolution. In addition, microwave heating can enhance the cumulative gas production by 43.9% by promoting gas desorption and seepage. The sensitivity analysis showed that the microwave operating at 2450 MHz under lower powers is preferred to maintain efficient stimulation and to avoid overheating. Furthermore, the stimulation efficiency of microwave heating increases with the initial permeability and gas pressure. Outcomes of this study can provide valuable insights into microwave-assisted CBM recovery.

Coal seam degasification by underground drilling and its effect are directly related to mining safety and coalbed methane (CBM) recovery efficiency. The coal seam permeability is a critical parameter affecting the gas drainage effect and the evolution of gas flow field around the borehole. However, the distribution and evolution of coal permeability during gas drainage remains unclear and has resulted in failure of gas drainage radius determination and borehole arrangement in field applications. In this work, we propose an improved coal permeability model, which incorporates the coal deformation, gas diffusion in matrix, and gas flow in fracture. For the dual-porosity attribute of CBM reservoir, two gas pressures, namely, fracture and matrix gas pressures, and the swelling stress caused by gas adsorption have been introduced to improve the effective stress principle. Furthermore, because the classical unipore diffusion model fails to calculate the diffusion coefficient of coal masses, we propose a multistage pore model, which considers a widely distributed pore structure. Based on the multistage pore model, a dynamic diffusion coefficient is introduced in the permeability model. The improved permeability model is then compared with an analytical solution and field data. With the verified model, we analyzed the sensitivity of coal permeability to the key parameters in the model. The results show that the permeability increases with the increase of Langmuir adsorption constants a and b , initial permeability k 0 , and diffusion coefficient D 0 , and the decrease of the attenuation coefficient λ . From the analyses, we find that when a relatively large value is used for a , b , k 0 , and D 0 , and a relatively small value is given to λ , a phenomenon of permeability rebound, which is thought to be related to the matrix shrinkage, is observed during gas drainage. This phenomenon contributes to the improvement of gas drainage effect. Therefore, certain measures, such as protective seam mining and hydraulic flushing, can be adopted to accelerate the appearance of the permeability rebound. In addition, the determination of the effective drainage radius (EDR) is discussed from the perspective of dual-porosity medium. Both the fracture and matrix gas pressures are used to determine the EDR, and the results show that the EDR determined by matrix gas pressure matches well with the field test result. Therefore, it is concluded that the matrix gas pressure is the reasonable index for the determination of the EDR.

Faults are one of the important factors influencing coal and gas outburst in outburst coal seams. In this paper, 3# and 8# coal seams in Xinjing Coal Mine were taken as the research objects in the hope of revealing the mechanism of how faults affect coal seam. The pore structure and its resultant change on gas emission of coal samples at different distances from the faults were studied by means of scanning electron microscope, nuclear magnetic resonance and adsorption/desorption experiments. The results show that the fault significantly influences the pore structure. The coal sample closer to the fault has a greater number of well-developed macropores and fractures as well as better connectivity between pores and fractures. Besides, compared with 8# coal seam, 3# coal seam which is more notably impacted by the fault possesses a more developed pore structure at the micron scale. As the distance from the fault increases, the gas adsorption volume declines while the desorption hysteresis coefficient grows on the whole. The coal sample closer to the fault adsorbs a larger volume of gas and meanwhile desorbs gas more easily, so it is more prone to coal and gas outburst. The initial velocity of gas emission falls gradually as the distance from the fault increases. The desorption hysteresis coefficient generally drops with the rise of the initial velocity of gas emission. Judged by the influence scope of the fault, 3# coal seam possesses a much higher outburst risk than 8# coal seam. The research results reveal the mechanism of how faults affect coal pore structure and gas sorption characteristics, which can provide a basis for the prevention and control of gas dynamic disasters in fault-containing coal seams.

Hydraulic slotting, an efficient technique for underground enhanced coal bed methane (ECBM) recovery, has been widely used in China. However, its pressure relief mechanism is unclear. Thus far, only limited research has been conducted on the relationships among the mechanical properties, flaw parameters, and crack propagation patterns of coal after hydraulic slotting. In addition, because of the limitations of test methods, an in-depth information is not available for this purpose. In this work, numerical models of specimens containing combined flaws are established based on particle flow code method. Our results provide insights into the effects of flaw inclination angle on the mechanical properties, crack propagation patterns, and temporal and spatial evolution rules of stress field in specimens containing combined flaws during the loading process. Besides, based on the initiation position and underlying mechanism, three types of crack initiation modes are identified from the failure processes of specimens. Finally, the crack propagation pattern is quantitatively described by the fractal dimension, which is found to be inversely proportional to the uniaxial compressive strength and elastic modulus of the specimen. To verify the rationality of the numerical simulation results, laboratory tests were conducted and their results match well with those obtained from the numerical simulation.

The phenomenon that gas permeability decreases because of the water retention is known as water blocking effect (WBE), especially in the later period of gas drainage, the WBE is more serious. In order to improve the coal seam gas drainage performance as required in the real coal mine, experimental study was carried out for removing the water blocking effect (WBE) through adopting the nuclear magnetic resonance (NMR) method, and this study is conducted from two aspects of the pore negative pressure and surfactants. Results show that: with the increase of pore negative pressure, the variation of the T2 cutoff value (T2c) experiences three stages: “stable I-reduction-stable II”. When the pore negative pressure in the coal seam is less than 360.9 Pa, the external water in the coal seam can plug the coal pores and block the gas flow passage thus the WBE occurs consequently; When it gets larger than 769.68 Pa, the external water in the coal seam will be separated from the coal pores, conducive to the gas flow passage. The WBE in coal pores larger than 68.9 μm is called the temporary WBE, for which the water can be extracted, while the WBE in coal pores smaller than 68.9 μm is called the permanent WBE and the water cannot be removed by the pore negative pressure, unless the characteristics of fracturing fluid is changed. Anionic and nonionic surfactant can remove the WBE, which is beneficial to gas drainage. The cationic surfactant solution can inhibit the release of WBE, preventing gas migration in the coal seam and decreasing gas drainage efficiency. This study offers theoretical guidance for removing the WBE and enhancing gas drainage performance in the coal mine.

A semi-confined duct with an explosion suppression device was designed to investigate experimentally the active explosion suppression by non-premixed nitrogen and ABC powder for a premixed methane/air mixture. The duct was 20 m long with cross-sectional area of 0.08 m × 0.08 m. The results show that the total time from accepting flame signal to forming jet curtain barrier is about 117 ms, and the time from accepting flame signal to depositing ABC powder is 919 ms. The flame-arrival time differences between a flame transducer in front of spurting suppressant position and the triggering flame transducer in the suppression experiments are all between 117 ms and 919 ms, which indicates that the active explosion suppressions by spurting ordinary and superfine ABC powders are both successful. Spurting nitrogen and ABC powder has an obvious suppression effect on the explosion overpressure and the flame propagation speed. The maximum overpressure and the flame propagation speed at each point for spurting superfine ABC powder are both almost smaller than that of ordinary ABC powder. It indicates that the suppression effect of spurting superfine ABC powder and nitrogen was more apparent than that of ordinary ABC powder and nitrogen. After the first gas explosion propagation is controlled successfully, the multiple explosions propagation can still be suppressed absolutely by the surplus ABC powder and nitrogen. Research results will provide a new method for the suppressions of gas explosions in mine roadways and gas transmission pipelines.

The low permeability of coal seams is the primary difficulty for gas production. Hydraulic cutting is one of the most effective technologies for increasing permeability and stimulating gas production. This paper investigates the hydraulic cutting styles in boreholes and the optimal equivalent borehole diameter after discharging the pulverized coal particles. This paper introduces the hydraulic cutting equipment and cutting styles and determines that the cutting cylinder is the most suitable style for a soft coal seam that is prone to gas outburst. The numerical simulation suggests that the radius of the stress-relaxed range increases quickly when the equivalent borehole radius is small, and the rate slows when the borehole radius becomes large. Then the result was verified by a field experiment in Pingmei coalfield. The gas drainage flowrate and stress-relaxed range greatly increased after initially outputting 2 tons of coal from the borehole, whereas the second outputting 2 tons of coal did not appreciably increase the stress-relaxed range and the flowrate. Too much or too little coal discharge is not optimal, and the borehole radius is recommend to be between 0.3 m and 0.4 m after the hydraulic cutting considering the pros and cons in Pingmei coalfield. The results are helpful for the engineering application of the hydraulic cutting technology, for increasing coal seam permeability, and for stimulating gas production. The method for finding the optimal coal discharge in this paper is also helpful for other coal mines.

Although many permeability models have been proposed in last few decades to characterize the permeability evolution in poroelastic coalbed methane (CBM) reservoirs, none of the existing model is capable of defining the failure-involved coal permeability behavior which is the common situation for deep coal in-seam boreholes and over matured CBM reservoirs. In this study, a conceptual model named equivalent fractured coal (EFC) was initially proposed to characterize the structure of plastic deformation with new fracture generation. Based on the EFC model, the fracture dilation with deviatoric stress evolution within coal bulk is quantified by an exponential function, and a plastic deformation-based coal permeability (PDP) model was then proposed by incorporating the quantified fracture generation and dilation into a poroelasticity-based permeability model we built previously. Then the PDP model was validated by matching it with the tested permeability data in the unloading-flow experiments. The experiment results show that at the initial stage, the permeability increases slowly with a decrease of the confining pressure, which is mainly caused by the increase of fracture aperture. Later, with the further decrease of the confining pressure, the permeability increases by orders of magnitude, this is dominantly resulted from the generation of large amounts of new cracks. In addition, the results show that the PDP modeled permeability evolution well agreed with the experimental measured results for the whole process of unloading. This suggests that the PDP model cannot only predict the permeability variation under fracture generation and dilation process during plastic deformation, but also can describe that in the case of elastic deformation. The primary implications for this study is to provide a new mechanism-based approach to predict the permeability behavior in deep CBM reservoir during primary depletion or coal seams disturbed by underground coal mining.

The relationship between breakdown energy and the porosity growth rate were quantitatively investigated through nuclear magnetic resonance (NMR) and fractal dimension analysis. It can be found that the porosity of electrically broken coal sample increased, and the growth rate of macro-pores is much greater than that of meso-pores and micro-pores, which can promote the extraction of coalbed methane. Furthermore, the porosity growth rate of macro-pores change obviously with the increase of electrical breakdown energy, representing that larger energy can lead to more fractures. Micro-pores and meso-pores can be easily changed by small energy, while larger energy is conducive to the formation of cracks. Some meso-pores and micro-pores will be transformed into macro-pores, resulting in negative growth rates of meso-pores and micro-pores, indicating that electrical breakdown helps to expand primary fractures and generate new fractures. According to the pore size, there are two types of fractal dimensions, namely, adsorption pores and seepage pores. The results indicate that the value of fractal dimensions of adsorption pores is smaller than 1, verifying that adsorption pores do not follow the characteristics of fractal. However, the value of seepage pores lies within the range of 2–3 and boasts high fitting degrees, suggesting that seepage pores follow the characteristics of fractal. The value of DA increases while that value of DS decreases, which represents that electrical breakdown is more conducive to enhancing connectivity between macro-pores and tiny fractures. Moreover, there is no obvious positive correlation or negative correlation between fractal dimension and electrical breakdown energy, indicating that fractal dimension is the intrinsic property of the pore surface.

Hydraulic fracturing can increase coal seam fractures and reduce gas outburst risks, but the main challenge of its implementation in coal mines is to achieve uniform fracturing and gas reduction. In this paper, we studied the mechanism of water-driven gas release through two aspects: the H2O molecule is more easily adsorbed by coal than CH4, and water can decrease the gas adsorption capacity of coal. Comparative experiments of conventional hydraulic fracturing and blasting-fracturing technologies were carried out in #8 coal mine in Pingdingshan, China. We found that fractures and water contents around the blasting-based fracturing hole were more uniform than the conventional fracturing hole. After hydraulic fracturing, low water content coal usually has high gas content, which confirmed that gas in wet areas was displaced by water and driven to dry areas. To improve the uniformity of hydraulic fracturing, we proposed the uniform fracturing technology by combining blasting-fracturing holes and control holes. Blasting before hydraulic fracturing will increase the uniformity of fracturing and water distribution, and the control holes are used to monitor and release gas driven by water injection. The effective wetting range of conventional hydraulic fracturing was from −6 to 6 m along the strike direction and from −3 to 9 m along the dip direction with 94.71 m3 water injected. While the effective wetting range using uniform fracturing was increased to from −21 to 18 m along the strike direction and from −12 to 18 m along the dip direction with78.13 m3 water injected. Furthermore, number of boreholes to be drilled was largely reduced from 1300 to 132 when using the uniform fracturing technology to replace the dense hole technology in a 300 m long roadway. This research has suggested that uniform fracturing can effectively reduce gas outburst risk in gassy mines in China.

Gas extraction in destressed coal seams is one of the effective measures to improve degasification efficiency and reduce gas disasters in deep coal seams. In order to reveal the influence of damage zone induced by stress relief on gas extraction of slotted boreholes, a multi-field model incorporating stress, damage, gas diffusion and gas flow in heterogeneous coal was established. By using this model, the stress relief induced coal damage around slotted boreholes was simulated with the example of Guhanshan Coal Mine. Besides, the effects of homogeneity coefficient, Langmuir volumetric strain constant and overburden stress on coal seam damage and gas extraction were analyzed. The following results were drawn: The rationality and engineering applicability of the model were verified by comparing the numerical simulated results with the field test data in Guhanshan Coal Mine. The stress relief induced coal damage zone around slotted boreholes rises gradually with the increase of the homogeneity coefficient and overburden stress, and the coal seam permeability rises gradually with the increase of Langmuir volumetric strain constant and the decrease of overburden stress. The reasonable borehole distance is determined to be 8 m which is obtained based on the variations of total gas production and gas pressure with different borehole distances. Therefore, reasonable borehole distances under varying conditions can be determined by integrating different geological conditions of coalmines with the proposed simulation method. The simulation results can provide theoretical guidance for the layout of stress relief boreholes and the improvement of gas extraction efficiency in the future.

Formation microwave heating treatment (FMHT) is a potential method for degassing coalbed methane. Camera pictures and ultrasonic method were used to evaluate the crack propagation on the surface and in the interior of the coal cores, respectively. The results suggest that the fracture induced by microwave energy would remarkably increase with increased heating time. New fractures and pore sizes produced by microwave energy decreased P- and S-wave velocity in coal cores. The amplitude of the frequency spectrum of coal cores decreased after microwave energy treatment. The frequency distribution domain in the frequency spectrum of coal cores was also affected by microwave energy treatment. The density, bulk modulus and shear modulus of coal decreased with microwave energy treatment. Specifically, the slopes of declines in the bulk modulus and shear modulus were larger than that in the coal density. The increasing fracture network could improve the permeability of coal for degassing coal seams. In a word, the microwave energy has potential for assisting coalbed gas drainageing.

In practice, adsorption stress significantly influences the effective stress of a coalbed and thus the permeability during coalbed methane (CBM) recovery. However, in current coalbed permeability models, this stress is simply calculated from the adsorption strain and is rarely determined by experimental methods. In this paper, a tri-axial rock test system was used to measure this stress. Based on the experimental results, a Langmuir-like equation is able to accurately describe the changes in adsorption stress with gas pressure, and the adsorption stress with respect to adsorption amount is well described using a linear relationship. In addition, an effective horizontal stress model under uniaxial strain conditions is established. These models are directly established based on the experimental results and do not contain any uncertain parameters, and they are able to truly reflect the coal stress induced by gas adsorption and gas pressure. Finally, the evolution of coal adsorption stress with gas pressures and adsorption amount were analyzed, and the coal seam anisotropy was discussed. The models proposed in this paper shed light on the development of coalbed permeability models.

Degassing of a coal bed via cross-measure boreholes from an underlying drainage gallery (UDG) is a reliable method for safeguarding a gate road against gas outbursts. Predicting the influence of the UDG on the gate road plays a key role in the optimal design of roadways to ensure safe excavation, support, and maintenance. In this paper, numerical simulations were conducted to assess the effects of in situ stress and the roadway layout on coal bed methane capture effectiveness and the mechanical behavior of a gate road. The results indicate that the influence of the UDG on the gate road increases as the angle between the major horizontal stress and the gate road is increased or when roadway separation is decreased. After excavating the UDG, the permeability distribution across the coal bed can be divided into distinct zones. The gate road should correspond as closely as possible to the high permeability zone that develops in the rib area of the UDG, and permeability enhancement measures should be implemented in the low permeability zone. Staggering the roadways can prevent stress concentrations during the excavation of the gate road, but staggered roadways exhibit asymmetric distributions of plasticity and deformation. Therefore, roadway support in cases of staggered roadways should be biased toward the sidewall close to the adjacent excavation. These results provide important insights for layout planning and efficiency maximization of the UDG. Field test results matched well with the simulations.

Hydraulic slotting is an effective method for enhanced coalbed methane (ECBM) recovery, and it has been widely employed in China. Although there have been many studies of this technique, the influence of slot parameters on the strength characteristic of the coal, which is an important factor that affects the permeability enhancement effect, has rarely been studied. Thus, only limited information is available regarding the pressure relief and permeability enhancement mechanisms of this technique. In the current study, the influence of flaw parameters on the compressive strength of a precracked sample under biaxial compression is discussed. The results indicate that an increase in the flaw length and width has a negative effect on the compressive strength, whereas an increase in the flaw inclination angle has a positive effect on the compressive strength. The results of the response surface methodology (RSM) indicate that the interactions among the flaw parameters have a significant influence on the compressive strength. The propagation patterns of cracks are quantified by the fractal dimension, which is used to explore the mechanism of compressive strength variation with changes in the flaw parameters. The study results indicate that the variation in the flaw parameters changes the propagation pattern of cracks, resulting in different compressive strengths. In addition, an opposite variation trend of the compressive strength and fractal dimension with flaw parameters is also observed. The research results are expected to guide the field application of hydraulic slotting.

Coal bed Methane (CBM), a primary component of natural gas, is a relatively clean source of energy. Nevertheless, the impact of considerable coal mine methane emission on climate change in China has gained an increasing attention as coal production has powered the country's economic development. It is well-known that coal bed methane is a typical greenhouse gas, the greenhouse effect index of which is 30 times larger than that of carbon dioxide. Besides, gas disasters such as gas explosive and outburst, etc. pose a great threat to the safety of miners. Therefore, measures must be taken to capture coal mine methane before mining. This helps to enhance safety during mining and extract an environmentally friendly gas as well. However, as a majority of coal seams in China have low-permeability, it is difficult to achieve efficient methane drainage. Enhancing coal permeability is a good choice for high-efficiency drainage of coal mine methane. In this paper, a modified coal-methane co-exploitation model was established and a combination of drilling–slotting-separation–sealing was proposed to enhance coal permeability and CBM recovery. Firstly, rapid drilling assisted by water-jet and significant permeability enhancement via pressure relief were investigated, guiding the fracture network formation around borehole for high efficient gas flow. Secondly, based on the principle of swirl separation, the coal–water–gas separation instrument was developed to eliminate the risk of gas accumulation during slotting and reduce the gas emission from the ventilation air. Thirdly, to improve the performance of sealing material, we developed a novel cement-based composite sealing material based on the microcapsule technique. Additionally, a novel sealing–isolation combination technique was also proposed. Results of field test indicate that gas concentration in slotted boreholes is 1.05–1.91 times higher than that in conventional boreholes. Thus, the proposed novel integrated techniques achieve the goal of high-efficiency coal bed methane recovery.

Coal and gas outbursts comprise the predominant hazards in Chinese coal mines. A large number of accidents have occurred during coal roadway excavation. In this study, we propose the application of a cross-borehole hydraulic slotting technique for preventing and controlling coal and gas outburst disasters during coal roadway excavations. In this technique, a high-pressure water jet is applied in a coal body to cut a slot. This helps to increase the permeability of the coal seam, improve the gas-extraction efficiency of the borehole, and prevent and control coal and gas outburst disasters during excavations. In the field test in which hydraulic slotting was employed, the average mass of coal that was discharged from a slotted borehole is 8.2 t; the weight of the largest discharged mass is 16 t. The diameter of the slotted borehole is 12.87 times the diameter of a conventional borehole, which effectively improves the borehole's influence range. After half a month of gas extraction, the average gas-extraction concentration for the slotted borehole is 26%, whereas the average gas-extraction concentration of the conventional borehole is 7%, that is, the gas-extraction concentration for the slotted borehole is approximately 3.7 times the gas-extraction concentration of the conventional borehole. After four months of gas extraction, the residual gas content and gas pressure for the 11091 transportation roadway satisfy the requirement of the State Administration of Work Safety. A conventional borehole requires a minimum period of six months to prevent and control outbursts, whereas a slotted borehole requires a minimum period of four months. These results suggest that the technology can improve the efficiency of gas drainage, reduce the gas drainage time, and is a potential application for preventing and controlling future gas disasters.

Reducing gas content via cross-measure boreholes is one of the primary gas control technologies in China, where most outburst-threat coal seams are soft and highly gassy. Regardless of the significant costs associated with drilling boreholes, the gas drainage rate remains low because of the low permeability of the soft coal seam and the small influence zone of a single borehole. In this paper, the effect of increasing borehole diameter on coal seam permeability is discussed and a new method for drilling large diameter cross-measure boreholes by using the water-jet technique is proposed. Numerical modeling results indicate that the plastic zone and the effective influence zone of one borehole expand as borehole diameter increases, and the interaction between adjacent boreholes is strengthened. The field test shows that when the borehole diameter is 1.0 m, the effective influence zone radius reaches 4 m which is 2.67 times larger than that of an ordinary borehole. After using the new method, the number of cross-measure boreholes per hundred meters and the length of cross-measure boreholes per meter can reduce by 32.5% and 42.9%, respectively. In addition, the gas drainage rate reaches 52.1%, and the monthly excavation length of coal roadway increases from 50–70 m to 109 m.

Based on the difficult situation of gas drainage in a single coal bed of high gas content and low permeability, we investigate the technology of pulsating hydraulic pressure relief, the process of crank plunger movement and the mechanism of pulsating pressure formation using theoretical research, mathematical modeling and field testing. We analyze the effect of pulsating pressure on the formation and growth of fractures in coal by using the pulsating hydraulic theory in hydraulics. The research results show that the amplitude of fluctuating pressure tends to increase in the case where the exit is blocked, caused by pulsating pressure reflection and frictional resistance superposition, and it contributes to the growth of fractures in coal. The crack initiation pressure of pulsating hydraulic fracturing is 8 MPa, which is half than that of normal hydraulic fracturing; the pulsating hydraulic fracturing influence radius reaches 8 m. The total amount of gas extraction is increased by 3.6 times, and reaches 50 L/min at the highest point. The extraction flow increases greatly, and is 4 times larger than that of drilling without fracturing and 1.2 times larger than that of normal hydraulic fracturing. The technology provides a technical measure for gas drainage of high gas content and low permeability in the single coal bed. Keywords: Gas drainage, Pulsating hydraulic fracturing, Fatigue damage, Permeability improvement

Hydraulic slotting technique has been widely adopted in China for underground enhanced coal bed methane (ECBM) recovery. However, the related fundamental research of this technique is insufficient enough to get a comprehensive understanding of its action mechanism. In the current work, numerical models of specimens with various flaw inclination angles are built with particle flow code in two dimensions ( PFC 2D ). The relationship between mechanical properties and flaw inclination angle is analyzed, and the results show that (1) with an increase in the flaw inclination angle θ, the peak axial strain appears a decreasing trend after an initial increase and the peak lateral strain decreases gradually. With regard to the peak volumetric strain, when θ 30°, the peak volumetric strain is negative and the dilatancy occurs. (2) The elastic modulus and the initial damage coefficient show opposite variation trend with θ. Both of the relationships satisfy the Boltzmann function. (3) The peak strength and the initiation stress increase with θ. Furthermore, the damage constitutive equation of specimens with different flaw inclination angles under uniaxial compression is established with the consideration of the initial damage, loading damage, the stress–strain curves and crack number. The theoretical curves match well with the numerical curves and the achievements reported by other authors. The results of this study are expected to provide crucial guidance for the application of hydraulic slotting in ECBM recovery.

The study analyzed the evolution equation of the effective stress–permeability coefficient and the mechanism of pressure relief and permeability enhancement by water jet slotting. A FLAC 3D seepage module has been adopted for the computational analysis of the evolution of the coal effective stress, permeability enhancement rate and the pore pressure induced by the slots. Field experiment examined that the effects of slotting permeability enhancement and the dynamic features of methane drainage from boreholes. It was found by simulation that the effective stress in slotted coal was significantly reduced and the reduced area coverage was four to seven times larger than that in a conventional borehole. The pore pressure varied logarithmically with time during methane drainage. It was found in the field experiment that the methane drainage capacity was six times higher than a conventional borehole, and the significant region for methane drainage lay within 5.0 m, the transition region for methane drainage lay between 5.0 and 7.0 m, and the marginal region for methane drainage was above 7.0 m. The results of this research have provided guidance on the theories and parameters of applying water jet slotting technology to methane drainage from underground coal mines.

Because most coal mines in China have high gas content with low permeability, it is necessary to increase the efficiency of the CBM extracting technology. Enhanced coalbed methane (ECBM) recovery is a suitable option in this regard. This paper demonstrates a novel ECBM extraction technology, which involves the integration of hydraulic slotting (HS) and hydraulic fracturing (HF). The efficiency of the method is evaluated by field test. The slots produced by the HS method are used to control the extension direction of fracture cracks, and the efficiency of CBM extraction from the single drilling is improved by performing slotting drilling in coordination with fracturing drilling. The numerical software Coupled Analysis of Flow and Solid Mechanics in Rock Failure Process Analysis (RFPA2D-Flow) was used to analyse the direction of the crack propagation in the HF process. The coal-crack-propagation process, hydraulic gradient evolution, acoustic emission phenomena, and the change in the stress of coal body with the propagation of the fracturing crack were studied. The results of the study show that because the water pressure around the fracture bore is affected by the slots created, the hydraulic gradient in the horizontal direction is larger than that in the vertical direction, and the cracks more easily extend along the direction of the larger hydraulic gradient following the initiation of the cracks. After HF, a fully unloading area, a transition pressure-relief area, and the original stress area around the fracturing boreholes are formed. Results of the field test show that once the ECBM extraction is completed, the coal-seam disturbance range of the single drilling increases, and the CBM extraction efficiency is significantly improved. This novel ECBM extraction technology is a very promising method for improving the effect of the CBM extraction in the field.

Pulse hydraulic fracturing (PHF) is a new stimulation technique to enhance effectively permeability of coal seams, increasing the efficiency of gas extraction. However, the mechanism of PHF is still unclear, especially the effects of the key parameters. In this study, a series of experiments using a triaxial loading system were conducted to investigate the effect of pulse frequency on the fracture propagation and extension during PHF. The changes of pressure and acoustic emission (AE) energy rate during PHF were analyzed, and the relationships between frequency and pressure amplitude, fracture initiation pressure and duration were discussed. The results indicated that: (1) The changes of pressure during PHF can be divided into three periods: rising (R), plateau (P) and declining (D), and the P is the key period. (2) A lower frequency corresponded to a smoother pressure curve with a longer P period. In this case, the sample was “softened”, the fracture initiation pressure was decreased and more complicated fractures may be created. A higher frequency corresponded to a more seriously fluctuating pressure curve with a shorter P period. In this case, fractures may be easily formed due to the rapid increasing of pressure. (3) The grading PHF could take both the advantages of lower frequency and higher frequency, and thus could strike a balance between the fracture extension effect and the working efficiency. This study can provide a foundation for the improvement of pulse hydraulic fracturing in coal mines.

The variation of methane adsorption property of coal after the treatment of hydraulic slotting and methane pre-drainage was investigated by testing the seven coal samples obtained from the coal seam #10 of Yangliu Mine. The methane isothermal adsorption experiments were conducted to measure the adsorption constants and the Mercury Injection Capillary Pressure (MICP) and Nitrogen Gas Adsorption (N 2 GA) were organically combined to depict the pore size distribution (PSD). It is revealed that non-significant variation occurs in the various indexes of proximate analysis. With the increase of borehole distance, adsorption constant a presents a tendency toward enlargement and adsorption constant b has an opposite variation tendency, which is consistent with the change tendency of the curvatures of the curves. Remarkable variation occurs in pore size distribution of coal samples and adsorption pore proportion decreases from 55.38% to 33.27% with the decrease in borehole distance. The curves of coal parameters ( a , b , X abs ) present a characteristic of boundness and nonlinearity and the variation amplitude is “gentle-drastic-gentle”, which obeys the Boltzmann equation. There exists a significant sectionalized feature after the treatment of hydraulic slotting and methane pre-drainage. That is, region of significant influencing, region of transition and region of non-significant influencing. The gas pressure decreases and the effective stress increases after the treatment, which controls the adsorption property of coal. Hydraulic slotting and methane pre-drainage are sequential processes and each is a dominate factor that changes the methane adsorption property in relevant process. The results can provide reliable theoretical support for the application of hydraulic slotting.

In order to understand the kinetic characteristics of coal gas desorption based on the pulsating injection (PI), the research experimentally studied the kinetic process of methane desorption in terms of the PI and hydrostatic injection (HI). The results show that the kinetic curves of methane desorption based on PI and HI are consistent with each other, and the diffusion model can best describe the characteristics of methane desorption. Initial velocity, diffusion capacity and ultimate desorption amount of methane desorption after PI are greater than those after HI, and the ultimate desorption amount increases by 16.7–39.7%. Methane decay rate over the time is less than that of the HI. The PI influences the diffusion model parameters, and it makes the mass transfer Biot number Bi′ decrease and the mass transfer Fourier series F0′ increase. As a result, PI makes the methane diffusion resistance in the coal smaller, methane diffusion rate greater, mass transfer velocity faster and the disturbance range of methane concentration wider than HI. Therefore, the effect of methane desorption based on PI is better than that of HI. Keywords: Pulsating injection, Hydrostatic injection, Methane desorption, Kinetic characteristics

The paper presents a Monte Carlo simulation to study the adsorption characteristics of methane molecule on coal slit pores from different aspects. Firstly, a physical model of adsorption and desorption of methane molecules on micropores was established. Secondly, a grand canonical ensemble was introduced as the Monte Carlo simulation system. Thirdly, based on the model and system, the molecule simulation program was developed with VC++6.0 to simulate the isothermal adsorption relationship between the amount of molecule absorption and the factors affecting it. Lastly, the numerically simulated results were compared with measured results of adsorption coal samples of two different coal mines with a laboratory gas absorption instrument. The results show that the molecule simulations of the adsorption constants, the adsorption quantity, and the isothermal adsorption curve at the same and different coal temperatures were in good agreement with those measured in the experiments, indicating that it is feasible to use the established model and the Monte Carlo molecule simulation to study the adsorption characteristics of methane molecules in coal. Keywords: Methane, Adsorption, Molecular simulation, Slit pore, Isothermal adsorption curve

As for the coal seam with high stress, high gas and low permeability, a single technology cannot prevent the complex dynamic disasters. Because of this, the study proposes a new method of pressure-relief and permeability-increase technology of the high liquid–solid coupling blast. Through coal seam injection and charging structure change, the paper fully works out the dual functions of the water and explosion. Using the theoretical calculation, numerical simulation and physical experiments, we obtained that the initial blasting stress, displacement and overpressure of the liquid–solid coupling blast are much better than that of ordinary blasting. The technology has been used in the relative coal mine, and the application results show that the technique has effectively prevented the coal and gas outburst, which has a wide range of application. Keywords: High stress, High gas, Low permeability coal seam, Liquid–solid coupling blast

Variable frequency, a new pattern of pulse hydraulic fracturing, is presented for improving permeability in coal seam. A variable frequency pulse hydraulic fracturing testing system was built, the mould with triaxial loading was developed. Based on the monitor methods of pressure sensor and acoustic emission, the trials of two patterns of pulse hydraulic fracturing of single frequency and variable frequency were carried out, and at last fracturing mechanism was analyzed. The results show that the effect of variable frequency on fracture extension is better than that of single frequency based on the analysis of macroscopic figures and AE. And the shortage of single frequency is somewhat remedied when the frequency is variable. Under variable frequency, the pressure process can be divided into three stages: low frequency band, pressure stability band and high frequency band, and rupture pressure of the sample is smaller than that of the condition of single frequency. Based on the Miner fatigue theory, the effect of different loading sequences on sample rupture is discussed and the results show that it is better to select the sequence of low frequency at first and then high frequency. Our achievements can give a basis for the improvement and optimization of the pulse hydraulic fracturing technology. Keywords: Variable frequency, Pulse hydraulic fracturing, Fracture, AE, Fatigue

To improve the gas extraction efficiency of single seam with high gas and low air permeability, we developed the “fracturing-sealing” integration technology, and carried out the engineering experiment in the 3305 Tunliu mine. In the experiment, coal seams can achieve the aim of antireflection effect through the following process: First, project main cracks with the high energy pulse jet. Second, break the coal body by delaying the propellant blasting. Next, destroy the dense structure of the hard coal body, and form loose slit rings around the holes. Finally, seal the boreholes with the “strong–weak–strong” pressurized sealing technology. The results are as follows: The average concentration of gas extraction increases from 8.3% to 39.5%. The average discharge of gas extraction increases from 0.02 to 0.10 m3/min. The tunneling speeds up from 49.5 to 130 m/month. And the permeability of coal seams improves nearly tenfold. Under the same conditions, the technology is much more efficient in depressurization and antireflection than common methods. In other words, it will provide a more effective way for the gas extraction of single seam with high gas and low air permeability. Keywords: Coal gas, High-energy gas fracturing, “Fracturing-sealing” integration, Pressure relief and permeability increase, Gas extraction